Trap geometry: changes

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Predicting the occurrence of oil and gas traps
Chapter Predicting preservation and destruction of accumulations
Author Alton A. Brown
Link Web page
Store AAPG Store

We often assume that a structure remains static when charged by petroleum. Traps may be charged during structural growth, and accumulations can be partially or completely spilled by later structural deformation.

Traps charged during structural growth are not destroyed by spillage as long as the trapping geometry is maintained during deformation because petroleum migrates with the structural closure much faster than the rate of structural growth.[1] Conversely, if structural closure is destroyed during deformation, spillage occurs rapidly.

Paleofluid contacts may be tilted where spillage results from structural tilting. For example, Prudhoe Bay field, charged during the Late Cretaceous and tilted during the late Eocene[2] resulted in a tilted paleo oil-water contact.

Change in a fold trap

Figure 1 Continued growth of a foreland-sloping duplex preserves an accumulation in an early duplex but displaces the accumulation relative to the reservoir rock. Modified from Mitra;[3] courtesy AAPG.

Figure 1 shows how continued growth of a foreland-sloping duplex preserves an accumulation in an early duplex but displaces the accumulation relative to the reservoir rock. The stippled area outlining the initial accumulation is fixed relative to the rock. The solid area on the lower figure marks the accumulation at the top of the structure after movement.

Similarly, where the axis of a fault-bend fold on a hanging wall is fixed relative to the bend of the fault on the foot wall, the actual rock occupying the fold changes during movement along the fault. However, the position of the trap remains approximately fixed relative to the footwall and the fault bend.

Change in a fault trap

Figure 2 Spillage resulting from movement on a sealing fault.

Traps in which faults form part of the closure are especially susceptible to spillage during structural growth because movement on the fault may result in leakage. Movement on the fault is also likely to juxtapose permeable lithologies across the fault at some point in the movement. Figure 2 shows spillage resulting from movement on a sealing fault.

As the fault displaces the units, an early-charged trap (A, at t = 1) is juxtaposed against a sandstone at some later time (B, at t = 2). This probably will result in rapid spillage. If further fault movement restores favorable seal juxtaposition (C, at t = 3), additional petroleum charge will be needed to fill the new trap.

Evaluating spillage

Structural spillage is avoided if trapping geometry is maintained during deformation after charging. Structural closure must be maintained at all times during subsequent deformation. Throws on faults likely to cut the seal at the accumulation should be less than the seal thickness to avoid spillage by juxtaposition across the fault plane.

Spillage potential can be evaluated by combining geohistory analysis and structural analysis. Geohistory analysis (combined analysis of burial, thermal, and generation history) of gathering areas for prospects gives the range of charging times for the prospect [essentially the time of generation in nearby gathering areas[4]]). Structural analysis, using balanced structural cross sections as well as cross-cutting and superposition relationships, gives the range of times for trapping geometry formation and failure.

See also

References

  1. Hubbert, M., K., 1953, Entrapment of petroleum under hydrodynamic conditions: AAPG Bulletin, vol. 37, p. 1954–2026.
  2. Atkinson, C., McGowen, J., Block, S., Lundell, L., Trumbly, P., 1990, Braidplain and deltaic reservoirs, Prudhoe Bay field, Alaska, in Barwis, J., McPherson, J., Studlick, J., eds., Sandstone Petroleum Reservoirs: New York, Springer-Verlag, p. 7–30.
  3. Mitra, S., 1986, Duplex structures and imbricate thrust systems: geometry, structural position, and hydrocarbon potential: AAPG Bulletin, vol. 70, p. 1087–1112.
  4. England, W., A., Mann, A., L., Mann, D., M., 1991, Migration from source to trap, in Merrill, R., K., ed., Source and Migration Processes and Evaluation Techniques: AAPG Treatise of Petroleum Geology Handbook of Petroleum Geology, p. 23–46.

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