1. NORMAL FAULTS, ASSOCIATED STRUCTURES AND HYDROCARBON TRAPS.
GROUP 3
RACHEL EVANS
FRANCIS EZEH
CHU’KA CHIZEA
NICK PAPANICOLAOU
Supervisor: Dr Noelle Odling
School of Earth Science
University of Leeds.
December, 2005

2. PRESENTATION OUTLINE NORMAL FAULTS FORMATION OF NORMAL FAULTS
TYPES
CHARACTERISTICS
GEOMETRIES
FORMATION OF NORMAL FAULTS
STRESS AND STRAIN REGIMES
BASINS & ASSOCIATED STRUCTURES
HYDROCARBON STRUCTURES AND PROSPECTIVITY
CASE EXAMPLES
DISCUSSION AND CONCLUSIONS

3. Definition of normal faults
Hanging wall moves down relative to the footwall.
Fault surface dips more steeply than 45o.
Created under tensional stress

4. ASSOCIATED NOMENCLATURE
FOOTWALL IS THE BLOCK BELOW THE FAULT PLANE
HANGING WALL IS THE BLOCK ABOVE THE FAULT PLANE
HEAVE IS THE MAXIMUM HORIZONTAL DISPLACEMENT
THROW IS THE MAXIMUM VERTICAL DISPLACEMENT
DIP IS THE ANGLE BETWEEN THE FAULT PLANE AND HORISONTAL
(Butler, R. 2003)

5. TYPES OF NORMAL FAULTS LOW ANGLE NORMAL FAULTS LISTRIC GROWTH FAULTS
DOMINO AND IMBRICATE NORMAL FAULTS.
CONJUGATE NORMAL FAULTS.

6. LOW ANGLE NORMAL FAULTS.
Detachment confined to crust: Extension Balanced by compression.
Gulf Coast and Perido Fold Belt.
Detachment Fault cuts whole lithosphere: Ductile shear zone at 10-15km.
Basin and Range Pronvince.

7. (Twiss, R.J & Moore, E.M. 1992)

8. GULF COAST EXAMPLE
(Twiss, R.J & Moore, E.M. 1992)

9. BASIN AND RANGE PROVINCE
(Twiss, R.J & Moore, E.M. 1992)

10. (Butler, R. 2003)

11. GROWTH FAULTS Form at same time as sedimentation (syn-sedimentary).
Sediment thickness decreases away from normal faults.
Fault dip shallows with increasing depth.
Associated with roll-over anticlines in syn-depositional settings.
Also associated with synthetic and antithetic faults.
Forms collapsed crest structures when detached faults can’t accommodate sediment load.
Growth index (ratio of sediments on both sides of major growth faults).

12. (Twiss, R.J & Moore, E.M. 1992)

13. CONJUGATE NORMAL FAULTS
Fault planes dip towards each other.

14. STRESSES
Stress is a pair of equal forces acting on a unit area of a body.
The magnitude of the stress is:
Stress = Force / Area
The force of gravity can give rise to a stress
Gravity makes an important contribution to the stress field governing the formation of faults and folds.
In order to understand the concept of stress we must first define “force”: a force is the product of a mass and its acceleration. Since force is a vector quantity, it can be represented as an arrow whose length specifies the amount and the orientation and direction is given by the arrow itself.
In rock deformation, we usually neglect any overall acceleration of a body and treat the system of forces as closed, i.e. opposing forces cancel out. (Newton’s 3rd law)
We can now define stress: a stress is a pair of equal forces acting on unit area of a body. Thus a stress results from a force acting on a surface surrounding or within a body, and comprises both the force and the reaction of the material on the other side of the surface.
The magnitude of the stress is:
Stress = Force / Area
The force of gravity can give rise to a stress which is measured by calculating its effect across a surface. Gravity makes an important contribution to the stress field governing the formation of faults and folds.
Newton’s 3rd law states that: “For a body at rest or in uniform motion, to every action there is an equal and opposite reaction”

15. A force F acting on a surface within a body can be resolved into:
a normal stress (σ)
a shear stress (τ).
Principal stress planes,
Principal stress axes,
σ1 greatest
σ2 intermediate
σ3 least.
Where the principal stresses are equal, the state of stress is said to be hydrostatic.
at depth, hydrostatic pressure is termed lithostatic.
A force F acting on a body can be resolved into:
a normal stress (σ) acting perpendicular to a surface within the body, and
a shear stress (τ) acting parallel to the surface. Shear stress can then be resolved into two further components, (τ1 -τ2) at right angles to each other.
The three mutually perpendicular planes on which the shear stress is zero are called principal stress planes, and the normal stresses across them are called principal stresses axes.
Where the principal stresses are equal, the state of stress is said to be hydrostatic.
In a system with unequal principal stresses, it is convenient to recognise a mean stress P, which represents the hydrostatic stress component of the stress field.
P=(s1+s2+s3)/3.
The remaining part of the stress system is referred to as the deviatoric stress component, which consists of three deviatoric stresses:
S1-P
S2-P
S3-P
These deviatoric stresses measure the amount of distortion in a body, whereas the hydrostatic stress component controls the change in volume.
In rocks at depth, stresses that are hydrostatic and due solely to the weight of overlying rock are termed lithostatic.

16. STRESS AXES AND FAULTS σ1 σ3 σ3 σ2
Stress is a force applied over an area. One type of stress, that we are all used to, is a uniform stress, called pressure. A uniform stress is a stress wherein the forces act equally from all directions. In the Earth, pressure due to overburden is a uniform stress, and is sometimes referred to as confining stress.
If stress is not equal from all directions then we say that the stress is a differential stress. Three kind of differential stresses occur:
Tensional stress (or extensional stress), which stretches rocks;
Compressional stress, which squeezes rock; and
Shear stress, which has a result of slippage and translation.
When rocks deform they are said to strain. Strain is the change in
shape, and/or
volume of a material.

17. STRESS AXES AND FAULTS σ1 σ3 σ3 σ2
Stress is a force applied over an area. One type of stress, that we are all used to, is a uniform stress, called pressure. A uniform stress is a stress wherein the forces act equally from all directions. In the Earth, pressure due to overburden is a uniform stress, and is sometimes referred to as confining stress.
If stress is not equal from all directions then we say that the stress is a differential stress. Three kind of differential stresses occur:
Tensional stress (or extensional stress), which stretches rocks;
Compressional stress, which squeezes rock; and
Shear stress, which has a result of slippage and translation.
When rocks deform they are said to strain. Strain is the change in
shape, and/or
volume of a material.

18. MOHR DIAGRAM Can obtain: Maximum & minimum stresses
Orientation of principal axes (note that the angle is 2Θ)
The angle in which failure occurs
The Mohr diagram is a convenient way of portraying the relationship in 2D between shear stress, hydrostatic pressure and the angle of failure at the point where failure occurs.
The centre of the circle is (σ1+σ2)/2 i.e. the mean stress or hydrostatic component
The radius of the circle is SQRT((((σ1-σ2)/2)^2)+(T^2))) i.e. the stress difference
From Mohr’s Circle we can obtain:
max & min stresses (C & R)
orientation of principal stress axes (from the angle measured from σ1 to the materials axis, note that it is 2Θ)
information on the strength of the material in question (from the failure envelope)

19. FAILURE CRITERIA τ = c + μ * σ ( Coulomb criterion )
T0 is the tensile strength of a material, fracturing occurs when tensile stresses exceed this tension fracture envelope, which is the boundary between stable and unstable states of tensile stress, given by sn*=T0
Coulomb’s criterion implies that the shear stress acting on the fracture plane to promote failure is opposed by the compressive stress acting across the fracture plane to prevent failure, but this scarcely is the case
More accurately,
Griffith’s criterion is based on the suggestion that failure results from the propagation and linking of minute defects in the material σt = tensile strength (a negative value ) of the material.
τ = c + μ * σ ( Coulomb criterion )
τ2 = I 4σt (σt + σ) I ( Griffith’s criterion )

20. STRAIN
Is the geometrical expression of the amount of deformation caused by the action of a system of stresses on a body.
Strain is the change
in shape (distortion) and
in volume (dilation),
or a combination.
If the amount of strain in all parts of a body is equal then it is called homogenous strain
In the case of heterogeneous strain, straight lines become curved and parallel lines non-parallel.
In addition, for convenience, rotation is used as an extra component of strain.

21. Strain can be measured in two ways:
Either by a change in length of a line (linear strain or extension)
Or by a change in the angle between two lines (angular strain or shear strain)
The principal strain axes are projected in an ellipsoid, the strain ellipsoid, which can be regarded as a deformed sphere.
e1,max, e2,inter and e3,min represent the stretches along the axes and are known as principal strains.

22. MATERIAL BEHAVIOR
It is dependant on several factors. Amongst them are:
Temperature
Confining Pressure
Strain rate (time)
Composition
Another aspect is the presence or absence of water.
It is dependant on several factors. Amongst them are:
Temperature – At high temperatures molecules and their bonds can stretch and move, thus materials will behave in a more ductile manner, whereas at low temperatures materials behave in a brittle manner.
Confining Pressure – At high confining pressures materials are less likely to fracture because the surrounding pressure tends to hinder the formation of fractures, but at low confining pressures materials will have a brittle behavior and tend to fracture sooner.
Strain rate – At high strain rates materials tend to fracture, and at low strain rates materials will seem more resistant to fracturing.
Composition – Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas and calcite are more ductile. This is due to the chemical bonding that binds each mineral. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock.
Another aspect is the presence or absence of water. Water appears to weaken the chemical bonds and to form films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in a brittle manner.

23. Elastic Deformation – Ductile Deformation – Fracture –
When a rock is subjected to increasing stress it passes through 3 successive stages of deformation.
Elastic Deformation –
wherein the strain is reversible,
Ductile Deformation –
wherein the strain is irreversible,
Fracture –
irreversible strain, wherein the material breaks.

24. STRENGTH OF THE MATERIALS
Two limiting stresses can be mentioned here:
Yield strength = above which permanent deformation occurs
Failure strength = above which failure occurs

25. STRAIN RATE
In laboratory experiments the effect of the applied stress is instantaneous,
Whereas in nature, the same effect will probably take more than tenths, hundreds, millions of years…?
So aside of the failure of the materials there will be elastic flow, deformation, e.t.c.

30. ASSOCIATED BASINS AND REGIONAL STRUCTURES
How all the previous statements correlate to basins?
BASINS ASSOCIATED WITH EXTENSIONAL DYNAMICS
RIFT BASINS
MID-OCEAN RIDGES

31. STRUCTURES ASSOCIATED WITH NORMAL FAULTS.
FOLDS.
Rollover Anticlines
Drag Folds
COMMONLY PRESENT AS SYSTEMS OF MANY ASSOCIATED FAULTS.
Synthetic faults
Usually smaller and parallel to the major fault and have same direction of dip.
Antithetic faults
In conjugate orientation to major faults and have opposite dip.
Ring faults
Concentric normal faults developed as surficial rock collapse into subsurface cavity: Calderas.
Strike-Slip faults
HORSTS AND GRABENS

32. SYNTHETIC AND ANTITHETIC
(Allen, P.A. & Allen, J.R. 1990)

33. HORSTS 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 grabens and the uplifted blocks form horsts.
In areas where tensional stress has recently affected the crust, the grabens may form rift basins and the horsts may form linear mountain ranges.

34. HORST AND GRABEN TOPOGRAPHY
The East African Rift Valley is an example of an area where continental extension has created such a rift.
The basin and range province of the Western U.S. is also an area that has recently undergone crustal extension.

35. ASSOCIATION OF HYDROCARBONS WITH NORMAL FAULTS
Settings for normal fault traps occur in two principal geological settings:
Fault-bounded grabens or half grabens
Forelands of compressional basins
Faults are rarely on their own a trapping mechanism but can have intrinsic association with other trap geometries.
Percentage of potential structures decrease rapidly with increasing distance from faults.
Fault-bounded grabens: faulting usually precedes the accumulation of hydrocarbons. In few cases do the faults alone cause the traps.
Foreland: Cutting sequences on the forelands of basins. Normal faults particularly well developed where the sequences where sequences thicken rapidly towards basinward into successions e.g. deltaic.
Normal faults associated with other trap geometries such as drape structures, unconformity traps and diapirs, fault-drag structures.
Percentage of structures which produce hydrocarbons drops of rapidly with increasing distance away from faults.

36. TRAPPING STRUCTURES ASSOCIATED WITH NORMAL FAULTS
Schematic diagram to see the other features with which normal faults are associated, e.g. anticlines, drapes, sedimentary sequences onlap so it is see that not all the hydrocarbon reservoirs are actually related directly to the normal faults.
(North, F.K. 1985)

37. Fluid seals or conduits?
Faults can act to both inhibit and enhance fluid flow.
Act as conduits for flow (not traps)
During reactivation of faults (after accumulation)
If potentiometric surface of the reservoir rock is above the topography.
More likely to act as fluid seals
Gouge/natural mudcake
Fluid pressures on either side of faults
Juxtaposition of lithologies

38. Standard Juxtaposition
Common possible juxtapositions of lithologies across faults include:
The reservoir rock juxtaposed across the fault within the hydrocarbon column.
One sandstone body juxtaposed across the fault not within the hydrocarbon column.
Two different reservoirs juxtaposed across the fault within the hydrocarbon column.
The reservoir rock juxtaposed across the fault with another lithology.

39. Hydrocarbon Prospectivity
(North, F.K. 1985)
(Allen, P.A. & Allen, J.R. 1990)

40. Hydrocarbon Production
Although normal faults can be useful to act as traps they can also restrict the production rate of a hydrocarbon reservoir.
Miri Oilfield, NW Borneo. Numerous oil accumulations, generally on the upthrown side of antithetic faults.
(North, F.K. 1985)

41. CASE EXAMPLE (1): NIGER DELTA BASIN
COMPOSED OF THREE MAJOR LITHOLOGIC UNITS.
GROWTH FAULTS DEVELOP DUE TO INTERPLAY OF SUBSIDENCE AND SEDIMENTATION (Weber., et al 1978).
LARGE REGIONAL GROWTH FAULTS DIVIDE BASIN INTO DISCRETE DEPOBELTS.
HYDROCARBON RESERVOIRS ARE FORMED BY THE DEVELOPMENT OF ROLLOVER ANTICLINES.
RESERVOIRS ARE ALSO FORMED BY THE JUXTAPOSITION OF SANDS AGAINST SHALE AS SECONDARY GROWTH FAULTS AND ANTITHETIC FAULTS DEVELOP.
MAJOR GROWTH FAULTS SERVE AS MIGRATION PATHWAYS.

42. RESEVOIRS BOUNDED BY MAJOR GROWTH FAULTS.
(North, F.K. 1985)
RESEVOIRS BOUNDED BY MAJOR GROWTH FAULTS.
RESEVOIRS ALSO DEVELOPS AT CREST OF ROLLOVER ANTICLINES.

43. CASE EXAMPLE (2): VIKING GRABEN, NORTH SEA.
(Glennie, K.W. 1998)

44. CASE EXAMPLE (2): VIKING GRABEN, NORTH SEA.
Plays are grouped according to reservoir age and their relationship to 3 main rift-related tectonic phases relative to the main Late Jurassic rift event.
Close relationship between play type and rifting.
Rifting controlled the thickness and facies distribution in the upper Jurassic syn-rift succession, including its widespread organic-rich marine source rock (Glennie., 1998).
Rifting also determined the distribution of mature Upper Jurassic source rocks following post-rift thermal subsidence (Glennie., 1998).

45. CONCLUSIONS Normal faults:
Influence the distribution of sediment infill (e.g. Viking Graben).
Can be important in hydrocarbon exploration.
Can form the trapping structure for hydrocarbon reserves when acting as seals; usually in combination with other structures.
Can also make production of hydrocarbon reservoirs more difficult.

46. REFERENCES
Allen, P.A. & Allen, J.R. (1990) Basin Analysis Principles & Applications. Blackwell.
Butler, R. et al. (2003).
Glennie, K.W. (1998) Petroleum Geology of the North Sea:basic concepts & recent advances. Blackwell Science: 4th edition.
North, F.K. (1985). Petroleum Geology. Allen & Unwin.
Twiss, R.J. & Moores, E.M. (1992) Structural Geology. W.H.Freeman
Weber, K.J. et al. The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures, , Offshore Tech. Conf., Houston paper OTC 3356.