Similitud Estructural
V. Presentación de la herramienta gráfica Sim23
post-depositional deformation of strata which includes folding, faulting, piercement, or a combination of any of these. In contrast, closure in a stratigraphic trap exists solely as a result of stratigraphic elements (Foster and Beaumont, 1992) such as changes in facies, changes in porosity/permeability, or by simple erosion.
The combination trap type is perhaps more complex because closure is achieved by a combination of stratigraphic and structural influences.
Structural traps
The rearrangement of strata into a geometrical feature known as a structural trap is achieved by the syn- to post-depositional deformation of strata. Most attempts to subdivide structural traps seek to differentiate between folded, faulted and piercement structures (Figure 58), which is dependent upon the quality of the available information. According to Biddle and Wielchowsky (1994) traps formed by gently dipping strata beneath an erosional unconformity are often excluded from the structural trap category. The most straightforward subdivision of structural traps is to differentiate on the basis of fold dominant versus fault dominant, with piercement structures forming a third category. However, in reality pure fault traps are process. Examples of tectonically derived fold-dominant structures include: the arches/domes, thrust-fold assemblages, and the compressive-block trap types of Harding and Lowell (1979) (Figure 60); the buckle-and-thrust and bending fold traps of North (1985); the compressional anticlines of Selley (1985); the fault-related fold structures and the fault-free fold 憀ift off?
and chevron/kink band structures of Biddle and Wielchowsky (1994). The non-tectonic category of fold-dominated structures includes those formed by drape folding, slumps, and differential compaction. The term fold will be retained herin, however, because the term describes the general structural style of the trap.
Figure 59. A 慸omal?
structure in plan view, o/w=oil water contact.
Figure 58. Categories of structural trap (A to D) (from Biddle and Wielchowsky, 1994).
Figure 60. Structural trap types from Harding and Lowell (1979).
Chapter 4—Reservoir, Trap, and Basin
48 Examples of fold-dominated structural traps
Tectonically derived
Fold-dominated traps formed by compression are most likely to be found adjacent to an active plate margin (i.e., subduction), or associated with the transcurrent movement of two plates.
The Laojunmiao oil field, in the Gansu Province of the People
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Republic of China, has been in production since 1941, and contains more than 800 wells, which produce oil from the Miocene Baiyanghe Formation. The Laojunmiao traps (Figure 61) consist of asymmetrical folds formed during the Neogene by basinward thrusting (Guangming and Jianguo, 1992). Closure on the fold is approximately 800 m and the northern flank of the fold is bisected by normal and reverse faults.The Portachuelo field, in the Talara Basin of Peru, is geologically complex, (Figure 62) although interpretations have evolved throughout time it is now recognized as a highly segmented reservoir (Roe and Millar, 1992). Initially, water distribution was considered typical for a folded (anticlinal) trap, especially since gas was encountered towards the top of the section.
However, gas-oil and oil-water contacts were inconsistent. Reservoir pressure data strongly supported the interpretation that the Salinas reservoir was initially charged as a pure fold structure (Figure 62 [A]) that was subsequently 慺lattened? by post-early Eocene faulting (Figure 62 [B]). Roe and Millar (1992) estimate that 40%
of the original oil may have been lost from the reservoir during this 慺lattening? event.
Unlike the previous two examples, the fold-dominated traps within the Oligo-Miocene Asmari limestone of Iran are less complex and significantly larger. The Asmari limestone, which reaches a thickness of more than 300 m, was folded into structures with
wavelengths of 20 to 100 km and amplitudes of 2 to 5 km (Figure 63), during the Alpine-Himalayan orogeny. The overlying evaporite, being weak under compression, flowed away from the rising anticlinal crests, accumulating in the flanks. Because the Asmari limestone underwent ductile deformation, folding occurred without much faulting. Most of the traps are the result of compressive forces along a plate margin.
Non-tectonically derived
This type is often included under the class of fold-dominant structures, although there is a high degree of control by the underlying surface. Such traps include those formed over a preexisting depositional feature, a paleotopographic surface, or a structurally deformed basement complex. (B) faulting (from Roe and Millar, 1992).
Figure 63. Cross-sections through numerous oil fields in western Iran, showing the presence of compressional folds within the Asmari limestone, with amplitudes of 2 to 5 km and wavelengths of 20 to 100 km (from Lees, 1953; reprinted by permission of Oxford University Press; www.oup.com).
Chapter 4—Reservoir, Trap, and Basin
depends upon the nature of the underlying lithology. A compactional fold may form over a (block) faulted basement in which closure of the trap is achieved by the differential burial and compaction of sediments on the crest and flanks of the faulted basement (Figure 64), whereas a drape structure occurs over a non-faulted surface (e.g., reef). One of the best examples of a drape structure over a reef occurs in the Leduc oil/gas field, Western Canada, in which the Devonian dolomite reservoir rock (D2) has been draped over the (non-compatible) D3 reef reservoir separated by sealing shale. Traps within the Viking Graben, North Sea, were initially ascribed to the formation of
compactional anticlines (Blair, 1975). More recently (Kirk, 1980), it has been suggested that the actual trapping mechanism is not caused by the convexity of overlying sediment, but by the truncation, by an unconformity, of homoclinally dipping strata.
Fault-dominant structural traps Faults can greatly influence the viability of a given trap, not only through the geometrical rearrangement of strata, but also through the creation of seals or leaks as discussed in the previous section on Seals. Fault planes can act as petroleum conduits or barriers to fluid movement and the variability in sealing capacity of faults is well illustrated within the Piper field in the Scottish North Sea (Figure 65), in which some fault planes seal (A), whereas other fault planes (B and C) do not (Williams et al., 1975; Maher et al., 1992). Fault-dominant traps are sub-divided into three categories: normal-, reverse-, and strike-slip fault-dominated traps.
Normal faults
Normal faults are extensional in genesis and often occur in subsiding basins. Normal faults represent a regular feature of many sedimentary basins and are part of the basin-forming process. Most of these faults are syn- to post-depositional, listric faults that dip towards the basin. If the fault plane dips in the same direction as the regional dip, the faults are synthetic, or 慸own the basin faults? (Figure 66). Synthetic faults are also growth faults, caused by the flexing of lithosphere due to sediment loading.
Faults cross-cutting the regional dip, or cutting homoclinal sequences, are antithetic.
Synthetic and/or antithetic faults are often responsible for the placement of impermeable rocks against reservoir rocks and represent true fault traps. The Hibernia field, offshore Newfoundland (Arthur et al., 1982), contains both synthetic and antithetic sealing faults.
Figure 64. A schematic cross section showing the formation of a compactional drape (area within red oval) over a faulted basement.
Figure 65. Cross-section through the Piper field of the North Sea, showing sealing (A) and non-sealing (B and C) fault planes (after Maher et al., 1992).
Figure 66. Synthetic and antithetic faults in the Hibernia field, offshore Newfoundland (from Arthur et al., 1982).
Chapter 4—Reservoir, Trap, and Basin
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Figure 68. A three-dimensional visualization of the structural contours overlying salt diapir and wall structures. All structures have discordordant contacts. Guglielmo et al.
(1997) refer to deep elongated diapirs as 憌alls,? they also differentiate between low salt walls overlain by symmetrical half grabens termed 憇alt ridges? and low salt walls overlain by asymmetric half grabens that are called 憇alt rollers.? The mean trend of the salt walls and rollers is approximately perpendicular to the direction of extension (from Guglielmo et al., 1997).
Domes and piercement structures Piercement structures
These regionally large structures represent the folding of overlying rock through the buoyant, upward movement of low density material (e.g., salt) that disturbs and disrupts overlying strata, often creating a variety of traps (Figure 67).
Salt domes and diapirs are, therefore, generally considered prospective areas of interest for petroleum exploration since salt has the potential to create a number of structural traps (Halbouty, 1967; Harding and Lowell, 1979). Salt has a density approximately equal to that of recently deposited sand and clay, however in response to compaction the density of clay and sand increases, whereas the density of salt does not. At some depth, (between 800 to 1200 m) the density difference between surrounding lithologies and the salt is great enough to initiate the (upward) movement of salt, known as halokinesis. This ductile, halokinetic movement causes the salt to form swells, domes, pillows, and eventually piercement structures (e.g., diapirs, walls) that often have the appearance of a dome or plug, although in 3-D the shapes are more convolute, sometimes appearing as a 憌all? (Figure 68).
If the salt rises gently without piercement, the structure is a salt dome; if the salt intrudes overlying sedimentary layers it is a piercement structure, known as a diapir and/or ridge.
Such structures are recognized as being typically associated with a variety of traps and trapping mechanisms, as shown in Figure 67; in which (1) is a simple domal trap draped over the salt, (2) graben fault trap over the dome, (3) porous cap rock, (4) flank 憄inch-out? sediments and sand lens, (5) trap beneath overhang, (6) trap uplifted and
buttressed against salt plug, (7) unconformity, (8) fault trap downthrown away from the salt, (9) fault trap downthrown toward the salt.
Figure 67. Generalized salt diapir and potential traps showing the common possible types of hydrocarbon trap associated with salt domes and diapirs: [1] simple domal trap draped over the salt, [2] graben fault trap over the dome, [3] porous cap rock, [4] flank 憄inch-out? sediments and sand lens, [5]
trap beneath overhang, [6] trap uplifted and buttressed against salt plug, [7] unconformity, [8] fault trap downthrown away from the salt, [9] fault trap downthrown toward the salt (after Halbouty, 1967, with kind permission of Gulf Publishing Co).
Chapter 4—Reservoir, Trap, and Basin
Numerous examples of dome and piercement structures occur within the Gabon Basin (West Africa), Scotian Shelf (Canada), Ekofisk and Cod fields in the North Sea, and the Texas Gulf Coast Basin (for example see Halbouty, 1979).
Domes and piercement structures are often readily recognizable in seismic, as shown by the structures in the Kraka and Dan fields of the Danish North Sea (Jorgensen, 1992), where Paleocene strata overlies Permian Salt (Figure 69).
Domal structures have the potential to create a number of trap types and consequently remain structures of significant exploration interest, because uplift may create abundant radial faults in the overlying strata, as exemplified by many of the domal structures of the Texas Gulf Coast Basin (King and Lee, 1976).
There are many large fields in which domal and piercement structures play a significant part.
However, there are also smaller fields that have produced steadily since discovery. For example, the Barbers Hill Dome, Texas, U.S.A., (Figure 70) was discovered in April 1916 and had produced nearly 130 million barrels by the end of 1984 when production ceased (Handbook of Texas Online, 2005).
Figure 69. Domal structure in the Kraka and Dan fields of the Danish North Sea, (left) structural map (top of chalk) and (right) an interpreted seismic line (from Jorgensen, 1992).
Figure 70. The Barbers Hill Dome, Texas, U.S.A., in section showing the location of known traps. Note the scale bar (from Halbouty, 1967, reprinted with kind permission of Gulf Publishing Co.).
Chapter 4—Reservoir, Trap, and Basin
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