Importance of structure geology in petroleum industries
Prepared by
Tarek Abdellatif Ali
Senior Geologist
General Petroleum Company
 introduction
 Principals of structure geology
 Hydrocarbon Traps and trap style
 Fault seal integrity
 STRUCTURE GEOLOGY REDUCES DRILLING OIL AND GAS PROBLEMS
 Reservoir Geomechanics
 Fracture reservoirs
 Conclusion
Introduction
Petroleum is a compound of concentrated hydrocarbon components and other few quantities of
inorganic components containing sulfur, oxygen, nitrogen, and some metals. Petroleum includes
both oil and hydrocarbon gas. (Petroleum geoscience / by Jon Gluyas and Richard Swarbrick,
2006)
A petroleum system concept was defined by Magoon and Dow in 1994 as a pod of active source
rock and all genetically related oil and gas accumulations. It includes all the geologic elements
and processes that are essential if an oil and gas accumulation is to exist.
The elements include source rock, reservoir rock, overburden, seal rocks and trap while the
processes include generation, migration and accumulation of hydrocarbon as well as trap
formation.
These elements and processes are known as magic five ingredients.
The source rock is as a sedimentary rock that contains sufficient organic matter that when it is
buried and heated it will produce petroleum (oil and gas).
The environment of high productivity of organic matter and then petroleum includes many
types of environment such as coastal upwelling, swamps, shallow seas, lakes and deep marines.
The reservoir rock is a container that has sufficient void spaces which allow to fill with fluids.
Reservoir rocks should have connected pore spaces to permit fluids to flow.
There are common types of reservoir rocks which may be well-sorted sandstones or carbonate
such as limestone and dolomite and some rare of basement rocks like fractured granites.
Unconventional reservoirs also are common where shales and tight rocks contain petroleum
(oil and gas).
Overburden rocks are that stratigraphic sedimentary rocks that overlies the source rock, seal
rock, and reservoir rock.
The thickness and age of overburden rock provides a history of the rate of burial of a source
rock toward and through the increasing temperature domains of the basin.
The seal rocks are fine-grained or crystalline of low permeability rocks. Typical examples include
muds tone/shale, cemented limestone, chert, anhydrite, and salt (halite).
The importance of seals to prevent the hydrocarbon from escaping to the surface.
Trap is simply the geometry of the sealed petroleum-bearing reservoir.
Traps can be divided into three broad categories; structural, stratigraphic and combination of
them.
Processes of petroleum include generation where the organic matter has transformed into
petroleum, migration where the hydrocarbon migrates internally in source rock which is known
as primary migration and externally to the carrier bed and then reservoir which is known as
secondary migration, and accumulation of hydrocarbon where it entrapped in reservoir.
See figure (1).
FIGURE (1) PETROLEUM SYSTEM ELEMENTS AND PROCESSES
Principals of structure geology
Structure geology is the study of architecture of earth’s crust.in other word the structure
geology is the study of the history of deformation (strain) of rocks in three dimensions to
understand the affected stress.
These stresses applied mainly due to tectonic activities of the earth.
Deformation: is a change in shape, geometry and location of the rock body.
By kinematic analysis, a deformation may occurred due to pure shear or simple shear stress.
Pure shear (known as coaxial strain) occurs when principle axes of finite strain ellipse remain at
Principle axes of finite strain ellipse remain at same orientation through progressive strain.
Simple Shear (known as non -coaxial strain) when principle axes of finite strain ellipse rotate
through each stage of progressive strain.
Figure (2) kinematic strain types
Deformational Structures Types:

Non-tectonic structures: (also known as syn-sedimentary structures or primary
structures) which originate close to the Earth´s surface, most likely due to gravitational
forces.
These structures may be: bedding features, mud cracks, ripple marks or vesicles.
 Tectonic structures are related with regional stress-field as the response to geodynamic
(tectonic) processes.
The most important in petroleum industries is tectonic structures which are divided into three
types of deformations on the basis of different stress regimes, these are: compressional,
tensional and shearing figure (3).
FIGURE (3) DEFORMATION TYPES
The rheology of the rocks is a main factor in the form of deformation and dependent on the rheology
the deformation can be divided into three forms, brittle, ductile and brittle-ductile deformation.
Brittle deformation occurs when a rock deforms by fracturing into joints with no surface
displacement and faulting with surface displacement.
Joints commonly form when the surface of a volcanic rock cools and contracts figure (4).
FIGURE (4) JOINTS
A fault is a surface or narrow zone with parallel displacement and offset of layers either side of the
fracture and can be classified by kind of movement that has occurred along them into:
Normal Fault: it occurs when a part of rock moves parallel to the fault plane in the downward relative
to the original rock, the moved part called hanging-wall and the original rock called foot-wall figure(5).
FIGURE (5) FAULT TERMS &NORMAL FAULT
Thrust fault: (A reverse fault is a high angle thrust fault) it occurs when the hanging-wall moves
up along a fault plane relative to the foot-wall figure (6).
FIGURE (6) THRUST FAULT
(LIFT) & REVERSE FAULT (RIGHT)
Strike-slip Faults: are steep faults or vertical faults along which the rocks on either side have
moved horizontally in opposite directions.
It is described as left-lateral (sinistral) when the lift block moves in the direction of the sight and
right lateral (dextral) when the right block moves to the direction of the sight figure (7).
FIGURE (7) Strike-Slip Fault Left-Lateral (LIFT) & Strike-Slip Fault Right-Lateral (RIGHT)
Horst and Graben: Due to the tensional stress responsible for normal faults, they often occur in
a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped
blocks form Graben and the uplifted blocks form Horst figure (8).
FIGURE (8) Horst and Graben
Half-Graben: A normal fault that has a curved fault plane with the dip decreasing with depth
can cause the down-dropped block to rotate figure (9).
FIGURE (9) Half Graben
Ductile Deformation
When rocks deform in a ductile manner, instead of fracturing to form faults or joints, they may
bend or fold, and the resulting structures are called folds.
Folds are resulted from compressional stresses or shear stresses acting over considerable time.
Because the strain rate is low and/or the temperature is high, rocks that we normally consider
brittle can behave in a ductile manner resulting in such folds.
We recognize several different kinds of folds.
Monoclines: are the simplest types of folds. Monoclines occur when horizontal strata are bent
upward so that the two limbs of the fold are still horizontal.
Anticlines: are folds where the originally horizontal strata has been folded upward, and the two
limbs of the fold dip away from the hinge of the fold.
Syncline: are folds where the originally horizontal strata have been folded downward, and the
two limbs of the fold dip inward toward the hinge of the fold. Synclines and anticlines usually
occur together such that the limb of a syncline is also the limb of an anticline.
Brittle-Ductile Deformation
The Ductile - Brittle Deformation occurs along strong movement while a simple shear stress
reveals formed second order wrenching structures.
It begins by ductile deformation (folds) and ends by through-going fault (strike slip fault)
passing by complexity of structure styles figure (10).
FIGURE (10) Wrench-Tectonics
Hydrocarbon Traps and trap style
Hydrocarbon traps form where permeable reservoir rocks (carbonates, sandstones) are covered
by rocks with low permeability (cap-rocks) that are capable of preventing the hydrocarbons
from further upward migration.
Traps require two fundamental components, they are a reservoir rock in which to store the
hydrocarbons and a seal
Traps are usually classified according to the mechanism that produces the hydrocarbon
accumulation. The two main groups of traps are those that are formed by structural
deformation of rocks (structural traps), and those that are related to depositional or diagenetic
features in the sedimentary sequence (stratigraphic traps).
Many traps result from both of these factors (strati-structural or combination traps).
The significant types to talk about are structural or combination traps.
Structural traps
Structural traps are caused by structural features. They are formed as a result of tectonics.
Structural traps are primarily the result of folding and (or) faulting, or both.
Anticlinal (fold) and dome traps:
A dome: an impervious cap rock and a porous reservoir rock; closure occurs in all directions to
prevent leakage and named four-way closure.
Anticlinal (fold): The simplest type of trap is formed when a sandstone bed that is overlain by
tight (i.e. low permeability) shale is folded into an anticline.
The crest of the anticline must have an apical culmination (i.e. a peak) somewhere along the
fold axis so that hydrocarbons can be trapped figure (11)
Salt domes: Strata around the salt dome curve upward creating traps against the sealing salt
layers figure (12).
Salt domes form when salt is less dense than the overlying rock, and the salt moves slowly
upwards due to its buoyancy. For this to happen, there must be a minimum overburden and the
thickness of the salt deposits must be more than ~100 m. Upward movement of salt through
the sedimentary strata, and associated deformation is termed halokinetics or salt tectonics.
Movements may continue for several hundred million years.
Traps may form:
(1) in the strata overlying the salt dome, (2) in the top of the salt domes (the cap rock - caused
by brecciation and dissolution), (3) in the strata that curve upward against the salt intrusion, (4)
due to stratigraphic pinch-out of strata around the salt dome:
Fig. (11) Anticlinal Fold Trap Type
Fig. (12) Ideal Salt-Dome Traps
Fault Trap: The fault plane must have a sealing effect so that it functions as a fluid migration
barrier for reservoir rocks. There are several common types of fault trap:
1) Normal faults: commonly associated with graben (rift) structures.
2) Strike-slip faults: these may not be sealed due to incremental movements, but
basement-controlled strike-slip faults commonly produce good anticlinal structures in
overlying softer sediments.
3) Thrust faults: commonly associated with compressional tectonics (e.g., the Front Ranges
in Alberta).
4) Growth faults: Growth faults typically form in sediments that are deposited rapidly,
especially at deltas. Faulting occurs during sedimentation (i.e. syndepositionally), such
that the equivalent strata on the downthrown side will be thicker than on the upthrow
side.
The throw between corresponding strata declines upwards along the fault plane. Minor
fault planes with an opposite throw (antithetic faults) may also form in the strata that
curve inward towards the main fault plane. The fault plane is commonly sealed,
preventing further upward migration of oil and gas. Fault traps may also form when
sandstone beds are offset against the fault plane. Some petroleum traps, however, form
in "roll-over" anticlines on the down-faulted block. Growth faults may reduce pore water
circulation in sedimentary basins; consequently, under-compacted clays, which may
develop into clay diapirs, are often associated with growth faults.
Well
Fig (13) Normal Fault Trap
Fig (13) Thrust Fault Trap
Fig (14) Strike-Slip Fault Trap
Fig (15) Growth Fault Trap
In most common a fault traps are associated by folds such as in thrust belts and inverted
basins, also in rifting system where rollover anticlines are formed figure (16).
Fig (16) Combination of Fault and Fold Trap
Combination traps
Many of world’s hydrocarbon traps are not simple but combine both structural and
stratigraphic elements.
Map View
Map View
Cross-Sectional View
Cross-Sectional View
Fig (17) Combination of Fault and Stratigraphic Trap (Left) Fold and Stratigraphic Trap (Right)
Other Types of Structural Traps
Fractured basement rock that projects upward locally into overlying shale can also provide
good traps figure (18).
Fractured, well-cemented rocks (limestone and chert) along faults may also form good traps of
limited lateral extent figure (18).
Fig (17) Fractured Basement Trap (Left) Fractured Tight Rock Along Fault Trap (Right)
Fault seal integrity
Fault sealing is now recognized as one of the key factors controlling hydrocarbon reservoir
trapping and behavior during production. Fault seal represents a significant unknown in any risk
analysis associated with both hydrocarbon exploration and development strategies.
Define geometry
of fault array
Establish sub-seismic fault
density and fault zone
structure
Test models against
hydrocarbon contact
levels if known
Map seal
distributions on fault
planes which might
form compartment
Assess sealing mechanism
and fault rock properties
Evaluate critical
juxtapositions and seal
distributions
Model reservoir flow and the impact of faults on
drainage
Fault seal processes
There are five groups of processes which can operate individually or which may combine to
alter the pore structure during or after deformation events.
1) Deformation induced porosity collapse by disaggregation, mixing and grain boundary
sliding without large scale cataclasis. This group of processes dominates deformation
of unconsolidated or unlithified material and is therefore characteristic of the
deformation at shallow depths of burial.
2) Diffusive mass transfer: This process involves the redistribution of material away from
sites of high stress by dissolution, transport and precipitation.
3) Cataclasis: Processes which induce grain size reduction by fracturing are grouped
together as cataclasis and dominate the deformation associated with faulting in
lithified rocks.
4) Cementation: Faults may act as conduits for fluid flow during deformation and are
therefore susceptible to mineralization.
5) Clay or phyllosilicate smearing: describe deformation induced shearing of clays or
phyllosilicates.
Lindsay et al. (1993) have identified three types of clay smear associated with:
a) Abrasion by movement past sandstones.
b) Shearing and ductile deformation between hanging-wall and footwall cut-offs of
shale beds.
c) Injection of clays during fluidization.
The fault seal types and associated fault-rock types can be divided into two broad categories:
a) Juxtaposition seals: These seals are associated with the cross-fault juxtaposition of
lithologies with different petrophysical properties, e.g., sand on shale or sand on
cemented unit.
b) Fault rock seals: These seals are associated with the type of fault rock developed during
deformation.
Fig. (19) Cemented Faults and Fractures
Fig. (19) Clay Smear
Fig. (20) SEM Image of a Cataclastic Fault Gouge in a Clean Sandstone.
STRUCTURE GEOLOGY REDUCES DRILLING OIL AND GAS PROBLEMS
Drilling operation is the first and the most costly phase in petroleum industry as, according to API
(1991), it represents nearly one fourth of the total oil-field expenditure and its efficiency is
mandatory as it will affect the whole investment technically and financially.
Drilling well problems
Borehole instability: Borehole instability in petroleum is well defined as the undesirable
condition of an open hole interval that does not maintain its gauge size, shape or structural
integrity, consequently driving the drilling operation into major problems and difficulties that will
lead to additional costs and nonproductive time which increases with its severity.
Borehole instabilities pose significant challenges to drilling and completion operations,
particularly in regions with weak bedding planes and pre-existing fractures where formations
have strong anisotropies.
Present day in-situ stresses acting on the freshly drilled borehole is therefore likely to be
substantial.
After drilling, it is important that the borehole stays in good shape if it is to success- fully perform
its intended function, e.g. produce hydrocarbons.
Borehole instability will severely compromise this function and reduce the working life of the well
figure (21).
Fig. (21) Types of Borehole instability
Loss of Mud Circulation: Loss of circulation is the undesirable flow of portion or whole mud into
the formation.it causes many problems which may be Formation damage, wellbore instability,
Insufficient hole cleaning and then extra costs.
Several aspects cause loss of mud circulation among them inherently fractured formations and
induced fractured formation due to present-day stresses, and also fault zones figure (22).
Fig. (22) Types of Loss of Mud Circulation
Reservoir Geomechanics
Reservoir geomechanics has become a powerful tool to better understand the reservoir when
geophysics, sedimentology, petrophysics or any other reservoir discipline is not enough to
explain the reservoir response to different scenarios during its productive life.
Geomechanics not only are the key to understand the reservoir properties but also to control
the drilling problems.
A geomechanical reservoir model requires knowledge of the in-situ stress state (magnitudes
and orientations), the reservoir pressure and the rock strength properties.
In-situ stress state:
Overburden stress (σov)
Overburden stress (σov) magnitude is equivalent to the weight of overlying fluids and rock. It
can be calculated by integrating the density of the materials overlying the depth of interest.
Minimum horizontal stress (σh)
It can be measured from leak-off test or minifracs, where a fluid is pumped into the wellbore to
pressurize a short interval of exposed rock until the rock gets fractured and the fracture is
propagated a short distance away from the wellbore by continued pumping. Orientation of the
minimum horizontal stress is mainly obtained from wellbore breakouts when an image log is
run in a vertical well.
Maximum horizontal stress (σH)
Once independent measurements of σov and σh are available, σH can be estimated from
analytical methods based on the breakouts widths in vertical boreholes or from derived
equations, which relate σh and mechanical properties. The maximum horizontal stress
orientation is perpendicular to the minimum horizontal stress.
Reservoir pressure:
Reservoir pressure is the pressure at which the fluid contained within the pore space of a rock is
maintained at depth.
Rock strength properties:
For small strains the elements of the stress and strains are linearly related. Depending on the
mode of the acting geological force and type of geological media the force is acting upon, three
types of deformation can result as well as three elastic properties that correspond to each type
of deformation, i.e., Young’s modulus (E), Bulk modulus (K), Shear modulus (G) and Poisson’s
ratio (ν).
Nearly all hydrocarbon reservoirs are affected in some way by natural fractures, yet the effects
of fractures are often poorly understood and largely underestimated.
Fractured Reservoirs
Nearly all hydrocarbon reservoirs are affected in some way by natural fractures, yet the effects
of fractures are often poorly understood and largely underestimated.
They include reservoirs with low hydrocarbon recovery, these reservoirs initially may appear
highly productive, only to decline rapidly.
In carbonate reservoirs, natural fractures help create secondary porosity and promote
communication between reservoir compartments.
The investigation of natural fractures should start during the exploration stage.
When developing and modeling fractured reservoirs, the ability to understand and predict the
characteristics of fracture and fault systems is essential.
The complexity of natural-fracture systems is captured in the descriptive, genetic and geometric
methods that geoscientists employ to classify natural fractures.
In the laboratory, fracture types are divided into two groups related to their mode of
formation: shear fractures that form with shearing parallel to the created fracture, and tension
fractures that form with tension perpendicular to the created fracture.
In the laboratory, shear and tension fractures form in consistent orientation with respect to the
three principal stress directions, namely the maximum compressive principal stress, σ1, the
minimum compressive principal stress, σ3, and the intermediate stress, σ2 figure (23).
Fig. (23) Principal stresses and the creation of fractures.
Extension fractures include natural tension fractures (formed in absolute tension) and
hydro-fractures. For extension fractures, the relative displacement is perpendicular to, and
away from, the fracture plane.
For shear fractures the relative displacement is parallel to the fracture plane. Shear fractures
with significant or large displacements are commonly referred to as faults.
For modeling rock fractures in reservoirs, three crack modes are used Extension fractures are
modeled as modeI cracks. Shear fractures are modeled as either mode II or mode III cracks
depending on whether the crack-tip displacement is perpendicular or parallel to the leading
edge (tip) of the crack. The various combinations of these basic types include mixed mode
cracks (“hybrid cracks”) where, commonly, modeI displacement is mixed with either mode II or
mode III displacement figure (24).
Fig. (24) Principal stresses and the creation of fractures.
Most, if not all, reservoirs contain fractures. It is the degree to which fractures influence fluid
flow through a reservoir that should dictate the level of resources needed to identify,
characterize and model fractures.
Fractured reservoirs are classified based on the interaction between the relative porosity and
permeability contributions from both the fracture and matrix systems.
In Type 1 reservoirs, fractures provide both the porosity and permeability elements.
Type 2 reservoirs have low porosity and low permeability in the matrix, and fractures provide
the essential permeability for productivity.
Type 3 reservoirs have high porosity and may produce without fractures, so fractures in these
reservoirs provide added permeability.
Type M reservoirs have high matrix porosity and permeability, so open fractures can enhance
permeability, but natural fractures often complicate fluid flow in these reservoirs by forming
barriers. Fractures add no significant additional porosity and permeability to Type 4 reservoirs,
but instead are usually barriers to flow.
Type G has been created for unconventional fractured gas reservoirs, such as Coal-Bed
Methane, and fractured gas-condensate reservoirs. Most Type G reservoirs fall within or near
the Type 2 reservoir classification figure (25).
Fig. (24) Naturally fractured reservoir classification system.
Conclusion
Structure geology is obviously one of the most important subjects for geoscientists working in
petroleum industry as they can identify the locations that may have traps such fold and faults
which are good for the accumulation of oil and natural gas.
Structure geology not only is the main role in petroleum exploration phase, but also in the
appraisal and development phase.
The study of reservoir characteristics and behavior depend in wide range on the understanding
its physical properties which affected by the structure regime.
The directional oil and gas wells depend mainly on understanding structure setting of the area,
also, its design should control by study of the structure to mitigate the drilling problems while
drilling.
References