Bài giảng Địa chất dầu khí - Chương 3: Bẫy

s infilled the basin. The evaporites completely covered the reefs, thereby providing an excellent seal for hydrocarbon entrapment. There is a wide range of net pays found in the Rainbow reefs ( Fig. 39 ). Some reefs are nearly full of oil and gas, while others contain a very small column of oil or gas at the very crest of the reef. Porosities and permeabilities also differ greatly from reef to reef as well as within individual reefs. Such changes are due to variations in lithofacies and diagenetic effects, and are typical features of reefal traps ( Fig. 40 , Cross-section of pinnacle reef showing complex lithofacies,Rainbow area, Alberta, Canada ). There are many other reef hydrocarbon provinces around the world, notably in the Arabian Gulf and Libya. In Libya, the Intisar reefs in the Sirte basin have been well documented Figure 39, Sơ đồ cắt ngang qua Trung rạn kỷ Devon, diện tích Rainbow, Alberta, Canada Schematic cross-section through Middle Devonian reefs, Rainbow area, Alberta, Canada Figure 39 Fig. 40 , Mặt cắt của rạn san hô cho thấy đỉnh cao lithofacies phức tạp, diện tích Rainbow, Alberta, Canada Cross-section of pinnacle reef showing complex lithofacies,Rainbow area, Alberta, Canada Figure 40 Diagnenetic Traps Diagenetic bẫy được hình thành bởi việc tạo ra độ xốp trung học ở một tảng đá không chứa bởi thay thế, giải pháp hoặc bẻ gãy với đá chặt chẽ không thay đổi gì hình thành nên con dấu cho cái bẫy (Rittenhouse, 1972).Một ví dụ về một cái bẫy diagenetic hình thành bởi thay thế là sông sâu trường ở Michigan, trong đó dolomitization của một tiền gửi đá vôi từ trước đã dẫn đến sự hình thành độ xốp intercrystalline phụ (Hình 41 Diagenetic traps are formed by the creation of secondary porosity in a non-reservoir rock by replacement, solution or fracturing with the tight unaltered rock forming the seal for the trap (Rittenhouse, 1972). An example of a diagenetic trap formed by replacement is the Deep River field in Michigan, in which dolomitization of a preexisting limestone deposit has resulted in the formation of secondary intercrystalline porosity ( Fig. 41 ). Figure 41 Sự phát triển của độ xốp giải pháp thường được kết hợp với đá cacbonat (Hình 42), nhưng có thể bao gồm đá cát và The development of solution porosity is commonly associated with carbonate rocks ( Figure 42 ), but may involve sandstones as well Figure 42 Bẻ gãy có thể gây ra độ xốp trung học ở bất kỳ đá giòn - dù cacbonat, đá sa thạch, đá phiến sét, lửa hay đá biến chất (Kostura và Ravenscroft, 1977). Xu hướng Spraberry ở phía tây Texas hình thức một loạt các bẫy diagenetic (với trữ lượng dầu khoảng một tỷ thùng) trong vòng một fairway sản xuất khoảng 240 km dài và 80 km rộng (Wilkinson 1953). Một bản đồ cấu trúc viền trên hình Spraberry sản xuất, một phần 300-mét-dày của Trung chặt kỷ Permi đá phiến sét, siltstones, đá vôi, và đá cát hạt mịn cho thấy trong lưu vực trung du miền Nam, các lĩnh vực sản xuất dầu có mối quan hệ ít để cơ cấu (Hình 43). Sản xuất đến từ các khu vực bị nứt trong sự hình thành Spraberry nếu không chặt chẽ Fracturing can cause secondary porosity in any brittle rock — whether carbonate, sandstone, shale, igneous or metamorphic rock (Kostura and Ravenscroft, 1977). The Spraberry trend in west Texas forms a series of diagenetic traps (with oil reserves of about one billion barrels) within a producing fairway about 240 kilometers long and 80 kilometers wide (Wilkinson 1953). A structure map contoured on the productive Spraberry formation, a 300-meter-thick section of tight Middle Permian shales, siltstones, limestones, and fine-grained sandstones shows that in the southern Midland basin, the areas of oil production have little relationship to structure ( Figure 43 ). Production comes from areas of fracturing throughout the otherwise tight Spraberry formation. Figure 43 Không chỉnh hợp liên quan đến bẫy Unconformity-Related Traps Các trầm tích và diagenetic bẫy địa tầng chỉ được coi là xảy ra trong các trình tự thông thường comformable, mặc dù họ cũng có thể xảy ra tại unconformities.Một nhóm lớn các bẫy địa tầng được thể hiện bằng những cái bẫy mà không chỉnh hợp một là điều cần thiết (Hình 44) (Levorsen, 1934) The depositional and diagenetic stratigraphic traps just considered occur in normal comformable sequences, although they may also occur at unconformities. Another major group of stratigraphic traps is represented by traps for which an unconformity is essential (Fig. 44) (Levorsen, 1934). Figure 44 Significantly large percentages of the known global petroleum reserves are trapped adjacent to major unconformities. In addition to being held in pure stratigraphic traps, many of these reserves are held in structural and combination traps as well. Unconformity-related traps can be subdivided into those which occur above the unconformity and those beneath ( Figure 45 , Schematic of traps located above and below an unconformity ). Above an unconformity: Shallow-marine or fluvial sands may onlap a planar unconformity. A stratigraphic trap can occur where such sands are overlain by shales and are underlain by impermeable rock which provides a seat seal. Onlapping updip pinch-out sands such as these could occur as sheets ( Fig.46 , Schematic of onlapping pinch-out sands- occurring as a sheet deposit ) Figure 45 , Schematic of traps located above and below an unconformity Figure 46 , Schematic of onlapping pinch-out sands- occurring as a sheet deposit Schematic of onlapping pinch-out sands- occurring as a sheet deposit ) , or as discrete paleogeomorphic traps ( Figure 47 , Schematic of onlapping pinch-out sands-occurring as a discrete paleogeomorphic sand ). One type of paleogeomorphic trap is represented by channels which cut into the unconformity; another occurs where sands are restricted within strike valleys cut into alternating hard and soft strata ( Figure 4 , Schematic of channel and strike valley sands above an unconformity ) It is important to note that closure is necessary along the strike of such traps, not just updip as shown in Figure 46 . In Figure 48 ( Schematic of sandstone pinch-out intersecting with a structural nose ), closure is provided by the intersection of a sandstone pinch-out with a structural nose. 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 50 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 5 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 cross-section, East Texas basin ). It has a length of some 60-70 kilometers and a width of nearly ten kilometers Figure 51 Figure 52 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 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 ). Figure 54 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 ). Figure 55, Structural contours on top of 'G2' sandstone, Main Pass Block 35, offshore Louisian Figure 55 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. 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 ).  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. 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 cross-section, Piper field, North Sea