03 chapter3 trap

Chapter 3: TRAP THÁI BÁ NGỌC - ĐH BK TP.HCM 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 THÁI BÁ NGỌC - ĐH BK TP.HCM • The highest point of the trap is the crest or culmination • The lowest point is the spill point A trap may or may not be full to the spill point • The horizontal plane through the spill point is called the spill plane • The vertical distance from the high point at the crest to the low point at the spill point is the closure THÁI BÁ NGỌC - ĐH BK TP.HCM • The productive reservoir is the pay • Its gross vertical interval is known as the gross pay This can vary from only one or two meters in Texas to several hundred in the North Sea and Middle East • Not all of the gross pay of a reservoir may be productive For example, shale stringers within a reservoir unit contribute to gross pay but not to net pay • Net pay refers only to the possibly productive reservoir (Figure 2, Facies change in an anticlinal trap, illustrating the difference between net pay and gross pay) THÁI BÁ NGỌC - ĐH BK TP.HCM Figure 1: Nomenclature of a trap using a simple anticline as an example THÁI BÁ NGỌC - ĐH BK TP.HCM Figure THÁI BÁ NGỌC - ĐH BK TP.HCM • A trap may contain oil, gas or a combination of the two The oil-water contact, OWC, is the deepest level of producible oil within an individual reservoir • ( Figure 3a , Fluid contacts within a reservoir in an oil-water system) • It marks the interface between predominately oil-saturated rocks and water-saturated rocks Similarly, either the gaswater contact, GWC ( Figure 3b , Fluid contacts within a reservoir in a gas-water system), • or the gas-oil contact, GOC ( Figure 3c , Fluid contacts within a reservoir in a gas-oil-water system) is the lower level of the producible gas The GWC or GOC marks the interface between predominately gas-saturated rocks and either water-saturated rocks, or oil-saturated rocks, as the case may be THÁI BÁ NGỌC - ĐH BK TP.HCM Figure THÁI BÁ NGỌC - ĐH BK TP.HCM • Source rock chemistry and level of maturation, as well as the pressure and temperature of the reservoir itself, are important in determining whether a trap contains oil, gas or both • In some oil fields (e.g Sarir field in Libya), a mat of heavy tar is present at the oil-water contact Degradation of the oil by bottom waters moving beneath the oil-water contact may cause this tar to form Tar mats cause considerable production problems because they prevent water from moving upwards and from displacing the produced oil THÁI BÁ NGỌC - ĐH BK TP.HCM • 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 THÁI BÁ NGỌC - ĐH BK TP.HCM 10 Figure 50 111 • This may be structural or stratigraphic but for many truncation traps, it may be provided by the irregular topography of the unconformity itself, such as a buried hill providing closure for a subcropping sandstone formation (Figure 51, Schematic of trap below unconformity, featuring closure provided by buried hill) • Many truncation traps have had their reservoir quality enhanced by secondary solution porosity due to weathering Secondary solution porosity induced by weathering is most common in limestones, but also occurs in sandstones and even basement rock Examples in limestones are found in Kansas, and in the Auk field of the North Sea • One of the best known truncation traps in the world is the East Texas field which contained over billion barrels of recoverable oil The trap is caused by the truncation of the Cretaceous Woodbine sand by the overlying impermeable Austin chalk ( Figure 52, Generalized west-east crosssection, East Texas basin) It has a length of some 60-70 kilometers and a width of nearly ten kilometers 112 Figure 51 113 Figure 52 114 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) 115 Figure 53 116 • 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 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, Schematic cross-section showing entrapment from both structural and hydrodynamic forces) 117 Figure 54 118 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 119 Figure 55 Figure 55, Structural contours on top of 'G2' sandstone, Main Pass Block 35, offshore Louisian 120 • 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 Carboniferousthrough-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 121 • Figure 56, Structural contours on top of Sadlerochit reservoir, Prudhoe Bay, Alaska 122 • 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) 123 Figure 57 Southwest-northeast structural cross-section, Piper field, North Sea 124 Exercise 125