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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 PRESENTATION 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 faults • Hanging wall moves down relative to the footwall. o. • Fault surface dips more steeply than 45 • Created under tensional stress 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) TYPES OF NORMAL FAULTS • • • • LOW ANGLE NORMAL FAULTS LISTRIC GROWTH FAULTS DOMINO AND IMBRICATE NORMAL FAULTS. • CONJUGATE 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 EXAMPLE (Twiss, R.J & Moore, E.M. 1992) BASIN AND RANGE PROVINCE (Twiss, R.J & Moore, E.M. 1992) (Butler, R. 2003) GROWTH FAULTS • Form at same time as sedimentation (synsedimentary). • Sediment thickness decreases away from normal faults. • Fault dip shallows with increasing depth. • Associated with roll-over anticlines in syndepositional 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 FAULTS Fault planes dip towards each other. 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. • 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 FAULTS σ1 σ3 σ3 σ2 STRESS AXES AND FAULTS σ1 σ3 σ3 σ2 MOHR DIAGRAM • Can obtain: – Maximum & minimum stresses – Orientation of principal axes (note that the angle is 2Θ) – The angle in which failure occurs FAILURE CRITERIA • τ = c + μ * σ ( Coulomb criterion ) • τ2 = I 4σt (σt + σ) I ( Griffith’s criterion ) 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 nonparallel. • 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 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 MATERIALS • Two limiting stresses can be mentioned here: • Yield strength = above which permanent deformation occurs • Failure strength = above which failure occurs 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. ASSOCIATED BASINS AND REGIONAL STRUCTURES • How all the previous statements correlate to basins? • BASINS ASSOCIATED WITH EXTENSIONAL DYNAMICS • RIFT BASINS • MID-OCEAN RIDGES 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 SYNTHETIC AND ANTITHETIC (Allen, P.A. & Allen, J.R. 1990) 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. 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. 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. TRAPPING STRUCTURES ASSOCIATED WITH NORMAL FAULTS (North, F.K. 1985) 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 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 Prospectivity (North, F.K. 1985) (Allen, P.A. & Allen, J.R. 1990) 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) 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. (North, F.K. 1985) • RESEVOIRS BOUNDED BY MAJOR GROWTH FAULTS. • RESEVOIRS ALSO DEVELOPS AT CREST OF ROLLOVER ANTICLINES. CASE EXAMPLE (2): VIKING GRABEN, NORTH SEA. (Glennie, K.W. 1998) 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). 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. REFERENCES • 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.