TRAP (cơ sở KHOA học địa CHẤT dầu KHÍ SLIDE TIẾNG ANH)

Chapter TRAP 3.1 Definitions and Concepts 3.2 Classification: four major types: Structural, Stratigraphic, Hydrodynamic and Combination 3.1.Definitions and Concepts •A trap is subsurface configuration of reservoir rock and cap rock or seal that has potential to concentrate petroleum in the pores of a reservoir rock •A trap is a geological feature of a reservoir rock that restricts the flow of fluids •A trap can content one or more reservoirs Figure 1: Nomenclature of a trap using a simple anticline as an example Figure 2 Figure 3 • Boundaries between oil, gas and water may be sharp ( Figure 4a , Transitional nature of fluid contacts within a reservoir- sharp contact • Gradational ( Figure 4b , Transitional nature of fluid contacts within a reservoirgradational contact) An abrupt fluid contact usually indicates a permeable reservoir Gradational contacts usually indicate low permeability reservoirs with high capillary pressure Figure 4 Figure 5 Figure 47-2, Schematic of channel and strike valley sands above an unconformity Figure 48 • The second group of traps associated with unconformities is truncation traps which occur beneath the unconformities (Figure 49 , Schematic of traps below unconformity) • Again, it is generally overlying shales which provide a seal (and often the source as well) for such traps As with onlap, pinch-out, and paleogeomorphic traps, closure is needed in both directions along the strike ( Figure 50, Schematic of trap below unconformity, featuring closure provided by the intersection of a dipping structural nose and a flat unconformity) Figure 49 Schematic of traps below unconformity Figure 51 3.2.3 Hydrodynamic Traps • In a hydrodynamic trap, a downward movement of water prevents the upward movement of oil or gas Pure hydrodynamic traps are extremely rare, but a number of traps result from the combination of hydrodynamic forces and structure or stratigraphy • An ideal hydrodynamic trap is shown in Figure 53 (Schematic cross-section of an ideal hydrodynamic trap) Figure 53 • A monoclinal flexure is developed which has no genuine vertical closure; oil could not be trapped within it in a normal situation Groundwater, however, is moving down through a permeable bed and is preventing the upward escape of oil Oil is trapped in the monoclinal flexure above a tilted oilwater contact Pure hydrodynamic traps like this, however, are very rare • There are a number of fields with tilted oilwater contacts where entrapment is a combination of both structure and hydrodynamic forces (Figure 54, Schematic cross-section showing entrapment from both structural and hydrodynamic forces) 3.2.4 Combination Traps • Combination traps result from two or more of the basic trapping mechanisms ( structural, stratigraphic, and hydrodynamic ) Since there are many ways in which combination traps can occur, a few examples must suffice for explanation • In the Main Pass Block 35 field of offshore Louisiana, a rollover anticline has developed to the south of a major growth fault (Hartman, 1972) (Figure 55, Structural contours on top of 'G2' sandstone, Main Pass Block 35, offshore Louisiana) • The rollover anticline, however, is crosscut by a channel Oil with a gas cap occurs only within the channel; thus, the trap is due to a combination of structure and stratigraphy Figure 55, Structural contours on top of 'G2' sandstone, Main Pass Block 35, offshore Louisian • An excellent example of a combination trap is provided by the Prudhoe Bay field on the North Slope of Alaska (Morgridge and Smith, 1972; Jones and Speers, 1976; Jamison et al., 1980; Bushnell, 1981) A series of Carboniferous-through-basal-Cretaceous strata were folded into a westerly-plunging anticlinal nose (Figure 56, Structural contours on top of Sadlerochit reservoir, Prudhoe Bay, Alaska) • This nose was truncated progressively from the northeast, and overlain by Cretaceous shales which acted as source and seal to the trap Oil and gas were trapped in reservoir beds subcropping the unconformity, primarily in the Triassic Sadlerochit sandstone Major faulting on the northern and southwestern side of the structure provided additional closure • Figure 56, Structural contours on top of Sadlerochit reservoir, Prudhoe Bay, Alaska • Fault-unconformity combination traps characterize the northern North Sea • Jurassic sandstone reservoirs exist in numerous tilted fault blocks which were truncated and overlain by Cretaceous shales The resulting traps include such fields as Brent, Ninian, and Piper A cross section through one of these, the Piper field, is shown in Figure 57 Southwest-northeast structural cross-section, Piper field, North Sea) Figure 57 Southwest-northeast structural crosssection, Piper field, North Sea Exercise ... 3.2.Classification Basically, traps can be classified into four major types: Structural, Stratigraphic, Hydrodynamic and Combination TRAP TYPES CAUSES Structural Traps Fold Traps: Compressional Folds... processes Tectonic Processes Fault Traps Tectonic Processes Stratigraphic Traps Depositional morphology or diagenesis Hydrodynamic Traps Water flow Combination Traps Combination of two or more... rock." Fold Traps Fold Traps (Compressional ) • Anticlinal traps which are due to compression are most likely to be found in or near geosynclinal troughs Examples of Compressional Fold Traps • 01-The