Chapte r 3: TRAP HCMUT-AU-2011 3.1 Definitions and Concepts 3.2 Clas s ific atio n: fo ur majo r type s : S truc tural, S tratig raphic , Hydro dynamic and Co mbinatio n 3.1.De finitio ns and Co nc e pts •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 Fig ure : No me nc lature o f a trap us ing a s imple antic line as an e xample Figure Figure • Boundaries between oil, gas and water may be sharp ( Figure 4a , Trans itional nature of fluid contacts within a re s e rvoir- s harp contact • Gradational ( Figure 4b , Trans itional nature of fluid contacts within a re s e rvoirgradational contact) An abrupt fluid contact usually indicates a permeable reservoir Gradational contacts usually indicate low permeability reservoirs with high capillary pressure Figure Figure Figure 47-2, Schematic of channel and strike valley sands above an unconformity Figure 48 • The second group of traps associated with unconformities is trunc atio n traps which occur beneath the unconformities (Figure 49, S che m atic of traps be low unconform ity) • 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, S che m atic of trap be low unconform ity, fe aturing clos ure provide d by the inte rs e ction of a dipping s tructural nos e and a flat unconform ity) Figure 49 S che m atic of traps be low unconform ity Figure 51 3.2.3 Hydro dynamic 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 (S che m atic cros s -s e ction of an ide al hydrodynam ic 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 mo no c linal fle xure above a tilted oil-water contact Pure hydrodynamic traps like this, however, are very rare • There are a number of fields with tilted oil-water contacts where entrapment is a combination of both structure and hydrodynamic forces (Figure 54, S che m atic cros s -s e ction s howing e ntrapm e nt from both s tructural and hydrodynam ic force s ) 3.2.4 Co mbinatio n 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, S tructural contours on top of 'G2' s ands tone , Main Pas s Block 35, offs hore Louis iana) • 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, S tructural contours on top of 'G2' s ands tone , Main Pas s Block 35, offs hore Louis ian • An excellent example of a combination trap is provided by the Prudhoe Bay field on the North Slope of Alaska (Morgridge and Smith, 1972; J ones and Speers, 1976; J amison et al., 1980; Bushnell, 1981) A series of Carboniferousthrough-basal-Cretaceous strata were folded into a westerly-plunging anticlinal nose (Figure 56, S tructural contours on top of S adle rochit re s e rvoir, Prudhoe Bay, Alas ka) • 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, S tructural contours on top of S adle rochit re s e rvoir, Prudhoe Bay, Alas ka • Fault-unconformity combination traps characterize the northern North Sea • J urassic 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 S outhwe s t-northe as t s tructural cros s -s e ction, Pipe r fie ld, North S e a) Figure 57 S outhwe s t-northe as t s tructural cros s -s e ction, Pipe r fie ld, North S e a Exercise ... cross-cutting faults Figure 10 • From a southwest-northeast cross section of the Wilmington field, we can see the broad arch of the anticline ( Figure 11, S outhwe s t-northe as t cros s -s e... outhwe s t-northe as t cros s -s e ction A-Z, W ilm ington fie ld) The main reservoir occurs beneath the Pliocene unconformity in Miocene- and Pliocene-age deep-sea sands Figure 11 Fo ld Traps ( Co.. .3. 1 Definitions and Concepts 3. 2 Clas s ific atio n: fo ur majo r type s : S truc tural, S tratig raphic , Hydro dynamic and Co mbinatio n 3. 1.De finitio ns and Co nc