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SOME USEFUL TERMS & EXPRESSIONS
PETROLEUM GEOLOGY, GEOCHEMISTRY, EXPLORATION, ETC.
From a number of different sources of different age and varying quality – should be used with care, but hopefully of some use
(some has been scanned – and may have errors due to poor scanning)
Notice that many terms may have different/alternative meanings/definitions for different users/in different connections.
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ACCUMULATION - An occurrence of trapped hydrocarbons. See DEPOSIT.
ACTIVE SOURCE ROCK - A source rock that generates petroleum, either biogenically or thermally. If a source rock
was active in the past, it is either inactive or spent today. See CONTROLS.
AEROMAGNETIC SURVEY - Measurements of the Earth's magnetic field gathered from aircraft. Magnetometers
towed by an airplane or helicopter can measure the intensity of the Earth's magnetic field. The differences between actual
measurements and theoretical values indicate anomalies in the magnetic field, which in turn represent changes in rock type
or in thickness of rock units.
ALGAE -One of the most primitive plants consisting of a single cell or a cell aggregate of low organisation and without
vascular system; algae live in both fresh and marine water and range from diatoms to large seaweeds.
AMORPHOUS KEROGEN -Kerogen which, when examined through a microscope, lacks distinct form or shape; often
used to describe oil-prone material. See KEROGEN.
AMPLITUDE ANOMALY – (See HYDROCARBON INDICATOR). An abrupt increase in seismic amplitude that can
indicate the presence of hydrocarbons, although such anomalies can also result from processing problems, geometric or
velocity focusing or changes in lithology. Amplitude anomalies that indicate the presence of hydrocarbons can result from
sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used
synonymously with hydrocarbon indicator.
AMPLITUDE VARIATION WITH OFFSET (AVO) - Variation in seismic reflection amplitude with change in
distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below
the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density,
velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and
seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model
other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might
show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique
solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gasfilled reservoir from a reservoir having only partial gas saturation ("fizz water"). However, AVO analysis using sourcegenerated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more
successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented
sediments. (See HYDROCARBON INDICATOR)
API GRAVITY -The standard American Petroleum Institute method for specifying the density of crude petroleum. The
formula used to calculate API gravity is:
°API = 141,5 -(131,5 * specific gravity at 60 °F)
 Low gravity (heavy oil) <20° API
 Medium gravity 27° API+
 High gravity (light oil) >35° API
APPRAISAL - The phase of petroleum operations that immediately follows successful exploration drilling. During
appraisal, delineation wells might be drilled to determine the size of the oil or gas discovery and how to develop it most
efficiently.
APPRAISAL WELL - See WELL
AQUIFER - A water-bearing portion of a petroleum reservoir with a water drive
AREA OF CLOSURE – See CLOSURE AREA
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ASPHALT/ASPHALTICS -Dark brown, viscous liquid or low-melting solid that contains asphaltenes and resins, is
soluble in carbon disulphide and insoluble in n-heptane. Asphaltenes occur as tiny colloidal particles dispersed in the oil,
and are commonly isolated in the laboratory by precipitation in pentane (see below). The higher the proportion of
asphaltene, the harder the residual asphalt.
ASPHALTENES -Constitute a general class of aromatic-type substances which are defined on the basis of their
solubility: Asphaltenes are soluble in carbon disulphide and insoluble in light alkanes such as n-pentane and n-heptane.
Asphaltene molecules carry a core of stacked flat sheets of condensed (fused) aromatic rings linked at their edges by
chains of aliphatic and/or naphthenic- aromatic ring systems. The condensed aromatic sheets contain NSO atoms and
probably vanadium and nickel complexes.
ASSOCIATED GAS – See GAS, NATURAL.
AVO - See AMPLITUDE VARIATION WITH OFFSET and HYDROCARBON INDICATOR
BASIN - A depression into which sediment is deposited. Also refers to the basin fill. Sometimes used to mean petroleum
province and sedimentary basin.
BIODEGRADATION -This term normally refers to the alteration of petroleum by living microorganisms, primarily
bacteria, which may occur when a reservoir is exhumed, when oil seepage reaches the surface or near surface, or when
meteoric water is introduced into an accumulation. Oils which have been biodegraded exhibit distinct compositional
changes, including:
-loss of n-alkanes
-increase in the content of asphaltenes
-increase in sulphur content and decrease in API gravity
BIOGENIC GAS -Methane which is produced by the action of bacteria on organic matter at relatively shallow depths.
Compositionally, it consists almost entirely of methane (99.9%) which is isotopically light when compared to thermogenic
gas.
BIOLOGICAL MARKER (BIOMARKER) -Compounds found in petroleum or rock extracts which possess a carbon
skeleton that indicates an unambiguous link with a natural product. The most common biomarkers in petroleum include
isoprenoids, triterpanes and steranes.
BITUMEN –The term is used in two distinct senses:
(a) A general name for the fraction of organic matter in sediments and rocks that is soluble in benzene, benzenemethanol, carbon tetrachloride, etc. (i.e., extractable organic matter) -a complex mixture of hydrocarbons,
asphaltenes, NSO-containing compounds and other heterocyclic non-hydrocarbons.
(b) An even more general name for various solid, semisolid, and liquid hydrocarbons.
BLOCK - part of a sedimentary basin, formed by a vertical prism of undetermined depth with a polygonal surface defined
by the vertices of its geographical co-ordinates, in which petroleum exploration and production activities are conducted
(definition found in a petroleum law). In some countries a Block is equal to the contract area, in other countries a Block is
the “grid unit” in a grid system used to
BRIGHT SPOT – See HYDROCARBON INDICATOR
BURIAL HISTORY -The burial of one or more horizons traced through time is known as burial history. The data are
usually represented graphically as depth-time plots (BURIAL HISTORY CHART) and are used in conjunction with
thermal models to estimate the maturation history of source rocks.
BURIAL HISTORY CHART - A burial history curve or geohistory diagram constructed to show the time over which
hydrocarbon generation occurs. Depicts the essential elements and the CRITICAL MOMENT for the PETROLEUM
SYSTEM (See this).
C1 -C4 -Geochemical analysis of methane (CI) through butane (C4 ) gases in cuttings, waters. Synonymous with cuttingsgas.
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C4 -C7 -Geochemical analysis of butane (C4) through heptane (C7), liquid hydrocarbons in sediments, oils, and waters.
Synonymous with gasoline-range hydrocarbons.
C4 -C19 -High-resolution chromatographic analysis of butane (C4) through nonadecane (C19), liquid hydrocarbons in
oils.
C15+ -Geochemical analysis for heavy liquid hydrocarbons (saturates and aromatics) and non-hydrocarbons (NSO
compounds and asphaltenes).
CARBOHYDRATES -A class of natural organic compounds composed of carbon, hydrogen, and oxygen in units of five
or six carbon atoms. They include sugars, starches and cellulose.
CARBON DIOXIDE -Gas, formula CO2, which is the fundamental form in which carbon is cycled through plants,
animals and carbohydrates.
CARBON NUMBER -Refers to the number of carbon atoms in a molecule.
CARBON PREFERENCE INDEX (CPI) -Generally the ratio of abundance of odd over even carbon number paraffins,
specifically calculated over the C24-C34 range. Hydrocarbons derived predominantly from terrestrial sources and/or at
low maturity exhibit a predominance of odd-numbered alkanes (resulting in a high CPI); this feature disappears with
increasing maturation level (CPI approaches 1.0).
CHARGE, HYDROCARBON -The amount of migrated oil and/or gas available to a trap.
CHEMICAL REACTION EQUILIBRIUM -A state of balance between two or more chemical reactions. The amount of
any substance being built up is exactly counter-balanced by the amount being used up in other reactions, so that
concentration of all participating substances remains constant.
CHEMICAL REACTION KINETICS -Study of the rates at which reactions occur.
CHROMATOGRAMS -Record of detector response (intensity vs. time) during gas chromatography. individual
hydrocarbons appear as peaks above a baseline, and may be identified and quantified. CHROMATOGRAPHY -A
technique for separating components in a mixture based on their physical or chemical properties.
CLOSURE AREA - The area of a trap that could hold oil or gas, such as the area within a trap above the spill point.
CLOSURE HEIGHT - The vertical distance between a trap's highest point and the lowest level that could hold oil or
gas.
COAL -. A concentration of fossilised organic matter which is greater than 50% of a rock by weight is called a coal. Vast
majority of coals originated in plant debris that accumulated in situ as swamp peats. Metamorphic classification ranges
from peat through lignites and bituminous coals to anthracites. Visual classifications usually designate two major coal
types: humic (including fusinite, micrinite, vitrinite, exinite, resinite) and sapropelic (including cannel and boghead). Chief
constituent typically is VITRINITE (woody remains, type III KEROGEN), and other constituents may include EXINITE
(herbaceous material, type II KEROGEN) and INERTINITE (charcoal).
COMMERCIAL DISCOVERY - a discovery of Petroleum which, under conditions including the market price, enables a
return on an investment in the development and production (definition found in a petroleum law)
CONDENSATE - See NATURAL GAS LIQUIDS. A product of condensation -known sometimes as distillate; contains
hydrocarbons in the C4 to C1O range. Condensates occur in gas reservoirs of great depth and high pressure, are normally
in vapour phase, but condense as reservoir pressure is reduced by production gas. Petroleum geochemists commonly use
this term to refer to liquid hydrocarbons which are dominantly gas and light oil.
CONDENSATE YIELD - is the yield (volume) of condensate per unit volume of wet gas.
CONTRACT AREA - the area within which the holder of an Exploration and Production right is authorised to explore
for, to develop and to produce Petroleum (definition found in a petroleum law)
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CONTROLS, GEOLOGIC -The essential factors controlling significant oil and gas occurrence. All must occur
adequately, coeval1y and contiguously, or else the chances for a field are eliminated. Hence, they are also the key RISK
FACTORS to be evaluated in assessment:
 SOURCE ROCK - A sedimentary rock (commonly shale or limestone) whose organic matter has been
transformed naturally by heat through time and burial into oil and/or gas. This transformation is called
GENERATION or MATURATION. The organic matter must be adequate in amount (concentration, areal
extent, thickness), type (quality), and maturation.
 RESERVOIR ROCK - A rock unit containing or potentially containing recoverable oil or gas in its small open
spaces. The reservoir rock must be adequate in thickness, porosity, and permeability. (POROSITY is the
percentage of small open spaces in a rock's bulk volume; PERMEABILITY is the capacity of a porous rock to
transmit fluids.) Risks are that the reservoir unit is missing by facies change, unconformity truncation, or faulting,
or that the porosity has been cemented or not developed by necessary solution or fracturing.
 MIGRATION - Migration is the process whereby hydrocarbons move from source rocks to reservoirs through
openings in rock. Migration is divided into three categories:
o primary migration - movement of organic materials within the source rock to its boundary where oil
and gas leave the source rock;
o secondary migration - movement from the source-rock boundary to the reservoir/trap (or are lost as
seepage), through permeable rocks (CARRIER BEDS) or faults or fractures. For migration to be
effective, permeable paths must be adequate and the trap must be present when the oil and gas migrate
(TIMING).
o tertiary migration where oil and gas move from one trap to another or are lost.
 TRAP - A barrier to migration that allows oil and gas to accumulate in a reservoir. The barriers commonly are
impervious rock (SEALS) above, below, and/or lateral to the reservoir rock. STRUCTURAL TRAPS result from
folding, faulting, or other rock deformation. The most common trap is an anticline or faulted anticline.
STRATIGRAPHIC TRAPS result from lithologic (facies) changes sometimes called POROSITYPERMEABILITY PINCHOUTS. A trap with both structural and stratigraphic aspects is called a
COMBINATION TRAP. A trap must have adequate size and seals. TRAP FILL is the (potential range of)
trapped hydrocarbon volume as a percentage of the porous volume under closure
 PRESERVATION - Once in the trap, protection of hydrocarbons from flushing, overcooking, or biodegradation.
FLUSHING is water washing. OVERCOOKING is overmaturation to less desirable products.
BIODEGRADATION is decomposition by micro-organisms.
 RECOVERABILITY - The ability to bring underground oil and gas to the surface. Factors that must be
adequate are the permeability of the reservoir, low VISCOSITY (resistance to flow) of oil, and reservoir DRIVE
(the motive force required to produce hydrocarbons).
CONVENTIONAL OIL - crude oil capable of flowing, has an °API value greater than 20
CRACKING - A process whereby larger hydrocarbon molecules are broken down into smaller hydrocarbon molecules.
Catalytic cracking is a refinery process which breaks down the molecules through the use of a catalyst which speeds up the
reaction. The term is also used to refer to the thermal decomposition of petroleum in the reservoir.
CRITICAL MOMENT - The time that best depicts the generation-migration-accumulation of hydrocarbons in a
PETROLEUM SYSTEM (See this). A map and cross section drawn at the critical moment best shows the geographic and
stratigraphic extent of the system.
CRUDE OIL - Unrefined oil, usually with gas in solution, as found in accumulations in the subsurface. (definition found
in a petroleum law: crude mineral oil, asphalt, ozocerite and all kinds of hydrocarbons and bitumens, both in solid and in
liquid form, in their natural state or obtained from Natural Gas by condensation or extraction, but does not include coal, or
any substance that may be extracted from coal.)
CUMULATIVE PRODUCTION - amount of fuels produced from start of production up to date
CUTTINGS-GAS ANALYSIS - A specific geochemical analysis; cuttings from drilling operations are canned with water
at the well site. In the lab the cuttings are homogenized in a blender, and any gases released are transferred to a
chromatograph. Methane through butane (CI to C4) are measured. Results are used to evaluate both kerogen type and
maturity.
CYCLIC -Refers to those compounds having ring structure:
(1) Homocyclic -only carbon atoms in the ring.
(2) Heterocyclic -one or more of the ring atoms are elements other than carbon, e.g. sulphur.
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DEAD OIL - Oil at sufficiently low pressure that it contains no dissolved gas or a relatively thick oil or residue that has
lost its volatile components.
DEASPHALTING - Refinery or lab procedure for precipitating asphaltenes out of complex hydrocarbon mixture (e.g., a
crude oil) by adding pentane, hexane, etc. The same process may occur naturally when methane and other gases move into
crude oil reservoirs.
DECOMMISSIONING PLAN - a plan for the closure of Petroleum Operations including the removal and disposal of all
installations.
DELPHI METHOD - One which utilizes the averaging of several expert opinions when evaluating the geological RISK
factors and/or the probability distributions of undiscovered resources.
DEPENDENT VARIABLE – One variable influenced by changes in another variable.
DEPLETION MID-POINT - The time at which half of the original reserves of the total potential has been produced
DEPOSIT - an ACCUMULATION of petroleum in a geological unit delimited by rock types through structural or
stratigraphical boundaries, interfaces between petroleum and water in the formation, or a combination of these, so that all
the petroleum concerned is in pressure communication through liquid or gas.
DEVELOPMENT (OF A FIELD) - the building and installation of installations for Production and transportation of
Petroleum, including the drilling of production wells (definition found in a petroleum law)
DEVELOPMENT AND PRODUCTION AREA - a part of the Contract Area which following a commercial Discovery
has been delineated according to the terms of the Exploration and Production Contract (definition found in a petroleum
law)
DEVELOPMENT PLAN - a plan for the Development and Production of Petroleum discovered in a Contract Area
prepared in accordance with this Law and the relevant provisions in the Regulations for Petroleum Operations and the
Exploration and Production Contract covering that Contract Area (definition found in a petroleum law)
DEVELOPMENT WELL -See WELL.
DHI - DIRECT HYDROCARBON INDICATOR (DHI). See HYDROCARBON INDICATOR
DIAGENESIS - Low temperature transformation of organic matter in sedimentary environments. Most diagenesis is
biologically-mediated, but may also include chemical transformations. Diagenesis precedes oil generation ,but includes
formation of biogenic gas.
DIESEL - Refined petroleum distillate boiling in the approximate range of 230 to 300 degrees Centigrade. Diesel contains
mainly hydrocarbons of 13 to 20 carbon atoms and is commonly used as an additive in oil-based drilling muds.
DIRECT HYDROCARBON INDICATOR (DHI) - See HYDROCARBON INDICATOR
DISCOVERY - a petroleum deposit or several petroleum deposits combined (ie they have been discovered in the same
exploration well) in which the existence of mobile petroleum has been made probable through testing, sampling or logging
(definition in the Law: the first Petroleum encountered in a reservoir or geological structure by drilling that is recoverable
at the surface by conventional Petroleum industry methods)
DISCOVERY RATE -The rate of hydrocarbon discovery expressed as barrels per year, number of discoveries divided by
number of wildcat wells drilled (for instance: in total or per year), discoveries per year, barrels per foot of drilling, or the
like.
DISSOLVED-ASSOCIATED GAS -See GAS, NATURAL.
DISTRIBUTION, PROBABILITY -A mathematical description of the variation in the relative likelihood of occurrence
of possible values of a variable (See GEOSTATISTICAL TERMS and ESTIMATE).
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CUMULATIVE FREQUENCY (PROBABILITY) DISTRIBUTION -One in which the frequency counts of each
interval are totalled for successive intervals, so that the last interval has the total count of the distribution. The
ASSESSMENT CURVE is an example.
EQUAL PROBABILITY DISTRIBUTION -Several discrete values having identical probabilities.
FREQUENCY DISTRIBUTION -The count of the number of observations in a sample falling within each of a
set of intervals spanning the range of possible values of the variable.
HISTOGRAM -A bar graph in which bar widths represent class intervals, and bar heights are proportional to
class frequencies.
LOGNORMAL DISTRIBUTION -A symmetrical "bell-shaped" distribution on a log scale with no absolute
limits. Plotted on an arithmetic scale it is asymmetrical, or skewed, with a long "tail" toward the higher values.
The median value is also the geometric mean. The arithmetic mean is greater than the median, and the median is
greater that the mode.
LOG-TRIANGULAR DISTRIBUTION - A triangular distribution based on the logarithmic scale of the variable.
This distribution has more probability associated with the interval on the lower side of the most likely than does
the TRIANGULAR DISTRIBUTION. In a SYMMETRICAL LOG-TRIANGULAR DISTRIBUTION the ratio
of the most likely value to the minimum is the same as the ratio of the maximum to the most likely. Such a
distribution is symmetrical on the log scale but asymmetrical with a long "tail" toward higher values when plotted
on the arithmetic scale. The arithmetic mean exceeds the median, which exceeds the mode.
NORMAL DISTRIBUTION -An arithmetically symmetrical "bell-shaped" distribution which has no absolute
limits. The mean equals both the median and mode.
RECTANGULAR DISTRIBUTION - One in which all values between the minimum and maximum have equal
liklihood of occurrence.
TRIANGULAR DISTRIBUTION -Represented by a most likely value of greatest probability that lies between
designated minimum and maximum values. Probability increases linearly from minimum to most likely, and
decreases linearly from most likely to maximum. In a SYMMETRICAL TRIANGULAR DISTRIBUTION the
most likely value is arithmetically centered between minimum and maximum and equals both the median and
arithmetic mean.
DRILL STEM TEST (DST) - See WELL TEST
DRIVE - See CONTROLS (RECOVERABILITY).
DRY - Barren of hydrocarbons; nonproductive. See RISK.
DRY GAS - See GAS, NATURAL. Natural gas as it comes from the well is classified in the field as dry gas, lean gas, or
wet gas, according to the amount of liquid vapours it contains. A dry gas contains less than 0.1 gallon gas liquid vapour
per 1000 cubic feet, and a wet gas 0.3 or more. Geochemically, dry gas is defined as follows:
(a) Reservoir gas: methane greater than 97% of total hydrocarbons.
(b) Cuttings gas: methane greater than 75% (sometimes 50%) of the total hydrocarbons.
DST (Drill Stem Test) - See WELL TEST
DYSAEROBIC - Term used loosely to describe water conditions which are between aerobic and anaerobic; source rocks
deposited under these conditions are frequently of fair to poor quality.
EARLY MATURE - The maturation stage when some oil and gas has been generated within a source rock prior to the
main stage of hydrocarbon generation. Frequently equated with vitrinite reflectance between 0.5 and 0.6%.
ECONOMIC RISK - See RISK.
EFFECTIVE SOURCE ROCK - A source rock that is generating or has generated and expelled petroleum (oil and gas).
See CONTROLS.
ESSENTIAL ELEMENTS - The source rock, reservoir rock, seal rock, and overburden rock of a petroleum system.
Together with the processes, essential elements control the distribution of petroleum in the lithosphere. See CONTROLS.
ESTIMATE – (See also DISTRIBUTION and GEOSTATISTICAL TERMS)
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LOW ESTIMATE – Is normally the P90 quantity, which means there is a 90-percent chance that an estimated
quantity, such as a prospective resources volume or associated value, will be equaled or exceeded. (some may use
“Low estimate” for other quantities, for instance the P95 quantity)
MEAN – Is the average of all results (not an actual trial)
MEDIAN - Is the P50 quantity, which means there is a 50-percent chance that a given variable (such as
prospective resources, porosity, net/gross, or water saturation) will be equaled or exceeded. The median of a data
set is a number such that half of the measurements are below the median and half are above.
MODE – Is the quantity that occurs with the greatest frequency in the data set and therefore is the quantity that
has the greatest probability of occurrence (“Most likely”)
HIGH ESTIMATE – Is normally the P10 quantity, which means there is a 10-percent chance that an estimated
quantity, such as a prospective resources volume or associated value, will be equaled or exceeded. (some may use
“High estimate” for other quantities, for instance the P05 quantity)
ETHANE - A gaseous two-carbon alkane (C2H6) occurring in natural gas and in coal gas.
EVENTS CHART - A chart for a PETROLEUM SYSTEM showing when the essential elements and processes took
place as well as the preservation time and critical moment of the system
EXCEEDANCE PROBABILITY - The chance that potentials are equal to or greater than a given value.
EXINITE - An herbaceous constituent of kerogens and coals.
EXPANSION FACTOR, GAS (1/Bg) – See FORMATION VOLUME FACTOR (FVF)
EXPLORATION WELL – See WELL
EXPULSION - The loss of oil and gas from a source rock due to the increasing effects of temperature and pressure.
EXTRACTABLE ORGANIC MATTER (EOM) - Organic materials (bitumen) which can be extracted from sediments
with organic solvents; usually only a small percent of the total organic matter.
FACIES - Distinctive, coeval, adjacent rock units. FACIES CHANGE - A change in lithology (distinctiveness); e.g., a
reservoir sandstone unit changes laterally to shale.
FATTY ACIDS - Organic compounds containing the carboxyl (-COOH) group bonded to an normal- alkyl group.
Probable petroleum precursors.
FAULT LEAK POINT - See FILL FRACTION.
FIELD - A single pool, or multiple pools in one location. ECONOMIC FIELD - One that is profitable to develop and
produce; synonymous with COMMERCIAL FIELD. FIELD SIZE - The estimated ultimate recoverable reserves of oil
and/or gas in a field. See also MINIMUM and MAXIMUM. SIGNIFICANT FIELD - One equal to or larger than some
minimum size that is meaningful for the area, the available historical data, and the assessment methodology used.
FILL FRACTION OR PERCENT - The fraction of total available trap volume (or trap area) that is occupied (or
postulated to be occupied) by in-place hydrocarbons. Definitions of trap volume may vary according to the chosen spill or
leak point:
 SYNCLINAL SPILL POINT - A dip reversal on the reservoir that defines the largest apparent trap.
 FAULT LEAK POINT - A permeable fault plane that could drain the contiguous part of the reservoir.
 RESERVOIR LEAK POINT - Leakage across a fault where the reservoir contacts a permeable formation.
FINDING RISK - See RISK.
FLASHPOINT - The lowest temperature at which a combustible liquid will give off a flammable vapour which can be
ignited and will burn momentarily.
FLATSPOT – See HYDROCARBON INDICATOR (DHI)
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FLUORESCENCE - Light emission by matter under influence of energy such as ultra-violet light. Fluorescence is
diagnostic of oil-prone kerogens at intermediate and low maturity; petroleum also exhibits this property.
FLUSHING - See CONTROLS (PRESERVATION).
FOLD - A bending of rock layers. ANTICLINE - An upfold (convex upward). SYNCLINE -A downfold (concave
upward).
FORMATION -A mappable sedimentary rock unit of distinctive 1ithology.
FORMATION VOLUME FACTOR (FVF) - For oil, the barrels of fluid in the reservoir required to produce one barrel
of stock-tank oil at the surface - i.e., reservoir bbl/stock-tank bbl.
 OIL SHRINKAGE FACTOR (1/Bo) is the reciprocal of the Oil FVF - i.e., stock-tank bbl/reservoir bbl. (The
decrease in volume is due mainly to removal of dissolved gas during oil production – due to the reduced pressure
at the surface compared to in the reservoir).
 GAS FORMATION VOLUME FACTOR (Bg) equals reservoir bbl/thousand standard cubic feet. In
assessment, its reciprocal (GAS EXPANSION FACTOR, 1/Bg) is used in calculating surface gas volumes from
reservoir volumes (the gas will expand when brought from reservoir pressure to surface pressure conditions)
 WET GAS SHRINKAGE FACTOR defines how much the wet gas shrinks because of the removal of the
condensate liquids phase. The factor corresponds to the remaining dry gas fraction or percentage (100% means no
shrinkage).
FORMATION WATER - Water naturally occurring in sedimentary strata; once thought synonymous with connate
waters, now recognised that most water in permeable, continuous beds is meteoric, although the dissolved salts may have
been carried over from the time of deposition.
FRACTURE - Any break in a rock.
FREQUENCY - The number of times a specified event occurs within a specified interval. The percentage frequency of an
event relative to the entire distribution is its probability of occurrence. FREQUENCY DISTRIBUTION - See
DISTRIBUTION.
FRONTIER AREA - One in which there has been little or no exploration drilling.
GAS – See GAS, NATURAL
GAS-CAP - The portion of a reservoir occupied by free gas.
GAS CHIMNEY- A subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas can cause
overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or pushdowns.See HYDROCARBON INDICATOR.
GAS CHROMATOGRAPHY (Gas-Liquid Chromatography) - A form of chromatography where the moving phase is a
gas (helium, hydrogen, etc.) and the stationary phase is generally a solid coated with a liquid in a coiled tube (column)
through which the gas flows. Used to analyse complex mixtures for their individual components.
GAS EXPANSION FACTOR (1/Bg) – See FORMATION VOLUME FACTOR (FVF)
GAS FORMATION VOLUME FACTOR (Bg) - See FORMATION VOLUME FACTOR (FVF)
GAS GRAVITY - The molecular weight of the gas divided by the molecular weight of dry air (28.96).
GAS HYDRATE - Solid consisting of a combination of water and gas (primarily methane) found in deep water settings.
Gas hydrates resemble ice in physical appearance and are stable only under a restricted range of temperatures and
pressures. The gas can be either thermogenic or biogenic in origin.
GAS, NATURAL - A mixture of gaseous hydrocarbons (gaseous petroleum); consists of the lighter paraffins, of which
the most abundant is methane (typically methane with lesser amounts of ethane, propane, butanes, pentanes, and possibly
some nonhydrocarbon gases). (Definition found in a petroleum law: all hydrocarbons which are in a gaseous state under
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normal atmospheric conditions, including wet gas, dry gas and residue gas remaining after the extraction of liquid
hydrocarbons.)
 ASSOCIATED-DISSOLVED GAS - Gas in contact with crude oil either as a free gas cap or in solution with the
oil. (Definition found in a petroleum law: Natural Gas which exists in a reservoir in solution with Crude Oil,
including what is commonly known as gas-cap gas which overlies and is in contact with Crude Oil.)
 NONASSOCIATED GAS - Free gas not in contact with crude oil in the reservoir.
 DRY GAS - Gas consisting almost entirely of methane. See DRY GAS.
The natural gas of commerce, as delivered to the pipelines, usually ranges between 900 and 1200 BTU per 1000 cubic feet
and has the following general composition:
Methane (CH4) 72.3 percent
Ethane (C2H6) 14.4 percent
Carbon dioxide (CO2) 0.5 percent
Nitrogen (N2) 12.8 percent
GAS/OIL RATIO (GOR) - The volume of dissolved gas per unit volume of oil (SOLUTION GAS/OIL RATIO). More
loosely, the ratio to oil of total associated-dissolved gas in a zone or prospect, or of the total associated-dissolved plus
nonassociated gas in a prospect, play, or other entity (TOTAL GAS/OIL RATIO). SATURATION GOR is the maximum
amount of gas that can be dissolved in oil at a specified temperature, pressure, and oil gravity. Oils with lower GOR's are
termed UNDERSATURATED.
GAS SATURATION - See SATURATION.
GAS SEEPS - Surface expressions of gas leakage.
GASOLINE - The fraction of petroleum boiling between 30 and 180 degrees Centigrade. Hydrocarbons in the gasoline
boiling range from C4 to C10.
GASOLINE-RANGE HYDROCARBONS - General term for the light hydrocarbons; often used synonymously with
C4-C7, but could also include portions of C8-C10 range. These compounds can occur in mature source rocks.
GAS WETNESS - The quantity of hydrocarbon gases (other than methane) in a gas sample, expressed as a percentage of
the whole sample. A gas is characterised as wet if it contains more than 5% ethane through butane. Wetness of the gas can
be related to its origin.
GENERATION - Synonymous with MATURATION. See CONTROLS (SOURCE).
GENERATION-ACCUMULATION EFFICIENCY - The ratio, expressed as a percentage, of the total volume of
trapped (in-place) petroleum for the PETROLEUM SYSTEM to the total volume of petroleum generated from the pod of
active source rock.
GENERATION-MIGRATION-ACCUMULATION - One PETROLEUM SYSTEM process that includes the
generation and movement of petroleum from the pod of active source rock to the petroleum show, seep, or accumulation.
The time over which this process occurs is the age of the petroleum system.
GEOCHEMICAL ANOMALY - A concentration of one or more elements in rock, soil, sediment, vegetation, or water
markedly different from the normal concentration in the surroundings. Specifically applied to abnormal concentrations of
hydrocarbons in soils.
GEOCHEMICAL FOSSILS - Biological markers.
GEOCHEMICAL MATERIAL BALANCE - A special form of volumetric assessment requiring estimates of amounts
of hydrocarbons generated, migrated and trapped, in relation to estimated trap volume. Also called assessment by TRAP
AND HYDROCARBON CHARGE VOLUMES.
GEOCHEMICAL PROSPECTING - Prospecting for reservoired oil and gas by analysing for hydrocarbons in the
surface soil or sediments, but also other geochemical techniques, such as the search for concealed deposits of metallic ores
by analysing soils, surface waters, and/or organisms for abnormal concentrations of metals.
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GEOCHROMATOGRAPHY - Process occurring during migration whereby clay minerals and organic matter
preferentially retard the polar fractions of migrating petroleum, resulting in an initial enhancement of the saturated
hydrocarbons.
GEOGRAPHIC EXTENT (of the PETROLEUM SYSTEM) - the area over which the petroleum system occurs defined
by a line that circumscribes the pod of active source rock and all the discovered petroleum shows, seeps, and
accumulations that originated from that pod. The geographic extent is mapped at the CRITICAL MOMENT; also called
the known extent.
GEOLOGIC CONTROLS - See CONTROLS.
GEOLOGIC RISK FACTORS - See CONTROLS and RISK.
GEOMETRY CORRECTION - The reduction of crestal reservoir thickness (thickest potential hydrocarbon column) in
an anticline or other trap to an average value representing the entire trap area. This correction accounts for thinning of a
hydrocarbon column at the trap edges.
GEOMETRIC FACTOR or GEOMETRIC CORRECTION FACTOR - A geometry adjustment correction factor that
accounts for the thinning of a full hydrocarbon column at the edge. Input parameters used to estimate the geometric
correction factor include trap shape, length-to-width ratio, potential reservoir thickness, and the height of the potential
trapping closure (potential hydrocarbon column height).
GEOPOLYMER - Polymers formed in the geosphere as a result of chemical combination of small molecules and
alteration of biopolymers. Geopolymers include fulvic acids, humic acids, and kerogens.
GEOSTATISTICAL TERMS - (See also DISTRIBUTION and ESTIMATE)
 GEOMETRIC MEAN - The nth root of the product of n values.
 HIGHSIDE POTENTIAL - See MAXIMUM.
 LOG MEAN - The arithmetic mean of the logarithms of sample values. Its antilog is equivalent to the geometric
mean.
 LOWSIDE POTENTIAL - See MINIMUM.
 MAXIMUM or HIGHSIDE or UPSIDE POTENTIAL - For an assessment curve, this is the largest value of a
distribution shown at some specified exceedance chance (e.g., 0.05, 0.01, 0.002 - which means 5%, 1% and
0,2%).
 MEAN - The arithmetic average, which is the sum of a set of sample values divided by the number of values in
the set.
 MEDIAN - The middle value of a distribution; the median is the point with 0.5 probability of being exceeded.
 MINIMUM or LOWSIDE POTENTIAL - For an assessment curve, this is the smallest value of a distribution
shown at some specified exceedance chance (e.g., 0.95, 0.99, 0.998).
 MODE -The most frequently occurring value (or interval) in a frequency distribution. Synonymous with MOST
LIKELY.
 MONTE CARLO - A procedure to simulate probability distributions by running many trials to obtain a range of
possible answers reflecting different combinations of values selected at random from within specified ranges of
input parameters.
 MOST LIKELY - In a probability distribution of a variable, the value associated with the highest probability.
Synonymous with MODE.
 N -The number of samples in a distribution.
GEOTHERMAL GRADIENTS - A measure of the rate of change (increase) of temperature with depth, usually
expressed in oC/km or oF/100'. The gradient is approximately constant below an upper surface (50-400 feet) above which
the temperature is affected by atmospheric temperature changes and by the circulation of ground water. Factors which can
disturb the gradient are permafrost, lithology, ant the presence of aquifers at depth. Gradients have a world-wide average
of 1.4°F /100', but may vary from 0.3°F/100' (Bahamas) to 42°F/100' (Imperial Valley).
GOOD OILFIELD PRACTICE - all those practices that are generally accepted in the international Petroleum industry
as good, safe, environmentally friendly, economic and efficient in exploring for and producing Petroleum (definition found
in a petroleum law)
GOR – See GAS/OIL RATIO
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GRAVITY SEGREGATION - A process which may occur within reservoirs, whereby heavier molecules accumulate at
the bottom of he reservoir while the top is accordingly enriched in light molecules; results in the prevalence of high API
gravity oils overlying lower API gravity oils; process is probably due to movement of gas to top of reservoir.
GROSS ROCK VOLUME - The gross reservoir rock volume of the prospect.
HEADSPACE GAS ANALYSIS - Analysis by gas chromatography of the light hydrocarbon gases which collect in the
headspace above canned cuttings. The abundance and composition of the gas can be used to indicate the presence of
source rocks, and also reservoired hydrocarbons.
HEPTANE VALUE - A thermal maturity parameter based on the relative abundance of various isomeric C6 and C7
(gasoline range) hydrocarbons.
HERBACEOUS MATERIAL - Kerogen typing term which includes cuticle, spores and pollen.
HIGH ESTIMATE – See ESTIMATE and DISTRIBUTION
HIGH-SULFUR OIL - Crude oil having total sulphur content of more than 0.5%.
HOMOLOG - A member of a series of organic chemical compounds of uniform chemical type. A homologous series
shows a regular gradation in physical properties, whereby the molecule of each member of the series differs from the
preceding one by a definite constant group of atoms.
HOPANES - Pentacyclic alkanes containing between 27 and 35 carbon atoms which dominate the triterpanes found in
sediments and crude oils. Hopanes are derived from bacteria and are useful in maturation assessment, and in oil-oil and
oil-source correlation. The parent compound, hopane, contains 30 carbon atoms and usually dominates distributions in
geologic samples; numerous isomeric forms of hopanes are also common, including homohopanes, diahopanes,
neohopanes, moretanes, and demethylated hopanes.
HUMIC ACIDS - Polymeric, high molecular weight organic acids which can be extracted from soils and near-surface
sediments with aqueous base. They are derived from the decomposition products of plant and bacterial remains during
diagenesis.
HUMIC SUBSTANCES - Series of dark yellow to black polymers consisting of a heterogeneous mixture of molecules
ranging in weight from 2,000 to over 300,000. They have lower carbon and higher hydrogen contents than kerogen; much
kerogen may be formed from humic substances.
HUMUS - Decomposed soil organic matter; consists basically of humic substances plus non-humic substances such as
lipids, amino acids, carbohydrates, phenols, quinones, etc.
HYDROCARBONS - Organic compounds composed exclusively of hydrogen and carbon atoms. In general assessment
usage, the term includes oil, gas, and natural gas liquids.
HYDROCARBON INDICATOR (HCI), (DIRECT HYDROCARBON INDICATOR, DHI) - A type of seismic
AMPLITUDE ANOMALY (see this), seismic event, or characteristic of seismic data that can occur in a hydrocarbonbearing reservoir. Although "bright spots," as hydrocarbon indicators are loosely called, can originate in numerous ways,
they are not all indicative of the presence of hydrocarbons. Criteria to distinguish true hydrocarbon indicators (sometimes
called HCIs) from other types of amplitude anomalies include:
 AMPLITUDE VARIATION WITH OFFSET (AVO) (See this)
 BRIGHT SPOT or dim spot(s) in amplitude as a result of variations in lithology and pore fluids, sometimes
occurring in groups of stacked reservoirs. A seismic AMPLITUDE ANOMALY (see this) or high amplitude that
can indicate the presence of hydrocarbons. Bright spots result from large changes in acoustic impedance and
tuning effects, such as when a gas sand underlies a shale, but can also be caused by phenomena other than the
presence of hydrocarbons, such as a change in lithology. The term is often used synonymously with hydrocarbon
indicator.
 change or reversal in polarity because of velocity changes, also called phasing
 conformity with local structures
 diffractions that emanate from fluid contacts
12

FLAT SPOT that represents a fluid (gas-oil or gas-water) contact, which can also show the downdip limit of the
reservoir in some cases
 GAS CHIMNEYS above leaking reservoirs
 shadow zones below the accumulation
 velocity push-down because of lower velocities of hydrocarbons than rocks
 difference in response between reflected pressure and shear energy.
Hydrocarbon indicators are most common in relatively young, unconsolidated siliciclastic sediments with large impedance
contrasts across lithologic boundaries, such as those in the Gulf of Mexico and offshore western Africa. An ongoing issue
in exploration for hydrocarbon indicators is the difficulty in distinguishing between gas accumulations and water with a
low degree of gas saturation ("fizz water").
HYDROGENATION - Chemical combination of hydrogen with unsaturated compounds, reducing oxygen and sulphur
content and converting chemically reactive, unstable hydrocarbons into stable hydrocarbons.
HYDROGEN INDEX - A parameter derived from Rock-Eval pyrolysis which measures the hydrogen richness of source
rocks. It is defined as the ratio of the S2 peak to total organic carbon content (S2/TOC) and is expressed in mg
hydrocarbons/g organic carbon. Elevated values are commonly associated with oil-prone kerogen; a direct relationship
exists between hydrogen index and the H/C atomic ratio.
HYDROGEN-TO-CARBON ATOMIC RATIO (H/C ratio)- The ratio of hydrogen to carbon atoms- viz., wt. H/atomic
wt. H to wt. C/atomic wt. C - in kerogens. Provides a measure of carbon condensation - i.e., the number of carbon-tocarbon bonds (H/C atomic ratio = 12[H/C weight ratio D. Commonly plotted versus oxygen-to-carbon atomic ratios (O/C)
on a Van Krevelen diagram to show kerogen type and maturity differences. A high H/C ratio (> 1.5) at low maturity
indicates oil prone organic matter.
HYDROUS PYROLYSIS - Pyrolysis in the presence of water, usually in a sealed vessel known as a "bomb". Hydrous
pyrolysis is used to simulate the maturation of source rocks in the laboratory.
IMMATURE SEDIMENTS - Sediments which are potential source rocks, but are incapable of generating significant
quantities of oil and gas because they have yet to attain a sufficient degree of thermal stress; may be due to insufficient
burial, insufficient time, or both - i.e., the time- temperature exposure is too low. Frequently equated with vitrinite
reflectance <0.5% and TAl <2.2, although the actual values will vary with organic matter type.
IMPROVED OIL RECOVERY (IOR) - a term used in various ways. It is often used for any increase in oil production
relative to what was envisaged at a given moment in time. This also includes measures which accelerate production and
improve profitability, without necessarily increasing total production. Another, more restricted use, is that improved oil
recovery is linked to specific ways of raising the total recovery factor for oil from a delimited deposit. One possible usage:
applies to methods and potentials for improving the recovery factor for oil through specific measures in clearly defined
reservoirs.
INACTIVE SOURCE ROCK - A source rock that has stopped generating petroleum, although it still shows some
petroleum generating potential. (See CONTROLS)
INDEPENDENT RISK - See RISK.
INDEPENDENT VARIABLE - One not influenced by changes in another variable.
INDIVIDUAL RISK - See RISK.
INERTINITE - A charcoal-like constituent of kerogens and coals.
INJECTION WELL - A well in which fluids are injected rather than produced, the primary objective typically being to
maintain reservoir pressure. Two main types of injection are common: gas and water. Separated gas from production wells
or possibly imported gas may be reinjected into the upper gas section of the reservoir. Water-injection wells are common
offshore, where filtered and treated seawater is injected into a lower water-bearing section of the reservoir.
IN PLACE (RESOURCES) - All oil and gas originally in a reservoir, including both recoverable and nonrecoverable
volumes. The NONRECOVERABLE volumes are left in the ground after the field is abandoned.
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INSPISSASITION - Evaporation of oil deposits. Loss of the gases and lighter fractions, leaves only the heavier oils and
asphalt.
INTERVAL VELOCITY - The velocity, typically P-wave velocity, of a specific layer or layers of rock, symbolized by
Vint and commonly calculated from acoustic logs or from the change in stacking velocity between seismic events on a
common midpoint gather.
ISOCHORE - A contour connecting points of equal true vertical thickness of strata, formations, reservoirs or other rock
units. A map that displays isochores is an isochore map. The terms isopach and isopach map are incorrectly used
interchangeably to describe isochores and isochore maps. Isopachs and isochores are equivalent only if the rock layer is
horizontal.
ISOCHRON - - A line joining points of equal time or age, such as a reflection in a seismic profile or contours in an
isochron map.
ISOCHRON MAP –
Either: i) A contour map that displays the variation in time between two seismic events or reflections.
or
ii) A contour map showing the travel times to one particular seismic event or reflection.
ISOPACH - See ISOCHORE.
ISOPRENE (UNIT) - Isoprene (2-methylbutane) is a five-carbon saturated hydrocarbon that forms the basic building
block for many plant products (e.g., natural rubber, resin). It is the fundamental unit for the synthesis of many of the
biological markers including isoprenoids, terpanes and steranes.
ISOPRENOIDS - A series of acyclic compounds composed of one or more "isoprene units". They are commonly derived
from plant and bacterial precursors and are regarded as biological markers. Geochemically, the most important isoprenoids
are the saturated hydrocarbons phytane and pristane.
ISOTOPES - Elements which have the same atomic number but differing atomic weights, and the same chemical
properties (e.g., C12,C13,C14). Relative abundance of isotopes is given by isotopic ratios expressed in parts per thousand
relative to a standard - expressed as Delta (d) values. Carbon and sulphur isotopes are useful in oil-oil and oil-source rock
correlation.
KEROGEN - The naturally occurring, solid, insoluble organic matter that occurs in source rocks and can yield oil upon
heating (through natural burial or laboratory pyrolysis). Typical organic constituents of kerogen are algae and woody plant
material. Kerogens have a high molecular weight relative to BITUMEN, or soluble organic matter. “Average shales”
contain approximately 1% organic matter, of which about 90% is kerogen and about 10% is bitumen. Bitumen forms from
kerogen during petroleum generation.
Kerogens are classified in basic kerogen types according to its chemical composition (based on C, H, 0 and S
compositions) or its visual (optical) properties:
 TYPE I: Typically, alginites consisting of mainly algal and amorphous (but presumably algal) kerogen and
highly likely to generate oil. High initial H/C ratios (generally 1.5 or more) and low initial O/C ratios (commonly
less than 0.1). Lacustrine oil shales are of this type.
 TYPE II (sub-divided into II and II-S kerogen): Commonly, exinites - mixed terrestrial and marine source
material that can generate waxy oil. Intermediate H/C and O/C ratios. This type includes many oil-prone marine
source rocks.
 TYPE III: Generally, vitrinites - woody terrestrial source material that typically generates gas. Low initial H/C
ratios (usually less than 1.0) and high initial O/C ratios (as high as 0.2 or 0.3). This type includes many gas-prone
nonmarine source rocks.
Type I & II kerogen produce oil when mature, gas when post-mature. Type III kerogen produces gas when mature.
LEAD – see PROSPECT
LEAK POINTS - See FILL FRACTION.
LEASE, LICENSE, CONCESSION - A contract authorizing oil and gas exploration, development, and production in a
given area for a specified time; also, the area or tract covered by such a contract. See LICENSING ROUND.
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LEVEL OF CERTAINTY - The measure of confidence. (For instance the measure of confidence that petroleum from a
series of genetically related accumulations originated from a specific pod of active source rock.) Levels of confidence used
may be i) known (!), ii) hypothetical(.), and iii) speculative(?), depending on the level of geochemical, geophysical
and geologic evidence.
LICENSING ROUND - An occasion when a governmental body offers exploration acreage for leasing by exploration
and production companies, typically in return for a fee and a performance or work obligation, such as acquisition of
seismic data or drilling a well. Exploration licenses are initially of limited duration (about 5-8 years) after which there
might be a requirement to return half or more of the licensed acreage to the state. If hydrocarbons are discovered, a
separate production license or production-sharing agreement is usually drawn up before development can proceed.
LIGHT HYDROCARBONS - Hydrocarbons which are gaseous and near gaseous at normal temperatures and pressures.
They range from methane (Cl) to octane (C8), including aromatic compounds, normal, iso-, and cyclic alkanes.
LIGNIN - A phenolic carbohydrate polymer which forms the matrix for the cellulose in the cell walls of many plants; a
precursor to vitrinite or huminite..
LIGNITE - A low rank of coal between peat and sub-bituminous coal. Lignite is commonly used as a drilling mud
additive.
LIPIDS - The term includes all organism-produced substances that are soluble in organic solvents such as chloroform and
acetone. From the petroleum geochemist's standpoint, the most important components of the lipids group are
hydrocarbons, waxes, terpenoids and pigments, but other substances such as fatty acids, esters and alcohols are included.
(Note that this definition is similar to that for bitumen - lipids are, in a sense, very immature bitumens.)
LIQUID CHROMATOGRAPHY (LIQUID-SOLID CHROMATOGRAPHY) - A form of chromatography where the
moving phase is a liquid and the stationary phase a solid. Used to fractionate an oil or extract into saturated hydrocarbons,
aromatic hydrocarbons and heterocompounds.
LIVE OIL - Oil that contains some gas in solution. (Related to Well Completions: Oil containing dissolved gas that may
be released from solution at surface conditions. Live oil must be handled and pumped under closely controlled conditions
to avoid the risk of explosion or fire.
LOM - Level of Organic Maturity (also Level of Organic Metamorphism), a maturity scale from 1-20 based on vitrinite
reflectance.
LOW ESTIMATE – See ESTIMATE and DISTRIBUTION
LOW-SULFUR OIL - Crude oil having total sulphur content of less than 0.5%.
LOW TO MEDIUM GRAVITY OIL - Oil with gravities in the range 200 to 270 API; most low-gravity oils «200 API)
likely have been biodegraded or water-washed.
MASS SPECTROMETRY - An analytical method for identifying and measuring hydrocarbon compositions.
Compounds introduced into a mass spectrometer at high vacuum are fragmented, ionised and ratios of mass to charge are
measured. These ratios are related to the chemical structure of the parent compound.
MATURATION - See CONTROLS (SOURCE). The process of hydrocarbon generation from source rocks under the
influence of heat; also used to refer to the increasing predominance of lighter hydrocarbons in oils with increasing thermal
exposure. The term is synonymous with thermal alteration.
MATURE (geochemical) - A source rock that has had sufficient thermal alteration of its kerogen to yield oil.
MATURE AREA (exploration) – An area where there has been extensive exploration drilling.
MATURITY - The thermal level required by a source rock to generate the hydrocarbon components normally found in
petroleum.
MEAN – See ESTIMATE and DISTRIBUTION
15
MEDIUM TO HIGH GRAVITY OILS - 270 to 350 API gravity; these are the most common and constitute the bulk of
the world's production.
METHANE (Marsh gas, CH4) - The first member of the aliphatic hydrocarbon series, it is the simplest, lightest and most
abundant hydrocarbon. Can be formed either from thermal decomposition of sedimentary organic matter or by bacteria.
METHANOGENIC BACTERIA - Anaerobic bacteria which produce methane by fermentation of organic matter are
known as methanogenic bacteria.
METHYLPHENANTHRENE INDEX (MPI)- A thennal maturity parameter based on the abundance of phenanthrene
and methylphenanthrene (3-ring aromatic hydrocarbons) compositions. The MPI is dependent on facies and kerogen type;
it is most accurate for Type III organic matter and has been shown to yield unreliable maturation estimates for Type II
kerogens.
METHYL STERANES - Steranes with an additional methyl group at either the 2-, 3- or 4-carbon position. The presence
of 4-methyl steranes in source rocks and oils is attributed to a dinoflagellate contribution and is usually associated with
lacustrine depositional environments.
MIGRATION (of hydrocarbons) - See CONTROLS.
MIGRATION (GEOPHYS.) - A step in seismic processing in which reflections in seismic data are moved to their
correct locations in the x-y-time space of seismic data, including two-way traveltime and position relative to shotpoints.
Migration improves seismic interpretation and mapping because the locations of geological structures, especially faults,
are more accurate in migrated seismic data. Proper migration collapses diffractions from secondary sources such as
reflector terminations against faults and corrects bow ties to form synclines. There are numerous methods of migration,
such as dip moveout (DMO), frequency domain, ray-trace and wave-equation migration.
During seismic processing, migration adjusts the location of events in seismic traces to compensate for dipping reflectors.
MODE – See ESTIMATE and DISTRIBUTION
MORETANES - Pentacyclic alkanes containing between 27 and 35 carbon atoms which are isomers of hopanes.
Moretanes are abundant at low maturity, and decrease with increasing maturation level; their abundance relative to hopane
is a common maturation indicator.
MUD GAS - Hydrocarbon gas present in the drilling mud; may either escape from the rock crushed by the bit, or from the
fonnation.
NATURAL GAS - See GAS, NATURAL.
NATURAL GAS LIQUIDS (NGL) -Generally propane and heavier hydrocarbons that are separated from natural gas as
liquids in field separators (LEASE CONDENSATE) or in processing plants. NGL yields are reported as bbl/million cu ft
of gas.
NERITIC - The zone in the sea between the lowest limit of the littoral zone and the top of the continental shelf.
NET/GROSS RATIO -The thickness of the net reservoir formation (the reservoir of sufficient quality, such as adequate
porosity and permeability) divided by the thickness of the gross reservoir formation. The non-net portion typically consists
of interbedded shales, siltstones, low permeability limestones, etc.
NET PAY THICKNESS -The thickness of a reservoir containing producible oil or gas, or the postulated such thickness
for a prospect. In assessments this should be an average thickness for the potentially productive area (see GEOMETRY
CORRECTION).
NICKELNANADIUM RATIO (NiN)- The ratio of nickel to vanadium in oils. The ratio is dependent on the oxidation
potential and pH of the water during depositional and thus is characteristic of depositional environment: NiN greater than
10 is associated with alkaline lacustrine environments, 10 to 1.0 acid lacustrine environments and less than 1.0 is typical of
marine environments.
NONASSOCIATED GAS -See GAS, NATURAL.
16
NON-HYDROCARBON GASES - Primarily carbon-dioxide (CO2), nitrogen (N2), and hydrogen sulphide (H2S), but
also helium, argon, and hydrogen.
NON-HYDROCARBONS - As used in organic geochemistry, these are compounds of carbon and hydrogen which, in
addition contain atoms of oxygen, sulphur, nitrogen, and/or other elements (synonymous with heterocompounds, NSO)
NONRECOVERABLE HYDROCARBON - See IN PLACE.
NON-THERMAL ALTERATION - Refers to changes in reservoired crude oils other than those resulting from heating;
specifically the action of gas (de-asphalting) and the action of waters (water washing and biodegradation).
NORMAL HYDROSTATIC PRESSURE - Hydrostatic pressure in porous strata and wells approximately equal to the
weight of a column of water whose depth is the depth under consideration.
NSO - Nitrogen, Sulphur, and/or Oxygen containing compounds isolated during liquid chromatography. Examples:
pyridine, sulphides, mercaptans, carboxylic acids, etc. NSO compounds commonly are regarded as having a structure of
fused aromatic rings similar to asphaltenes (synonymous with heterocompounds).
ODD-EVEN PREDOMINANCE - Refers to the prevalence of n-alkane (paraffin) molecules with odd numbers of carbon
atoms among high molecular-weight hydrocarbons (C24-C34). This attribute is common in immature sediments, but
generally not in oils or in mature source rocks.
OIL - A mixture of liquid hydrocarbons.
OIL-EQUIVALENT BARRELS (OEB or BOE) - Amounts of gas expressed as the energy-equivalent of oil, generally at
5600 to 6000 cubic feet per barrel. This ratio does not reflect the volume equivalency of the two fluids in reservoirs.
OIL FORMATION VOLUME FACTOR (Bo) - See FORMATION VOLUME FACTOR (FVF)
OIL FRACTION - In prospect assessment, the fraction of the in-place hydrocarbon volume occupied by oil and its
dissolved gas. (The free gas fraction is one minus the oil fraction.) In play assessment, the oil fraction is the fraction of the
number of potential fields that is expected to be oil fields. In volumetric basin assessment the oil fraction is the portion of
the total oil-equivalent hydrocarbon (oil plus gas plus NGL) that is oil.
OIL-OIL CORRELATION - The technique whereby oils areoils are grouped on the basis of a common source, although
the source itself is not necessarily known. Parameters which are used in correlations include carbon isotope ratios and
biomarker distributions.
OIL SATURATION - See SATURATION.
OIL SHALE - Shale containing enough kerogen to yield possibly commercial oil upon heating.
OIL SHRINKAGE FACTOR (1/Bo) - See FORMATION VOLUME FACTOR (FVF)
OIL-SOURCE CORRELATION - The correlation of an oil to its parent rock formation or source rock.
OIL WINDOW - The maturity zone over which oils are generated from source rocks is referred to as the oil window.
OLEANANE - A 30 carbon triterpane indicative of higher plant contribution in the source organic matter. Oleanane has
been linked to a flowering plant (angiosperm) source and has been found exclusively in Tertiary and Late Cretaceous oils
and source rocks. It elutes immediately before hopane in GC/MS (m/z 191) mass chromatograms.
OLEFINS (ALKENES) - Aliphatic hydrocarbons with the general formula CnH2n which contain a double bond, a
characteristic chemists have termed unsaturation. Olefins are present in living organisms but are rare in natural crude oils.
They are readily produced in refinery cracking processes and are important in the petrochemical industry.
OMT - See ORGANIC MATTER TYPE.
OPTICAL ACTIVITY - The property of certain compounds to rotate the plane ofplane-polarised light. In petroleum,
molecules having asymmetric carbons, in particular, biological markers, exhibit this property.
17
ORGANIC ACID - Compound containing the carboxyl (-COOH) group; usually equated with fatty acid although
definition includes other types such as naphthenic, aromatic, ketonic, etc. See also carboxylic acid.
ORGANIC CARBON - The noncarbonate carbon contained in kerogen; . In sediments and rocks, the amount of carbon
in compounds derived from living organisms; distinguished from inorganic carbon which may be in the form of
carbonates, graphite, carbides, etc.
 TOTAL ORGANIC CARBON (TOC) is reported as weight percent rock.
 ORIGINAL TOTAL ORGANIC CARBON (OTOC), the TOC of an immature sediment, is estimated by
multiplying the present TOC of a mature sample by a TOC MULTIPLIER to account for the oil and gas released
from a source rock during maturation.
ORGANIC FACIES - A distinctive assemblage of kerogen components which can be identified visually or has a
characteristic chemical composition. Organic facies can be predicted from depositional models and is of importance in
mapping the distribution of, and compositional variation in source rocks.
ORGANIC MATTER TYPE – See KEROGEN.
OTOC - See ORGANIC CARBON.
OVERBURDEN ROCK - The sedimentary rock above which compresses and consolidates the material below. In a
petroleum system the overburden rock overlies the source rock and contributes to its thermal maturation because of higher
temperatures at greater depths. An essential element of the PETROLEUM SYSTEM.
OVERCOOKING - See CONTROLS (PRESERVATION).
OVERMATURE - A source rock yielding gas rather than oil because of excessive thermal alteration of its kerogen.
OVERPRESSURED - Hydrostatic pressure exceeding the normal gradients of 0.43 psi/ft (9.8kPa/m) for fresh water, 0.47
psi/ft (10.9kPa/m) for normal sea water, and 0.53 psi/ft (12.0kPa/m) for more salty formation waters.
OXIDIZING CONDITIONS - Conditions under which free oxygen is present, thus favouring the destruction of organic
matter by oxidation and lack of preservation in sediments.
OXYGEN INDEX - A parameter derived from Rock-Eval pyrolysis which measures the oxygen richness of source rocks.
It is defined as the ratio of the S3 peak to total organic carbon content (S3/TOC) and is expressed in mg CO2/g organic
carbon. Together with the hydrogen index, the oxygen index can be used to assess kerogen type and level of maturation.
There is a direct relationship between oxygen index and O/C ratio.
OXYGEN- TO-CARBON RATIO (O/C) - The atomic ratio of oxygen to carbon determined by elemental analysis of
kerogens. The O/C ratios is commonly plotted versus hydrogen-to-carbon atomic ratios (O/C) on a Van Krevelen diagram
to show kerogen type and maturity differences. A high O/C ratio (>0.5) at low maturity, coupled with low H/C ratio
indicates oxidised organic matter with little liquid hydrocarbon potential.
P -Abbreviation for probability. P(.16) or P(16) are examples of probabilities given as decimal fractions and percentiles,
respectively.
PALEODRAINAGE AREA -The area being drained of hydrocarbons during the time of migration from source to trap.
PARAFFIN - See alkane (normal).
PARAFFIN-BASE CRUDE - Also wax-base; rich in paraffinic hydrocarbons of both high and low molecular weight,
hence is paraffinic rather than asphaltic.
PAY ZONE -Productive zone. See also NET PAY THICKNESS.
PEAK MATURE - The level of maturity associated with the maximum rate of hydrocarbon generation from kerogen.
n-PENTANE (C5H22) - A normal alkane hydrocarbon (paraffin) with a continuous chain five carbon
atoms long.
18
PERMEABILITY -See CONTROLS (RESERVOIR).
PETROLEUM - Synonymous with HYDROCARBONS. "...that naturally occurring mixture of dominantly
hydrocarbon substances - liquid, gas, or solid - which constitutes the commercial crude oil, natural gas and natural asphalt
of the petroleum industry." (Hedberg, 1964). A mineral oil occurring in subsurface rocks and at the surface which is a
naturally occurring mixture of hydrocarbon and nonhydrocarbon compounds. It may occur in the gaseous, liquid, or solid
state depending on the nature of these compounds and existent conditions of temperature and pressure. Commonly used
synonyms are hydrocarbon and oil and gas.
(Definition found in a petroleum law: Crude Oil or Natural Gas, or other hydrocarbons produced or capable of being
produced from Crude Oil, Natural Gas, oil shales or tar sands.)
PETROLEUM DEPOSIT - a separate accumulation of Petroleum in a geological unit limited by rock characteristics,
structural or stratigraphic boundaries, contact surfaces between Petroleum and water in the formation, or a combination of
these, so that all Petroleum comprised is in pressure communication through liquid or gas
PETROLEUM OPERATIONS - all or any of the activities related to Exploration, Development, Production, separation
and treatment, storage, transportation and sale or delivery of Petroleum to the point of export, or to the agreed supply point
in the Republic of Mozambique, and includes Natural Gas processing operations and the closure of all concluded activities
(definition found in a petroleum law)
PETROLEUM PROVINCE - A geographic location, frequently named, in which petroleum occurs; also referred to as a
petroleum basin or basin. Examples include the Williston basin, Zagros thrust belt, and the Paris basin.
PETROLEUM-SOURCE ROCK CORRELATION - A comparison of chemical properties and relative abundances of
individual chemical compounds present in source rocks and petroleum with the purpose of establishing a genetic
relationship between the petroleum and its source rock.
PETROLEUM SYSTEM - The essential elements and processes as well as all genetically related hydrocarbons that
occur in petroleum shows, seeps, and accumulations whose provenance is a single pod of active source rock. Also called
hydrocarbon system and oil and gas system.
PETROLEUM SYSTEM AGE - The time over which the process of generation-migration-accumulation of hydrocarbons in the system takes place on the EVENTS CHART.
PETROLEUM SYSTEM INVESTIGATION - An investigation that identifies, names, determines the level of certainty,
and maps the geographic, stratigraphic, and temporal extent of a petroleum system. It includes petroleum-petroleum and
petroleum-source rock geochemical correlation, petroleum system map, cross section, burial history and events charts,
table of hydrocarbon accumulations, and a mass balance calculation.
PETROLEUM SYSTEM LOGIC - The thought process required to develop an integrated interpretation of the process
of petroleum generation-migration-accumulation. In exploration, it is used to evaluate an exploration opportunity or
prospect.
PETROLEUM SYSTEM NAME - A compound name that includes the source rock in the pod of active source rock, the
reservoir rock containing the largest volume of petroleum, and the level of certainty of a petroleum system; for example,
the Mandal-Ekofisk(!) petroleum system.
PETROLEUM SYSTEM PROCESSES - The two processes of trap formation and generation-migration-accumulation
of petroleum. The preservation, degradation, and destruction of petroleum is omitted as a process because it generally
occurs after a petroleum system is formed (preservation time). Together with the essential elements, the processes control
the distribution of petroleum in the lithosphere.
PETROLEUM SYSTEM RECOVERY EFFICIENCY - A ratio, expressed as a percentage, of the ultimately
conventionally recoverable barrels of oil equivalent to the barrels of oil equivalent that could have been generated from a
pod of active source rock if the source rock were completely spent.
pH - Measure of alkalinity or acidity of a solution; the log of the reciprocal of the hydrogen ion activity i.e., water with a
hydrogen ion activity of 10-7, has apH = 7. Acidity, pH <7. Alkalinity, pH >7.
19
PHENANTHRENE - Aromatic hydrocarbon consisting of three fused benzene rings; an isomer of anthracene.
PHENOLS (Hydroxybenzenes) - One of the oxygen containing aromatic compounds found in coal tar and in crude oil.
They are closely related to alcohols; the aromatic nucleus exerts a strong modifying effect on the properties of the
hydroxyl group, causing phenols to be acidic.
PHOTIC ZONE - A depth range within which photosynthesis occurs in marine and lake waters. The photic zone is
limited by light penetration/turbidity and is seldom greater than 200 m.
PHYTANE - A common isoprenoid hydrocarbon containing 20 carbon atoms. Phytane is not found in living organisms
but is usually present in sediments and crude oils.
PHYTOPLANKTON - Unicellular photosynthetic organisms that live in marine or lake waters. They include algae,
diatoms and dinoflagellates.
PINCHOUT (PERMEABILITY-POROSITY) - See CONTROLS (TRAP).
PIPELINE DEVELOPMENT PLAN (for Oil or Gas Pipeline) - a plan for the construction and operation of an Oil
Pipeline or Gas Pipeline System comprising the pipelines, valve stations, pump stations, compressor stations and
associated installations required for the purpose of transportation of Petroleum (definition found in a petroleum law)
PIPELINE SYSTEM (Oil or Gas Pipeline System) - the pipeline(s) including valve stations, pump stations, compressor
stations and associated installations built for the purpose of Transportation of Petroleum (definition found in a petroleum
law)
PLAY -A group of geologically related prospects with similar hydrocarbon sources, reservoirs, and traps: A
geographically and stratigraphically delimited area where a set of specific geological factors is in place simultaneously,
thus making it possible to discover petroleum in producible quantities. Such geological factors are reservoir rocks, traps,
mature source rocks and migration paths, plus the conditionally that the traps were formed before the migration of
petroleum came to an end All fields, discoveries and prospects within the same play are characterised by the play's specific
set of geological factors and can therefore be distinguished from fields, discoveries and prospects of other plays
 Confirmed play. Contain a minimum of one discovery of production quantity of petroleum It is thus confirmed
that the critical factors are in place simultaneous for these plays
 Unconfirmed play. contain no discovered petroleum for the tine being This may be a result of having drilled one
or several dry well(s) in the play, or of not having started exploration activities
PLAY CHANCE -See CHANCE.
PLAY MAPS -Maps summarizing the critical geologic controls of hydrocarbons so as to define favourable areas within a
play. Good play maps help in quantitatively estimating both volume and risk factors.
POD OF ACTIVE SOURCE ROCK - A contiguous volume of source rock that is generating and expelling petroleum at
the critical moment and is the provenance for a series of genetically related petroleum shows, seeps, and accumulations in
a petroleum system. A pod of mature source rock may be active, inactive, or spent. (See CONTROLS)
POLYMER - A compound made up of one repeating structural unit (called a monomer), i.e., the combination of two or
more monomers makes a polymer (e.g., natural rubber is an isoprene polymer,
polyethylene is an ethylene polymer).
POOL - An underground accumulation of petroleum in a single and separate reservoir.
POROSITY - See CONTROLS (RESERVOIR).
PORPHYRINS - Pigments that occur in small quantities in animals and plants; in nature, usually derived from the green
colouring matter of plants (chlorophyll). Porphyrins are made up of four pyrrole rings linked together in an essentially
aromatic structure. They occur in crude oils and sediments (petroporphyrins), esp. in the asphaltene fraction.
POTENTIAL SOURCE ROCK - A source rock that contains adequate quantities of organic matter to generate
petroleum, but has not yet generated significant amounts of hydrocarbons due to immaturity (not reached the proper level
of thermal maturity to generate petroleum).
20
POUR POINT - Temperature below which an oil becomes semisolid and will no longer flow freely; an increase in
paraffin content will raise the pour point. Note: Pour point and viscosity are not necessarily related, i.e., an oil with a high
viscosity need not have a high pour point.
PRESERVATION - See CONTROLS.
PRESERVATION TIME - The time after generation-migration-accumulation of petroleum takes place that encompasses
any changes to the petroleum accumulations up to present day.
PRIMARY MIGRATION -See CONTROLS.
PRISTANE - A common isoprenoid hydrocarbon containing 19 carbon atoms. In contrast to phytane, pristane is found
both in living organisms and in sediments and crude oils.
PRISTANE/PHYTANE RATIO - The ratio of the relative abundance of the isoprenoids pristane and phytane. The ratio
is characteristic of organic facies and depositional environment; values less than 1.0 are often associated with carbonate
source rocks and oils while values greater than 4.0 are typical of terrigenous organic matter.
PROBABILITY - The perceived likelihood of occurrence of an event, i .e., the ratio of outcomes producing the event to
the total outcomes considered possible. Probability values range from 1.0 (certain to occur) to zero (certain not to occur).
 CONDITIONAL PROBABILITY -The probability of an event given that some other event has already occurred,
e.g., the chance for a field given that the geologic controls of oil and gas are favorable. (See SUCCESS RATIO.)
 MARGINAL PROBABILITY -The chance for at least a significant minimum amount. Synonymous with
ADEQUACY CHANCE. For prospects it is the PROSPECT CHANCE, and for plays it is the PLAY CHANCE.
PROBABLE RESERVES -See RESERVES.
PRODUCING RATE, WELL -The number of barrels of oil (or cubic feet of gas) that can be produced from a well in a
day.
PRODUCTION INDEX - A parameter derived from Rock-Eval pyrolysis, the production index is defined as the ratio of
S 1 divided by the sum of S 1 and S2 (S II S 1 +S2). This parameter can be used as both a maturation indicator and also to
assess the presence of in-migrated hydrocarbons (staining). Immature samples have a ration less than 0.10; mature samples
0.10 to 0.40. Elevated values in immature source rocks are sometimes associated with staining.
PROSPECT - A mappable, possible petroleum accumulation that is relying on a reservoir entrapped by sealing rocks and
charged with hydrocarbons The prospect is called a LEAD if it is a very low probability of existence of one of the three
factors. (Other definitions used for LEAD may relate to a potential mappable, possible petroleum accumulation but where
there the data coverage is insufficient for adequate mapping)
PROSPECT CHANCE -See CHANCE.
PROVED RESERVES -See RESERVES.
PYROBITUMEN – (See BITUMEN) A dark-coloured, solid, infusible natural-hydrocarbon complex often associated
with a mineral matrix; insoluble in water and relatively insoluble in organic solvents; with a hydrogen- to-carbon atomic
ratio of less than 0.53. One of the graphitic bitumens (see reservoir bitumen).
PYROLYSIS (Destructive distillation) - The decomposition of coal, wood, petroleum or other organic matter by heating
in the absence of air.
PYROLYSIS/GAS CHROMATOGRAPHY - An analytical technique which allows the products of pyrolysis to be
characterised using gas chromatography. This technique provides information on the organic facies of source rocks and the
likely hydrocarbon products generated on maturation.
Ro - Common abbreviation for vitrinite reflectance under oil immersion.
REARRANGED STERANES - See diasteranes.
21
RECOVERABILITY -See CONTROLS.
RECOVERY EFFICIENCY. Related RECOVERY FACTORS are expressed as barrels of stock-tank oil per acre-foot of
reservoir, or thousands of standard cubic feet of gas per acre-foot of reservoir, or standard cubic meters of oil or gas per
cubic meter of reservoir.
RECOVERY FACTOR - the proportion of oil and gas resources which can be recovered from a deposit to the resources
originally in place. It is used as a target figure for how effectively a field is drained. It is also used to compare deposits in
order to indicate the potential for improved oil recovery based on data from studies and experience.
RECTANGULAR DISTRIBUTION -See DISTRIBUTION.
REDUCING CONDITIONS - Conditions under which free oxygen is unlikely and thus the preservation of organic
matter is favoured. See BACTERIAL OXIDATION & REDUCTION
RESERVES, DISCOVERED – (See also RESOURCES)
 PROVED RESERVES -The estimated remaining quantities of petroleum that geologic and engineering studies
demonstrate with reasonable certainty will be recoverable from known reservoirs under existing economic and
operating conditions.
 PROBABLE RESERVES -The estimated quantities of petroleum, in addition to proved reserves, that geologic
and engineering studies indicate will likely be recovered from partially defined reservoirs under existing
economic and operating conditions. Reserve additions to known reservoirs may come from EXTENSIONS
(increased proved area) and/or REVISIONS (changed estimates based on new information or improved recovery
techniques).
 ESTIMATED ULTIMATE RESERVES (EUR) -Cumulative production plus proved reserves.
 PROBABLE FINAL RESERVES -Estimated ultimate plus proved plus probable reserves.
RESERVOIR LEAK POINT -See FILL FRACTION.
RESERVOIR BITUMEN – (See BITUMEN) Black, solid asphaltic particles or coatings in oil and gas reservoirs. Byproducts of the alteration (degradation) of pooled hydrocarbons. Divided into two subtypes according to the degradation
process:
(1) Graphitic bitumen (e.g., pyrobitumen) from thermal cracking or gas-deasphalting.
(2) Asphaltic bitumen (e.g., natural asphalt) from inspissation, water-washing, or oxidation.
RESERVOIR GAS - Gas within reservoirs, mayor may not be associated with oils.
RESERVOIR ROCK - See CONTROLS.
RESERVOIR THICKNESS - The possible range for the thickness of the reservoir, or the amount of vertical closure (See
CLOSURE HEIGHT) in a situation where structural amplitude is less than individual reservoir thickness.
RESOURCES (for a field) – (See also RESERVES) (different definitions exists for RESOURCES and RESERVES)
comprise the originally recoverable petroleum resources in a deposit, based on the current understanding of the quantities
in place and the recovery factor.
 RESERVES - comprise originally recoverable RESOURCES (see this) which the licensees have decided to
develop and for which the authorities have approved, or granted exemption from, a plan for development and
operation.
 RESOURCES ORIGINALLY IN PLACE - comprise the petroleum resources which, following mapping by
geological methods and calculation by geological and petroleum technological methods, are estimated to be in
place in a deposit. This estimate must state the quantities under sales terms.
 ORIGINALLY RECOVERABLE RESOURCES AND/OR RESERVES - comprise the total saleable or
deliverable resources and/or reserves from the start to the cease of production, based on current understanding of
the quantities in place and the recovery factor.
 QRIGINALLY-IN-PLACE GAS - divided into free gas and associated gas (dissolved in oil). Qriginally-inplace NGL means components dissolved in free gas, all of which will be converted to the NGL phase by current
or planned gas processing.
 ADDITIONAL RESOURCES - comprise the recoverable resources which may be added to the reserves of a
field by increasing the resources originally in place.
22
RESOURCES – (Different definitions exist.) New and reserve sources of potential petroleum supply, including both
discovered and undiscovered sources.
 CONTINGENT (STATIC) RESOURCES -Discovered petroleum that is potentially recoverable but currently
non-commercial.
 DISCOVERED RESOURCES -Proved plus probable reserves, plus contingent resources. Past plus future sources
of supply would also include past production.
 UNDISCOVERED RESOURCE BASE (UNDISCOVERED RECOVERABLE RESOURCES) - Potentially
recoverable petroleum volumes postulated to exist, regardless of present accessibility or economics, in fields
larger than some specified minimum size
 UNDISCOVERED ATTAINABLE POTENTIAL -That part of the undiscovered resource base deemed
accessible and economically and technologically findable, producible, and marketable.
 UNDISCOVERED NONATTAINABLE (NONECONOMIC) POTENTIAL – That part of the undiscovered
resource base deemed not accessible or economically or technologically findable, producible, and marketable.
RETROGRADE CONDENSATES - When the pressure in a gas condensate accumulation is reduced by production,
condensation may occur in the reservoir or in the well bore. This is known as retrograde condensation.
RISK FACTORS, -See also CONTROLS.
 RISK, GEOLOGIC -The chance that no significant oil or gas field exists. Also called EXISTENCE RISK or
CHANCE OF FAILURE or DRY RISK (= 1 – Chance of Discovery). (Chance of Discovery = 1 – Risk )
 RISK (ECONOMIC) -The chance that no commercial oil or gas field/accumulation exists. (The threat of loss.)
 RISK DECISION – weigh the level of investment against four considerations; net financial assets, chance of
success/failure, potential gain, and potential loss. The last three considerations must rely on estimates (made
under UNCERTAINTY) of the range of probabilities that some conditions may exist or occur.
 RISK (FINDING) -The chance that an existing field/accumulation might not be discovered.
 RISK (DEPENDENT OR GROUP) -The chance that all related zones (or prospects or plays) are dry if one is dry.
 RISK (INDEPENDENT OR INDIVIDUAL) -The chance that one zone (or prospect or play) is dry regardless of
the status of others.
R -Symbol for VITRINITE REFLECTANCE.
ROCK-EVAL - A pyrolysis technique which enables the chemical composition of a source rock, and hence its
hydrocarbon potential, to be determined. It involves two heating steps, the first to volatilize hydrocarbons in the source
rock (S 1), the second to pyrolyse the kerogen and convert the material to free hydrocarbons (S2). A third pyrolysis step
generates trapped CO 2 (S3). Maturation and source quality parameters are derived from the yields associated with each
step.
ROYALTY -Lease payment based on percentage of gross income or of total value of oil and gas produced.
S1, S2, S3 - Rock Eval pyrolysis parameters derived from the peak areas associated with the various pyrolysis steps.
SAPROPEL - The word sapropel is derived from the Greek word sapros meaning "rotten". Sapropel is an unconsolidated,
jelly-like ooze or sludge composed of plant remains, most often algae, macerating and putrefying in an anaerobic
environment. It has an amorphous and "fluffy" look, and is produced from organic matter through the action of bacterial
and possibly fungal attack. It also may be produced from precipitated humic acids.
SATURATED HYDROCARBONS - A saturated hydrocarbon is one in which the valences of all the carbon atoms is
satisfied by single bonds.
SATURATE FRACTION - The fraction of crude oils and sediment extracts which contain normal, branched and cyclic
alkanes, isolated during liquid chromatography.
SATURATION GOR -See GAS/OIL RATIO.
SATURATION - See WATER SATURATION.
 HYDROCARBON SATURATION - The hydrocarbon-bearing fraction of the pore volume of a reservoir.
 WATER SATURATION is one minus hydrocarbon saturation.
23
SEAL (ROCK) -See CONTROLS (TRAP). A shale or other impervious rock that acts as a barrier to the passage of
petroleum migrating in the subsurface; it overlies the reservoir rock to form a trap or conduit. Also known as roof rock and
cap rock.
SECONDARY MIGRATION -See CONTROLS.
SEISMIC AMPLITUDE ANOMALY –See DIRECT HYDROCARBON INDICATOR (DHI)
SHRINKAGE FACTOR, OIL or WET GAS - See FORMATION VOLUME FACTOR.
SIGNIFICANT FIELD -See FIELD SIZE.
SOLUTION GOR - See GAS/OIL RATIO. The amount of gas that is dissolved in an oil at a given temperature and
pressure.
SOURCE POTENTIAL - The ability of a source rock to yield oil or gas to a reservoir rock.
SOURCE ROCKS - See CONTROLS. Fine-grained rocks (shales or carbonates) which can yield gaseous or liquid
hydrocarbons to a reservoir rock.
SOUR GAS - Gas enriched in hydrogen sulphide.
SPENT SOURCE ROCK - A source rock that has generated and expelled all of its petroleum and is now overmature,
with no further potential for generation.
SPILL LEVEL -The lowest level to which oil or gas can be held in a trap before spilling out.
STABLE CARBON ISOTOPES - Carbon has two stable isotopes, 12C and 13C. The measure of the relative abundance
of these two isotopes is known as the stable carbon isotope ratio. Stable carbon isotopes are useful for maturation
assessment (gases) and also for correlation of oils and source rocks.
STANDARD DEVIATION -Square root of the arithmetic average of the squares of the deviations from the mean of a
population. About two-thirds of a normal population fall between the values at plus and minus one standard deviation, at
P(16) and P(84) respectively. The standard deviation is a measure of the spread or dispersion of a frequency distribution.
See VARIANCE.
STRATIGRAPHIC TRAP -See CONTROLS (TRAP).
STRATIGRAPHY - A branch of geology concerned with the study of the origin, composition, distribution, and
succession of rock strata.
STRUCTURAL TRAP -See CONTROLS (TRAP).
SUCCESS RATIO - In a PLAY, the conditional probability that some prospect is indeed an hydrocarbon
accumulation/potential discovery, given that the geologic controls of oil and gas are favorable. The success ratio may be
estimated as the expected number of fields exceeding a specified minimum significant size divided by the number of
prospects large enough to hold such fields.
SURFACE GEOCHEMISTRY - The detection of hydrocarbon accumulations at depth using surface manifestations,
such as seeps of oil and gas.
SWEET GAS - Gas containing primarily hydrocarbons and little or no hydrogen sulphide.
SYMMETRICAL DISTRIBUTION -See DISTRIBUTION.
SYNCLINAL SPILL POINT -See FILL FRACTION.
TAI -Abbreviation for THERMAL ALTERATION INDEX.
24
TAR, TAR MATS, TAR SANDS - The terms tar and pitch are commonly applied in the petroleum industry to any of the
black, sticky, liquid or solid residues produced in petroleum refining. In geochemical parlance the word "bitumen" could
be substituted for "tar" when referring to tar mats and tar sands.
TERRIGENOUS ORGANIC MATTER - Organic detritus derived from terrestrial plants and other land organisms;
distinguished from marine organic matter.
TERTIARY MIGRATION – See CONTROLS
TEST - See WELL TEST
THERMAL ALTERATION INDEX (TAI) - A colour index ranked from 1 to 5, and ranging from greenish yellow
through amber to brown to black, which describes the degree of thermal alteration/maturation (and resultant darkening)
experienced by organic matter not extractable from sediments by normal organic solvents (kerogens). Spores, pollen, and
leaf cuticle are the most reliable indicators for TAI.
THERMAL MATURITY - Sufficient thermal alteration in a source rock, either present day or in the past, to generate
petroleum. Three levels are early or low, peak or mid-, and late or high. The record of thermal maturity determined for a
source rock seldom reflects its current state of petroleum gener- ation, as thermal maturation levels are irreversible on
cooling. See CONTROLS.
THERMOGENIC GAS - Hydrocarbon gases which are generated from organic matter by thermal breakdown, usually at
depth. They are derived from mature, gas prone kerogens or post mature oil prone kerogen.
TIMING -See CONTROLS (MIGRATION).
Tmax - The temperature, in oC at which the pyrolysis yield of the S2 peak in Rock Eval pyrolysis of a source rock sample
reaches a maximum. Tmax is used as a maturation parameter, but is kerogen type dependent. For Type II kerogens,
significant hydrocarbon generation occurs between 435 and 460 oC, however, there is little variation in Tmax during
hydrocarbon generation of Type I kerogens.
TOC and TOC MULTIPLIER - See ORGANIC CARBON.
TOTAL ORGANIC MATTER (TOM) - All of the organic matter in a sample. As organic carbon can be determined
directly while TOM values must be derived using factors which "assume" certain amounts of oxygen, nitrogen and
sulphur, the use of organic carbon values is preferred in geochemistry. A factor of 1.22 is commonly used to derive TOM
from organic carbon, but this is, at best, an approximation.
TOTAL ORGANIC CARBON (TOC) - A measure of the organic carbon content in a rock, expressed as weight percent;
used as a fundamental parameter for classifying source rocks in conjunction with kerogen type and maturation. Most
hydrocarbon source rocks contain greater than 1.0% TOC.
TRANSFORMATION RATIO - A ratio related to the production index; it is defined as the difference between the
original hydrocarbon potential of a sample prior to maturation and the measured hydrocarbon potential divided by the
original hydrocarbon potential. Values range from 0 to 1.0.
TRAP - See CONTROLS (TRAP).
TRAP FILL - See CONTROLS (TRAP).
TRAP and HYDROCARBON CHARGE VOLUMES - See GEOCHEMICAL MATERIAL BALANCE.
TRIANGULAR DISTRIBUTION - See DISTRIBUTION.
TRUNCATION -The beveling of rock strata by erosion at an UNCONFORMITY.
UCM - Abbreviation for "Unresolved Complex Mixture", it refers to the part of the saturate or aromatic hydrocarbon
fraction of extractable organic matter which is not resolved into peaks during gas chromatography. Also informally known
as the hump, it becomes accentuated in biodegraded oils.
25
UNCERTAINTY, GEOLOGIC - Specifically, the imprecision in estimating the size range of prospects, discoveries and
fields. In the broader sense of being unsure or in doubt, uncertainty also encompasses risk.
UNCONFORMITY - A buried erosion surface.
UNDERSATURATED OIL - See GAS/OIL RATIO.
UNDISCOVERED ATTAINABLE POTENTIAL - See RESOURCE.
UNDISCOVERED RESOURCE BASE - See RESOURCE.
UPSIDE POTENTIAL - See MAXIMUM.
VAN KREVELEN DIAGRAM - The diagram used to display O/C vs. H/C ratios in elemental analysis of organic matter.
Rock Eval hydrogen and oxygen indices are displayed on a similar plot which is termed a modified Van Krevelen
diagram.
VAPOR PRESSURE - The pressure at which a liquid and its vapour are in equilibrium at a given temperature. The more
volatile the liquid the higher will be its vapour pressure.
VARIANCE - Square of the standard deviation.
VISCOSITY OF OIL - See CONTROLS (RECOVERABILITY). The ability to flow. Measured in arbitrary units by the
time required for a specified volume of oil to flow through a hole (Saybolt seconds) or a capillary tube (centipoises) of
definite size under controlled temperature conditions.
VISUAL ORGANIC MATTER - Dispersed organic matter visible with binocular microscope after digestion with HF
and HCI (to dissolve mineral matter), flotation, and centrifugation. Includes primary material such as cuticle (herbaceous,
woody), spores, pollen, microplankton, algae, fungi, plant resins; and secondary products such as sapropel, mineral
charcoal, polymers, pyrobitumen, and oil globules.
VITRINITE - Woody constituent of kerogens and coals. The dominant organic constituent of coal. Vitrinite forms the
familiar brilliant, black sands of coal. It results from the biochemical decomposition and coalification of plant debris.
VITRINITE REFLECTANCE (R ) - Widely used maturity indicator. The percentage of light reflected from vitrinite
particles immersed in oil under a microscope, at standard conditions (is a direct microscopic measurement made both on
the vitrinite maceral of coals and on kerogen extracted from rock samples). Higher reflectances indicate greater degrees of
thermal alteration.
VOLUME FACTOR - One of several parameters required to assess potential hydrocarbon volumes. Examples are net
pay thickness, productive area, and recovery factor for prospect assessment.
VOLUMETRIC ASSESSMENT METHODS - For prospects, the assessment of potential effective pore volume under
closure, with estimated hydrocarbon fill fraction. (See also GEOCHEMICAL MATERIAL BALANCE.) For basins, the
multiplication of sediment volume by a hydrocarbon yield factor such as bbl per unit volume.
WATER SATURATION - See SATURATION. The fraction of water in a given pore space. It is expressed in
volume/volume, percent or saturation units. Unless otherwise stated, water saturation is the fraction of formation water in
the undisturbed zone. The saturation is known as the total water saturation if the pore space is the total porosity, and the
effective water saturation if the pore space is the effective porosity. If used without qualification, the term usually refers to
the effective water saturation.
WATER- WASHING - A non-thermal alteration process affecting oils in reservoirs; waters flowing through the reservoir
carry away the more soluble hydrocarbons (chiefly the aromatics benzene and toluene, and the light paraffinic
hydrocarbons).
WAX - Commonly refers to the solid n-alkane (paraffin) hydrocarbon fraction recovered from petroleum. The term wax is
commonly used to refer to n-alkanes in petroleum containing greater than 25 carbon atoms; hence an oil enriched in these
components would be termed "waxy" (not to be confused with the term paraffinic, which refers to an abundance of nalkanes in petroleum without any implied carbon number preference).
26
WEDGE, FACIES-CYCLE - A body of rock bounded above and below by tectonically enhanced regional
unconformities or the tops of major nonmarine tongues. As defined here, wedges normally encompass several depositional
sequences. The ideal transgressive-regressive cycle includes, from base to top, facies successions from nonmarine to
coarse (sandstone or grain carbonate), to fine (marine shale or micrite), to coarse, and back to nonmarine. Different types
of
potentially productive coarse reservoir formations (plays) are identified by their distinctive vertical facies successions
within this cycle:
 WEDGE BASE - Fine over coarse (potential reservoir) over nonmarine.
 WEDGE MIDDLE - Fine over coarse over fine.
 WEDGE TOP - Nonmarine over coarse over fine.
 WEDGE EDGE - Nonmarine over coarse over nonmarine.
 SUBCONFORMITY - Any truncated part of a wedge unconformably underlying another wedge.
WELL  WILDCAT - a hole drilled on a geologic feature never before productive.
 APPRAISAL or DELINEATION WELL - a hole drilled to outline a new discovery.
 DEVELOPMENT or PRODUCTION WELL - a hole drilled for producing a field.
 EXPLORATION WELL – sometimes used as a synonym for WILDCAT well, sometimes used as a term
covering both WILDCAT and APPRAISAL wells.
WELL LOGS - Records obtained by lowering instruments in wells and recording continuously some physical property of
the rocks.
WELL TEST - Well tests conducted with the drillstring still in the hole. Often referred to as DST (drillstem test), these
tests are usually conducted with a downhole shut-in tool that allows the well to be opened and closed at the bottom of the
hole with a surface-actuated valve. One or more pressure gauges are customarily mounted into the DST tool and are read
and interpreted after the test is completed. The tool includes a surface-actuated packer that can isolate the formation from
the annulus between the drillstring and the casing, thereby forcing any produced fluids to enter only the drillstring. By
closing in the well at the bottom, afterflow is minimized and analysis is simplified, especially for formations with low flow
rates. The drillstring is sometimes filled with an inert gas, usually nitrogen, for these tests. With low-permeability
formations, or where the production is mostly water and the formation pressure is too low to lift water to the surface,
surface production may never be observed. In these cases, the volume of fluids produced into the drillstring is calculated
and an analysis can be made without obtaining surface production. Occasionally, operators may wish to avoid surface
production entirely for safety or environmental reasons, and produce only that amount that can be contained in the
drillstring. This is accomplished by closing the surface valve when the bottomhole valve is opened. These tests are called
closed-chamber tests.
Drillstem tests are typically performed on exploration wells, and are often the key to determining whether a well has found
a commercial hydrocarbon reservoir. The formation often is not cased prior to these tests, and the contents of the reservoir
are frequently unknown at this point, so obtaining fluid samples is usually a major consideration. Also, pressure is at its
highest point, and the reservoir fluids may contain hydrogen sulfide, so these tests can carry considerable risk for rig
personnel.
The most common test sequence consists of a short flow period, perhaps five or ten minutes, followed by a buildup period
of about an hour that is used to determine initial reservoir pressure. This is followed by a flow period of 4 to 24 hours to
establish stable flow to the surface, if possible, and followed by the final shut-in or buildup test that is used to determine
permeability thickness and flow potential.
WET GAS - A mixture of methane with significant amounts of ethane, butane, and propane; derived from mature source
rocks.
WET GAS SHRINKAGE FACTOR - See FORMATION VOLUME FACTOR
WILDCAT - See WELL.
YIELD - The fraction of OTOC by weight that is transformed into hydrocarbons.
 GROSS YIELD is the total generated;
 NET YIELD is that part released from the source rock.
ZONE - One of multiple pools (or postulated pools) in a field (or prospect).
27
ABBREVIATIONS & CONVERSION FIGURES OIL / GAS:
Notice: Quality control remains! Partly poor scanning….
GAS
NGL
LNG
1 Nm3
1 Sm3
1Sm3
1 Scuft
1 t.o.e.
- see oil (Propane -Butane)
- Dry gas (Metan - Etan)
= 1,056 Sm3
= 0,946 Nm3
= 35,55 Scuft
= 0,0283 Sm'
= 1000 Sm3
OIL
1 bbl
= 159 litres = 0,159 Sm3 = 0,135 ton
1 Sm3
= 6,29 bbl = 0,85 ton = 0,001 t.o.e.
1 ton
= 7,4 bbls (typical) = 1,176 Sm3
1 bbl/d
= 50 tons/year
Oil gravity
≈ 35º API = 850 kg/m3 (Varies with oil density)
Oil/NGL: number bbl / 6,29 = number Sm3 oil
1 Sm3/s
= 365 x 0.001 = t.o.e./year
ABBREVIATIONS:
°API
American Petroleum Inst1tute oil gravity
bbl
US barrel
bbl/d
barrels per day
Bg
gas formation volume factor
BHT
bottom-hole temperature
BOE
barrels oil-equivalent (also OEB)
G
109 (giga)
GB
109 barrels
GIP
gas in place
GOR
gas/oil ratio
HC
hydrocarbons
k
permeabi1ity
LNG
Liquefied Natural Gas (Dry gas)
LOM
1eve1 of organic metamorphism
MAX
maximum
MB
106 barrels
M bbl
106 barrels
MB/D
106 barrels /day
MCF /D 106 cubic feet/day
md
millidarcy
MIN
minimum
ML
most likely
N
number of samples in a distribution
NA
non-associated gas
N/G
net/gross ratio
NGL
natural gas liquids (Wet gas)
Nm3
Normal cubic metre
OE
oil-equivalent
OEB
oil-equivalent barre1s (also BOE)
OIP
oil in place
OMT
organic matter type
OTOC
original total organic carbon
P
probability
Pa
pascal
Ф
porosity (phi)
psi
pounds/square inch
Ro
vitrinite reflectance in oil
Scuft
(Scf) Standard cubic foot
3
Sm
Standard cubic metre
So
oil saturation
STB
stock tank barrel
STbbl
stock tank barrel
Std
standard
Sw
water saturation
Tº
temperature
TAI
thermal alteration index
TCF
1012
TOC
total organic carbon
t.o.e.
ton oil equivalents
TOM
total organic matter
TTI
time-temperature index
ABBREVIATIONS AND ACRONYMS
28
bbo billion barrels of oil
bo barrels of oil
boe barrels of oil equivalent (1 bo = 6000 dg)
Btu, British thermal unit. 1 Btu equals c. 1060 J
cfg cubic feet of gas (see sdg)
CGR condensate-gas ratio
DST drill-stem test
Fill flame ionization detection
FPD flame photometric detection
ft feet
GAB generation-accumulation efficiency
GC gas chromatography
GC-MS gas chromatography-mass spectrometry
GOR gas-oil ratio
Gt Gigaton
HC hydrocarbon
lWLC high-performance liquid chromatography
HI hydrogen index from Rock-Eval pyrolysis
H/C hydrogen/ carbon ratio from elemental
analysis
km kilometer
M thousand
m meter p
m3 cubic meter
mi mile '
m. y .million year time interval
Ma million years ago or million years of age
mW microwatt
OI oxygen index from Rock-Eval pyrolysis
O/C oxygen/carbon ratio from elemental analysis
PI production index from Rock-Eval pyrolysis
sdg standard cubic feet of gas (at standard temperature and pressure, STP)
t metric ton (tonne)
TAI thermal alteration index
TOC total organic carbon from Leco analyzer or carbon analyzer attached to Rock-Eval pyrolysis
tdg trillion cubic ft of gas
Tg teragram
TOC total organic carbon
TR transformation ratio
m time-temperature index
wt. % weight percent
UNIT CONVERSIONS
Energy
1 boe = 6000 cfg (energy equivalent)
Length
1 m = 100 cm
1 ft = 0.3048 m
Area
1 acre = 0.004046875 km2
1 mi2 = 2.589998 km2
1 km2 = 1 x 1010 cm2
Volume
1 bo = 42 gal (U.S.)
1 bo = 0.158987 m3
1 m3 = 6.28994 bo
1 m3 = 35.314 ft3
1 km3 = 1 x 1015 cm3
1 ft3 =0.028317 m3
Mass
1 Gt = 1015g
1 Tg = 1012g
300 API oil = 306.43lbs/bbl = 139.30 kg/bbl
400 API oil = 288.541bs/bbl = 131.15 kg/bbl
500 API oil = 272.581bs/bbl = 123.90 kg/bbl
GOR
ft3/bo = 0.178 m3/m3
m3/m3 = 5.614 ft3 /bo