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Structural traps

Images - Chapter 1 Sketch cross-sections of different types of faults. (a) Reversed fault caused by crustal compression. (b) Normal fault produced by crustal tension (note that for both traps to be sealed, the fault planes must be impermeable). (c) Growth fault. "H" indicates hydrocarbon accumulations.

Commercial accumulations of petroleum are found in sedimentary rocks where subsurface geometries are able to stop the upward migration of petroleum. Such subsurface geometries are known as petroleum traps. They can be classified in three groups: (i) structural traps, which are caused by folding or faulting, (ii) diapiric traps, which are caused by density differences, and (iii) stratigraphic traps, which are caused by sedimentation or erosion. This figure illustrates examples of structural traps due to faulting; the three main types of fault traps are illustrated in this figure. Note that an essential requirement for a commercial accumulation of petroleum is the occurrence of an impermeable cap rock (also known as a seal) which inhibits the upward movement of petroleum. In the examples of petroleum traps in this figure, the fault plane is the impermeable seal. (Adapted from Selley, 1983.)

Diapiric trap

Images - Chapter 1 Sketch cross-section showing some of the ways in which salt diapirs may trap petroleum. "S" indicates salt; "H" indicates hydrocarbon accumulation.

This figure shows an example of a diapiric trap. This type of trap results from the fact that as sediments are progressively buried, density and acoustic velocity generally increase, except in the situation in which the density of sediment layers locally decreases with depth. This situation causes the denser overlying sediment to move down and to displace the less-dense material. This material generally moves upward in subcircular domes, as described in the figure. We can see in this figure that this upward movement of the less-dense material gives rise to all sorts of types of potential traps. (Adapted from Selley, 1983.)

Stratigraphic traps

Images - Chapter 1 Examples of stratigraphic traps: A indicates a reef; B indicates a barrier-bar sand; C indicates a channel sand; D indicates an onlap sand pinchout trap; E indicates a truncation trap.

Stratigraphic traps are defined as those caused by deposition or erosion. They include traps which are not associated with unconformities (e.g., channels, barrier bars, and reefs) and those associated with unconformities like onlap pinchouts and truncations. This figure illustrates some of these types of stratigraphic traps. (Adapted from Selley, 1983.)

Other examples of structural traps

Images - Chapter 1 Structural traps. (A) Tilted fault blocks in an extensional regime. The seals are overlying mudstones and cross-fault juxtaposition against mudstones. (B) Rollover anticline on thrust. Petroleum accumulations may occur on both the hanging wall and the footwall. The hanging wall accumulation is dependent on a subthrust fault seal, whereas at least part of the hanging wall trap is likely to be a simple, four-way, dip-closed structure. (C) Lateral seal of a trap against a salt diapir and compactional drape trap over the diapir crest. (D) Diapiric mudstone associated trap with lateral seal against mud wall. Diapiric mud associated traps share many common features with that of salt. In this diagram, the diapiric mud wall developed at the core of a compressional fold. (E) Compactional drape over a basement block commonly creates enormous low-relief traps. (F) Gravity-generated trapping commonly occurs in deltaic sequences. Sediment loading causes gravity-driven failure and produces convex-down (listric) faults. The hanging wall of the fault rotates, creating space for sediment accumulation adjacent to the fault planes. The marker beds (grey) illustrate the form of the structure that has many favourable sites for petroleum accumulation. Adapted from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience. Oxford: Blackwell Science.

Other examples of stratigraphical traps

Images - Chapter 1 Stratigraphical traps. (A) 'Reef' oil is trapped in the core of the reef, with fore-reef talus and back-reef lagoonal muds acting as lateral seals and basinal mudstones as top seals. (B) Pinchout (sandstone) trap within stacked submarine fan sandstones. The upper surface of the diagram shows the plan geometry of a simple fan lobe. Lateral, bottom, and top seals are the surrounding basinal mudstones. (C) Channel-fill sandstone trap. The oil occurs in ribbon-shaped sandstone bodies. The top surface of the diagram shows the depositional geometry of the sandstone. Total seal may be provided by interdistributary mudstones or a combination of these and marine flodding surfaces. (D) Shallow marine sandstone bar completely encased in shallow marine mudstone. The upper surface of the diagram shows the prolate bar. (E) Subunconformity trap. The reservoir horizon is truncated at its up-dip end by an unconformity and the sediments overlying the unconformity provide the top seal. Lateral and bottom seals, like the reservoir interval, pre-date the unconformity. (F) Onlap trap. A basal or near-basal sandstone onlaps a tilted unconformity. The sandstone pinches out on the unconformity and is overstepped by a top seal mudstone. Adapted from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience. Oxford: Blackwell Science.

How does petroleum seismology work?

Images - Chapter 1 Snapshots of wave propagation through a structural trap (top) and the corresponding seismic data for horizontal and vertical arrays of sensors.

This figure shows an example of wave propagation through a subsurface model of the earth which contains a structural trap. Due to limited space, only six of these snapshots of wave propagation through this model are shown in this figure. The source used to generate waves in this example is an explosive source. We can recognize some of the reflections and transmissions at the various interfaces, especially for the snapshots corresponding to early time. For later times, pictures become very complex despite the relative simplicity of the geological model used in this example.

This figure also shows seismic data recorded by horizontal and vertical arrays of sensors. Notice that the various reflections and transmissions of energy in the snapshots are also captured by seismic data.

The source is then moved to another location, where the entire process of generating waves and recording them is repeated.

Seismic imaging

Images - Chapter 1 An example of reconstruction of the model of the subsurface using the seismic data in Figure 1-4. Only the data corresponding to the horizontal array were used in this reconstruction. (a) The portion of the model reconstructed by the imaging corresponds to the dotted square. (b) A result of imaging. Notice that all the boundaries between rock formations have been reconstructed and even the fault is reconstructed. However, the amplitudes at these boundaries are not identical because they describe different contrasts of physical properties between rock formations.

The seismic data recorded in this process are then imaged, based on arrival time and the magnitude of reflection energy, to obtain the model of the subsurface. In fact, the time it takes for the wave to travel from the source to the receivers is recorded in the seismic data. From these traveltimes we can reconstruct the depth of the reflector at which the recorded energy has been reflected. Furthermore, the magnitude of the reflected wave allows us to determine the contrast in physical properties which has caused the reflection. Thus we reconstruct the locations of the various discontinuities of our geological model and the contrasts of physical properties which characterize these discontinuities.

Oil provinces around the world

Images - Chapter 1 Distribution of offshore salt sheets

Salt is one of the most effective agents in nature for trapping oil and gas. Most of the petroleum accumulations in North America are trapped in salt-related structures, as are significant amounts in oil provinces around the world. (Adapted from Farmer et al., 1996.)

Exploring the subsalt

Images - Chapter 1 A geological model of a small region of the Gulf of Mexico contructed by the SMAART JV group, which included representatives of four major oil companies (BP, BHP, ChevronTexaco, ExxonMobil). "S" indicates salt sheets.

To gain a better insight into this problem, we have displayed in this figure an example of a geological model constructed by the SMAART JV group, which included representives of four major oil companies (BP, BHP, ChevronTexaco, and ExxonMobil).

The physical properties of salt---a density of 2.1 g/cc and a velocity of 4400 m/s or more---are in sharp contrast with the surrounding sediments, which are generally denser and have lower velocities. The strong contrasts in velocity and density at the sediment-salt interface acts like an irregularly shaped lens. In the past, petroleum seismologists have treated this contrast like a mirror, producing images that portrayed salt features as bottomless diapirs extending to the deepest level of seismic data. Once seen as impenetrable barriers to petroleum seismology probing, many salt structures are now proving to be thin blankets shielding rich reserves. Detecting these reserves requires more energy penetration than in traditional seismic acquisition. It also requires attenuating multiple reflections (i.e., unwanted reflections which arrive at almost the same time as the desired subsalt reflections but with higher energy than that of the desired reflections). Moreover, drilling through the salt is particularly difficult because the properties of salt---pseudoplastic flow under subsurface temperatures and pressures, and low permeability---make drilling through salt bodies like drilling through fluids. These difficulties increase the need for accurate imaging of the subsalt traps to reduce drilling risks. (From Bishop et al., 2001; Miley at al., 2001; and Stoughton et al., 2001.)

Exploring the sub-basalt

Images - Chapter 1 Voluminous basaltic basins (white ellipses) along the margins of the Altantic Ocean.

A significant number of hydrocarbon reserves are located below voluminous complex volcanic rock formations or sandwiched between two such volcanic rock formations. Most of these volcanic rock formations are basaltic, and the sedimentary layers located beneath them are broadly termed as sub-basalts. The offshore Atlantic Ocean area, for example, contains a number of hydrocarbon reserves in the sub-basalt formations. Some of these basins are currently the target of active oil and gas exploration, e.g., the basins on the Faroes, West Greenland, Brazil, Angola, and Namibia margins, as well as basins off mid-Norway. This figure shows voluminous basaltic complexes identified along prospective rifted continental margins. Notice that these basins stretch to very deep waters, in some cases to more than 2,000 m. Petroleum explorationists and producers working in these areas are faced with significant data- acquisition,processing and interpretation challenges, and huge drilling risks due to the depth of the water column and to the presence of voluminous complex volcanic rocks located above these basins. (Adapted from Coffin and Eldholm, 1992.)

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