PET312
BASIC PETROLEUM
GEOLOGY
Units: 3
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Brief overview of Course
The course basically is meant to
introduce students to petroleum geology
with particular interest in the origin of
petroleum, migration and eventual
accumulation due to geologic traps
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Course Objectives/Goals
At the end of the course, students should be
able to:
1. Know what minerals and rocks are,
sedimentary processes that supports
petroleum occurrence.
2. Know the origin of petroleum
3. Understand the concept of migration of
hydrocarbon from source rocks to reservoir
rocks
4. Understand how the various hydrocarbon
traps lead to accumulation of petroleum
3
Course Outline
1. Weathering, Erosion and Deposition
Geological cycles
Sedimentary processes
Sediment transport and deposition
2. Diagenesis
3. Reservoirs
4. Structural Geology and Petroleum
5. Origin, Migration and Accumulation of Petroleum
6. Sedimentary Basins
Basins in Nigeria –
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Recommended Reading/Text
1. Rocks and minerals – by Chris Pellant
2. An introduction to the study of mineralogy – by Bakewell, J.R
3. Elements of petroleum Geology – by Richard C. Selley
4. Introduction to physical geology – Plummer et. al.
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Weathering, Erosion and Deposition
This section explains that part of
the rock cycle that operates at,
or near, the Earth's surface. Such
processes are responsible for the
formation of hydrocarbon source
rocks, most reservoir units, and
seals.
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1.2 Physical and chemical weathering of rocks
Two distinct types of weathering are distinguished: physical weathering (or
mechanical disaggregation) and chemical weathering (or chemical
decomposition). Both processes, either separately or together, lead to the
formation of a wide range of sedimentary rocks.
 frost shattering: Such shattering of rocks into smaller fragments exposes a much
larger surface area to rainwater run-off, and so that they undergo chemical
weathering.
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Bowen reaction series 2
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Bowen reaction series 1
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The products of chemical weathering
Three products result from chemical weathering.
1) Leaching away of minerals
2) New minerals are formed thru leaching of existing minerals. An example of this process is
the formation of clay minerals through the weathering of feldspar and micas in granite.
3) Resistant minerals, such as quartz, are not chemically attacked at all. Remaining minerals
are known as residual minerals.
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The transport of weathered material
This points to the removal of residual products of weathering various transporting
media. (Wind, water, glacial, human)
 loads (bedload and suspended)
 sorting
 Energy of transportation (sorting, roundedness and angularity)
 distance of transportation
 nature of sediment in motion (i.e. Grain size)
The transport and erosion of sediment by wind and ice: Consequence is well
sorted sediment aggregate. Sediments end up with more rounded corners and
edges worn down much more rapidly due to the cushioning effect of water that is
absent.
On the converse, because sediments are embedded in ice and unable to move
they tend to retain their characteristic and consequently, true glacial sediments
are very poorly sorted and the fragments are characteristically angular. This
means that glacial sediments are not characterized by diagnostic grain shapes or
degrees of sorting.
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The transport and deposition of the soluble products of chemical
weathering:
Two major ways in which dissolved salts are removed are
(i) by the action of marine organisms and
(ii) by direct chemical precipitation.
a.) Many marine animals build shells composed of calcium carbonate by extracting
both Ca2+ and HCO3 from seawater to form the mineral calcite (CaCO3). Typical of
this is the formation of limestone
Formation of calcareous sediments in the deeper ocean basins from the
accumulation of calcareous microfossils that are far too small to be seen with the
naked eye (calcareous oozes). This accounts for the formation of fine-grained chalky
limestones.
Once exposed at the Earth's surface, limestones are very susceptible to chemical
weathering because they dissolve easily in rainwater that has been acidified by
atmospheric carbon dioxide.
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b.) Sediments formed from soluble cations by direct precipitation are comparatively
rare, but some are of great economic importance. The best known are salt deposits, or
evaporites, which form in isolated inland seas or in narrow, newly formed ocean basins
where circulation with the other masses of sea-water does not take place. The water
becomes saturated with respect to ions such as Na+, K+, M2+, Ca2+, CI-, (SO4)2- and (HCO3), and salts of these, such as halite (NaCl), gypsum (CaSO4.2H2O) and, of course, calcite
(CaCO3), crystallize out.
High evaporative rates are common in arid climates; thus, evaporites in rock sequences
are a good indicator of ancient arid climates.
A few limestones are precipitated directly from seawater but they are relatively rare.
They require warm shallow waters (CaCO3 is less soluble in warm water than cold
water) that are well agitated by waves or currents, and particles such as sand grains or
shell fragments act as nuclei for the precipitated CaCO3 to grow around. The resulting
calcareous grains are slightly well rounded and are termed ooids.
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EROSION AND MASS WASTING
These processes of wearing away and removal (or transport) of debris are known collectively
as erosion, a very apt term derived from two Latin words: ‘e' meaning ‘from' and ‘rodere'
meaning ‘to gnaw'. Thus erosion is the ‘gnawing away from' the land surfaces, which implies
a slow but very persistent process. Name some agents of erosion that you know?
Mass movement
The causes of mass movement: Weathered debris is usually stable on a slope until the
friction between the debris and the slope is overcome. In theory friction alone can hold
weathered debris on slopes up to 45°, but in practice movement may occur on slopes as
little as 1°, and so there must be other factors that act to overcome the friction. Triggering
mechanisms can be tectonic activity as, for example, in regions subjected to frequent
earthquakes.
The types of mass movement
“The importance of mass
movement is that it is the
means whereby weathered
debris can be moved from
hillsides and cliffs into a river,
onto a glacier, or into the sea
to begin a more substantial
phase of transport.”
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Facts
 When velocities are low, flow are laminar, and only fine sands, silts, and
clays can be moved; they are carried within the moving water that is in
suspension and form the suspension load. Fine-grained sediments carried
in suspension by laminar flows form suspension deposits when the current
velocity falls.
 When flow is turbulent, coarse sand grains and even pebbles can be
transported, but these are moved initially along a riverbed, and not in
suspension. This coarser sediment forms the traction or bed load of the
moving water. A very much higher velocity of water movement is needed to
lift the bed load into suspension.
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The erosion of sediments by moving fluids
A noticeably higher velocity is required to begin moving a
grain than that at which the grain was deposited. This is
because the friction between the grain and the riverbed must
be overcome first. Where wind transport is concerned, wind
speeds of the order of thirty times greater than those of
water are required to set grains of a given size in motion.
Finer silts and clays show a cohesive strength, which is due
to surface tension created by films of water between the
particles. These are called cohesive sediments, and
cohesion between the particles is increased by compaction of
the sediment.
Thus, once clay sediments are deposited, they are less likely
to suffer reworking than sandy sediments. Sand-sized
sediments are more likely therefore to be preserved if they
are buried by a layer of clay.
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Sediments and sedimentary bed forms resulting from
deposition in flowing water
The evidence of this is that, suspension and traction deposits
each lead to characteristic types of layering in sediments, which
are then preserved in sedimentary rocks.
 The nature of such layering will influence the vertical and
horizontal characteristics of reservoir units. When the discrete
layers are only a few millimetres thick, they are termed laminae
and the sediments or rocks are described as laminated.
 On the other hand, layers equal to or greater than 0.5cm are
called beds. Horizontal or planar laminae like these are
characteristic of suspension deposits. However, coarser horizontal
layering, known as planar bedding, occurs in some traction
deposits. Slow-moving water currents deposits coarse sands,
while at higher current velocities, the traction deposits are built
into a series of mounds on the bed beneath the current.
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Fig. The movement and deposition of sediment on a ripple.
Solid arrows represent transport in and deposition from the bed load. Broken arrows
represent transport in and deposition from the suspended load. Aqueous dune and
sand wave forms are produced by a similar pattern of sediment movement.
Bedform configurations
 cross-stratification
 cross-lamination
 cross-bedding.
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Flow
Bedforms
regimes
Lower flow Ripples, low
regime
planar beds,
sand waves and
sand dunes
Upper flow Upper planar
regime
beds, antidunes
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Two important points that are very relevant to the interpretation of sequences of
sedimentary rocks.
First, we can use cross-lamination or cross-bedding preserved in sedimentary
rocks to tell us which way the current was flowing when the original sediments
were deposited. This is referred to as a palaeocurrent direction. Such
information can be obtained from the subsurface if oriented cores are recovered,
or, for cross bedding, from dip-meter data. Knowledge of the palaeocurrent
direction is likely to aid prediction of the direction of elongation of reservoir sand
bodies.
Second, we can make estimates about how flow conditions may have varied with
time from the range of bed forms present in a sequence of sediments. Failure to
recognize whether or not cross-stratification represents a cross-section of a bed
form parallel to the flow direction can lead to the miscalculation of palaeocurrent
directions.
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Sedimentary bed forms resulting from oscillatory water movements
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Fig. Patterns of cross-stratification produced by tidal currents.
The arrows are proportional in length to the strength of opposing currents. (a) herring-bone
cross-stratification: the currents have approximately equal strengths. This form of crossstratification is produced only by larger bed forms and not produced by ripples. (b)
Reactivation surfaces within cross-stratification caused by opposing tidal currents: in this
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illustration, the flood tide is stronger than the opposing ebb tide.
Fig. The movement of sediment and water within a turbidity current.
In the head, the sediment and water are swept up and round, whereas turbulent eddies develop
in the body.
Turbidite sequence
Although the initial velocity of the current is a function of the density contrasts between current
and seawater and of the thickness of the flow, the decrease in gradient towards the base of the
continental slope leads to a rapid decrease in velocity so that deposition occurs. The resultant
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deposit is known as a turbidite.
Fig. The ideal Bouma turbidite sequence.
Units A to E were deposited in different flow conditions within a turbidity current.
Note that the width of the sediment column is proportional to the grain size. 25