the HC is the bottom water

Hydrocarbon Traps: Structural, Stratigraphic, Diapiric, Diagenetic, Hydrodyna amic, Astrobleme and, of course, Combinationn Trap: A geologic featuree in which petroleum can accumulate Seal: An impermeable rock medium that isolates a reservoir and retains petroleum, i.e., shale, evaporite. Morphology/ Nomenclature Highest point of the trap is the crest, or culmination Lowest point at which HC can be contained in the trap is the spill point Vertical distance from crest to spill point/plane is the closure Zone immediately beneath the HC is the bottom water Zone adjacent to the trap is the edge zone The productive reservoir within the trap is the pay Vertical distance from top of reservoir to OWC is gross pay Cumulative vertical thickness that can be produced is termed net pay Fluid Distribution Within the Trap Trap may contain oil, gas, or both OWC is the oil:water contact (deepest level of producible oil) GOC or GWC is the gas:oil or gas:water contact, respectively (lowest limit of producible gas) Gas always overlays oil (gas is less dense) Gradational fluid contacts indicate a low permeability with high capillary pressure Seals and Cap Rocks Trap must be overlain by an effective seal that prevents the vertical migration of the HC Shales are the commonest seal Evaporites are the most effective 1

Classification of Different Traps TYPE Structural: Stratigraphic: Diapiric: Hydrodynamic: Astrobleme: DESCRIPTION Fault and Fold related Due to change in lithology Viscous flow from depth Due to flowing water Meteorite impact traps Trap (Structural): one whose upper boundary has been made concave, as viewed from below, by some local deformation, such as folding, or faulting, or both, of the reservoir rock. The edges of a pool occurring in a structural trap are determined wholly, or in part, by the intersection of the underlying water table with the roof rock overlying the deformed reservoir rock. (Levorsen, 1967) Levorsen, A.I., Geology of Petroleum, 2 ed. Freeman, San Francisco, 1967. Formed by tectonic processes after the deposition of the beds involved Folding and Faulting Folding: Anticline: Structural bend in strata due to (usually) comressional stress in which the general shape is concave down with flanks dipping in opposing directions. When seen in outcrop, the age of the beds will get younger with distance from the axial plane. Usually found adjacent to a syncline. If the axis of the fold dips it is referred to as a plunging anticline. The most abundant hydrocarbon trap. Usually found to be asymmetric with the crest migrating with depth. HC may move down in a stratigraphic sense. Syncline: Conjugate fold to an anticline. Concave up with flanks dipping toward each other. When seen in outcrop, the age of the beds get younger with distance to the axial plane. 2

Compressional anticlines are due to compressive stress; folds are pushed upward Compactional anticlines are anticlinal-form folds that result from tensional stress; extension stress results in fault block sinkage (horst/graben type topography); overlying draping sediments sink with down-dropped blocks whereas sediments overlaying upthrown blocks form the culminations of the anticlinal folds Rollover anticlines and drag folds are pseudo-anticlinal type folds that occur along a fault plane (normal or reverse; recall anticlines are structurally defined as vertical-relief folds, so these do not occur with transform faults) due to the relative movement of the fault blocks; beds will sink into normal fault planes because of extension, and will drag along reverse fault planes due to friction + Productive drag folds are common in overthrust belts as in the Rocky Mountain region. Overthrust belts are characterized by multiple adjacent reverse faults each with fault planes whose dips die out with depth (become more horizontal) and meet at a common fault (gliding) plane known as a decollement Salt anticline: see section on diapiric traps 3

Domes: Circular concave down fold. Usually the result of sediments atop dome shaped basement rock or rising diapirs of clay or salt. Commonly produce multiply stacked reservoirs for obvious reason of their characteristic geometry. 4

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Faulting: Rollover anticlines are directly associated with growth faults; growth faults are fault zones characterized by normal faulting (extensional stress) and numerous antithetic minor faults Dip-slip faulting (normal and reverse) both contribute to HC trapping provided the fault is noncommunicative Noncommunicative faults do not permit the passage of fluids through the fault plane: can be due to either clay smearing along fault plane or sufficient offset on either side of fault to preclude continuous flow through common transmitting strata Noncommunicative faults are common in heavily faulted reservoirs and usually compartmentalizes the HCs into numerous individual traps Reservoir pressure differences across a fault plane will reveal whether or not the fault is communicative or not 10

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Trap (Stratigraphic): one in which the chief trap-making element is some variation in the stratigraphy, or lithology, or both, of the reservoir rock, such as a facies change, variable local porosity and permeability, or an up-structure termination of the reservoir rock, irrespective of the cause (Levorsen, 1967) Formed by changes in lithology Lithological variation may be depositional (e.g., channel, valley) or postdepositional (e.g., truncations, diagenetic changes) Channel traps: Shoestring traps: Pinchouts traps: Reef traps: Sedimentary environment eolian, fluvial, or delta (Continental/Coastal) Genetically related to crevasse-splay and mouth bar traps Can also be submarine Sedimentary environment is a barrier bar (Coastal) Sedimentary environment typically eolian, also deep marine (as in a submarine fan pinchout) Reefs are carbonate buildups Are arguably the most important stratigraphic trap Geometry is usually a domal pinnacle with an elongated antiformal platform Diagenetic traps: Oil or gas migrating up a permeable reservoir rock may be retarted upon reaching a cemented zone Oil of gas may trap in solution porosity within a cemented rock Oil may degrade and oxidize by bacterial action as it migrates upward and comes into contact with shallow zone meteoric waters Unconformity traps: Three types: Angular unconformity traps: HC accumulation in strata beneath erosional surface topped with impermeable layer in an angular unconformity sequence Disconformity traps: HC accumulation in strata within a nonchronostratigraphic sequence which is immediately overlain with an impermeable layer parallel to it and marking the unconformity Nonconformity traps: HC accumulation in fractured basement rocks in unconformity with overlying sediments HC may accumulate above the unconformity or below the unconformity (supraunconformity trap and subunconformity trap, respectively) depending on the stratigraphy 12

Trap (Diapiric): Traps resulting from the upward movement of mud or salt layers and doming of the overlying strata May give rise to many types of traps (i.e., fault or fold) Are differentiated from structural traps because due to local lithostatic movement rather than regional tectonic forces Clays and salts are less dense at depth than layers overlying them Behave viscously rises and may intrude upper layers Result in large plumes (diapirs): can be bulbous or mushroom shaped, which will affect the type of traps along the associated with it Same principle as a lava lamp 13

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Trap (Hydrodynamic): Hydrodynamic movement of waters down permeable beds will trap oils flowing upward provided the hydrodynamic force of the water is greater than the force due to the buoyancy of the oil These traps are rare but have been recorded (q.v., Delaware Basin, West Texas, and Paris Basin, France) Only achieved with hydrodynamic flow; if no flow, pressures would be hydrostatic and oil would rise above the water (because it is less dense) Hydrodynamic flow is important in considering HC migration down structures Usually observed in gentle flexures in monoclinally dipping beds which have closure but no vertical relief Trap (Astrobleme): Meteorite impact traps Oil found in highly deformed central uplift in large craters or in fractured and brecciated rim facies Unfortunately Chixculub has not produced; however, the Williston Basin (on Canadian/U.S. border) and the Ames Field (Oklahoma) are producing fields widely agreed to be associated with meteorite impacts The Lyles Ranch Field in South Texas is also believed to be associated with a large meteorite impact Further reading for the curiously inclined: LeVie, D.S., 1986. South Texas Lyles ranch field: Production from an astrobleme? Oil and Gas Journal, pp135-138, Curtiss, David K. and Wavrek, David A., 1998. Hydrocarbons in meteorite impact structures: oil reserves in the Ames feature. Mazur, Michael J. and Stewart, Robert R., 1997. The seismic expression and hydrocarbon potential of meteorite impact craters: current research. Conclusion: Combination Traps: As with many classification schemes in the field of geology, the practitioner should keep in mind there will often exist combinations of clearly defined types. Of the above traps anticlines are by far the most abundant hydrocarbon producer to date Examples of Combination Traps: 1) A channel rollover anticline trap, 2) Truncated tilted fault block unconformity trap, 3) Salt dome, growth fault trap 17

Glossary: Angular unconformity: An unconformity the erosional surface of which lies between nonparallel beds. Flat, impermeable younger beds in erosional contact with tilted, permeable, petroleum-bearing older beds form an angular unconformity trap. Anticline: An upwardly convex fold with limbs that dip away from the axial position. Antithetic fault: Conjugate, usually secondary faults, at approximately 60 degrees to a primary or synthetic fault. Apparent dip: Any dip measured at other than perpendicular to the strike of an inclined surface. Asymmetric fold: A fold with an inclined axial surface or nonidentical limbs. Axial plane: The planar axial surface of a fold. Axis (fold): A line which generates the form of a fold when moved parallel to itself in space. Block fault: A type of normal fault which separates the crust into fault-bounded blocks of different elevations. Closure: The vertical distance between the highest point on an anticline or dome and its lowest closed structural contour. Complex fault trap: A fault trap involving multiple faulting and or fault block tilting. Concentric fold: A parallel fold with constant orthogonal thickness of its folded layers. Deformation (strain): Changing shape by stress application: folding, shearing. Diapir: An anticlinal fold ridge or dome formed by the squeezing of shale, salt or other mobile material into the core of the feature. Dip: The departure normal to the strike in degrees of an inclined plane from the horizontal. 18

Dip slip: Fault movement parallel to the dip direction of the fault plane. Disconformity: An unconformity with sediments parallel to each other above and below the erosion surface which is not necessarily planar. Disconformity trap: A petroleum trap associated with a disconformity. Disharmonic fold: A nonparallel fold in which beds of varying competence demonstrate variable deformation intensity. Displacement: Relative movement of two sides of a fault. Dome: an anticlinal structure with all dips away from the apex. Downthrown block: The fault block which is displaced downward relative to the upthrown block. Down-to-the-coast fault: A fault type common in areas of rapid deposition where fault blocks are displaces downward in the direction of depositional transport. Drape fold: A fold in layered rocks produced by movement of underlying brittle rocks at high angles to the layers: a forced fold. Fault: A break in the Earth s crust along which there has been movement. Faulted anticline: An anticline disrupted or altered by faulting. Fault trap: A fault-controlled petroleum accumulation. Fold: A bent or curved stratum, cleavage plane or foliation. Foot wall: The underside wall rock of a fault, inclined vein or ore body. Fracture (stress): Stress-induced breaks in rock material occurring in conjugate sets. Geosyncline: A large, regional linear or basin like depressed area in which thick volcanic and sedimentary rocks accumulate. Related to plate margins and regional tectonism. Hanging wall: The side or block above an inclined fault. Heave (fault): The horizontal displacement of a fault. High relief salt anticline: A salt-induced, salt-cored, large relief anticline with strongly draped, often faulted strata over its apex. Nonconformity: An unconformity featuring sediment resting upon an eroded crystalline surface. Nonconformity trap: A petroleum trap resulting from a nonconformity. Overthrust trap: A fault or fold trap lying above a thrust fault. Overturned fold: A fold with an axial plane depressed below horizontal. Permeability barrier: A barrier to transmission of fluid through a medium. Plunging fold: A fold with a non-horizontal hinge line. 19

Recumbent fold: A fold with a horizontal axial plane. Reef trap: A petroleum accumulation retained by reef permeability surrounded by adjacent rock impermeability. Salt dome: A circular or elliptical, positive salt-cored structure which vertically penetrates of deforms the surrounding sediments. Seal: An impermeable rock medium that isolates a reservoir and retains petroleum, i.e., shale, evaporite. Shear: A stress-induced displacement along a zone or plane of failure. Similar fold: A fold in which bed thickness is greater in the hinge than in the limbs. Slip: Displacement along a fault surface. Strain: Deformation resulting from stress. Stratigraphic trap: A depositional, non-deformed petroleum trap. Stress: Applied force or pressure resulting in strain. Strike: The horizontal line of intersection between a dipping surface and horizontal plane. Structural trap: A petroleum trap formed by deformation. Subsidence: Downward movement of the Earth s surface. Subthrust trap: A petroleum trap beneath a thrust fault. Syncline: A downwardly convex fold with limbs that dip toward the axis. Synthetic fault: Minor faults of the same orientation and direction of displacement as associated major faults. Tar mat: Layer of heavy oil immediately above the bottom water that impedes the flow of water into the reservoir; are believed to have been produced the thermal degradation of oils and increased gas solution with associated precipitation of asphaltenes. Throw: The vertical displacement of a fault. Trap: A geologic feature in which petroleum can accumulate. Unconformity: An interruption of the geologic record manifest as an erosion surface bounded by rocks which are not immediately chronologically successive. 20