NORMAL FAULTS, ASSOCIATED NORMAL FAULTS, ASSOCIATED STRUCTURES AND STRUCTURES AND

HYDROCARBON TRAPS.HYDROCARBON TRAPS.

GROUP 3GROUP 3

•RACHEL EVANSRACHEL EVANS•FRANCIS EZEHFRANCIS EZEH•CHU’KA CHIZEACHU’KA CHIZEA

•NICK PAPANICOLAOUNICK PAPANICOLAOU

•Supervisor: Dr Noelle OdlingSupervisor: Dr Noelle Odling

School of Earth ScienceSchool of Earth ScienceUniversity of Leeds.University of Leeds.

December, 2005December, 2005

PRESENTATION OUTLINEPRESENTATION OUTLINE

• 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

Definition of normal faultsDefinition of normal faults

• Hanging wall moves down relative to the footwall.

• Fault surface dips more steeply than 45o.

• Created under tensional stress

ASSOCIATED NOMENCLATUREASSOCIATED 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)

TYPES OF NORMAL FAULTSTYPES OF NORMAL FAULTS

• LOW ANGLE NORMAL FAULTS

• LISTRIC

• GROWTH FAULTS

• DOMINO AND IMBRICATE NORMAL FAULTS.

• CONJUGATE NORMAL FAULTS.

LOW ANGLE NORMAL FAULTS.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.

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

GULF COAST EXAMPLEGULF COAST EXAMPLE

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

BASIN AND RANGE PROVINCEBASIN AND RANGE PROVINCE

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

(Butler, R. 2003)

GROWTH FAULTSGROWTH 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).

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

CONJUGATE NORMAL FAULTSCONJUGATE NORMAL FAULTS

Fault planes dip towards each other.

STRESSESSTRESSES

• 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.

• 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.

STRESS AXES AND FAULTSSTRESS AXES AND FAULTS

σ1

σ2

σ3

σ3

STRESS AXES AND FAULTSSTRESS AXES AND FAULTS

σ1

σ2

σ3

σ3

MOHR DIAGRAMMOHR DIAGRAM

• Can obtain:– Maximum & minimum stresses– Orientation of principal axes (note that the angle is 2Θ)– The angle in which failure occurs

FAILURE CRITERIAFAILURE CRITERIA

• τ = c + μ * σ ( Coulomb criterion )

• τ2 = I 4σt (σt + σ) I ( Griffith’s criterion )

STRAINSTRAIN

• 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.

• 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.

MATERIAL BEHAVIORMATERIAL 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.

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.

STRENGTH OF THE MATERIALSSTRENGTH OF THE MATERIALS

• Two limiting stresses can be mentioned here:

• Yield strength = above which permanent deformation occurs

• Failure strength = above which failure occurs

STRAIN RATESTRAIN 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.

ASSOCIATED BASINS AND REGIONAL ASSOCIATED BASINS AND REGIONAL STRUCTURESSTRUCTURES

• How all the previous statements correlate to basins?

• BASINS ASSOCIATED WITH EXTENSIONAL DYNAMICS

• RIFT BASINS

• MID-OCEAN RIDGES

STRUCTURES ASSOCIATED WITH STRUCTURES ASSOCIATED WITH NORMAL FAULTS.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

SYNTHETIC AND ANTITHETICSYNTHETIC AND ANTITHETIC

(Allen, P.A. & Allen, J.R. 1990)

HORSTS AND GRABENHORSTS 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.

HORST AND GRABEN TOPOGRAPHYHORST 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.

ASSOCIATION OF HYDROCARBONS ASSOCIATION OF HYDROCARBONS WITH NORMAL FAULTSWITH 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.

TRAPPING STRUCTURES TRAPPING STRUCTURES ASSOCIATED WITH NORMAL FAULTSASSOCIATED WITH NORMAL FAULTS

(North, F.K. 1985)

Fluid seals or conduits?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

Standard JuxtapositionStandard 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.

Hydrocarbon ProspectivityHydrocarbon Prospectivity

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

Hydrocarbon ProductionHydrocarbon 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)

CASE EXAMPLE (1): NIGER DELTA CASE EXAMPLE (1): NIGER DELTA BASINBASIN

• 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.

• RESEVOIRS BOUNDED BY MAJOR GROWTH FAULTS.• RESEVOIRS ALSO DEVELOPS AT CREST OF

ROLLOVER ANTICLINES.

(North, F.K. 1985)

CASE EXAMPLE (2): VIKING GRABEN, CASE EXAMPLE (2): VIKING GRABEN, NORTH SEA.NORTH SEA.

(Glennie, K.W. 1998)

CASE EXAMPLE (2): VIKING GRABEN, CASE EXAMPLE (2): VIKING GRABEN, NORTH SEA.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).

CONCLUSIONSCONCLUSIONS

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.

REFERENCESREFERENCES

• Allen, P.A. & Allen, J.R. (1990) Basin Analysis Principles & Applications. Blackwell.

• Butler, R. et al. (2003). www.earth.leeds.ac.uk/learnstructure• 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, 2643-2651, Offshore Tech. Conf., Houston paper OTC 3356.