Fault-dependent leak points, continuity, and charge

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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 Evaluating top and fault seal
Author Grant M. Skerlec
Link Web page
Store AAPG Store

In many basins the major control on hydrocarbon column heights is not the displacement pressure of top seals but fault-dependent leak points, the lateral continuity of seals, and charge.

Fault-dependent leak points

A comparison of top seal capacity determined from displacement pressures and actual hydrocarbon column heights in one African basin demonstrates that while top seals are capable of trapping more than depth::1000 ft of oil, most traps contain only a few hundred feet of oil.[1] Oil column heights in this basin are controlled instead by fault-dependent leak points.[2][1]

Similarly, only 5–10% of the fields in the Rocky Mountains and California are thought to have hydrocarbon column heights controlled by top seal capillary properties[3] The remaining 90–95% are controlled by some other factor, including charge, faults, and synclinal spill points.

Seal continuity

The lateral extent and continuity of top seals can create and destroy plays within basins. In many basins, thick, continuous, areally extensive sequences of shale or salt act as regional top seals. The Gippsland Basin and North Sea are only two of numerous examples. In some basins, this regional seal is lacking or limited. In the Gulf Coast, the North Frisco City field exists because the Buckner Formation anhydrite, a regional seal, is missing above a local basement high. Only where the regional seal is breached are hydrocarbons able to escape from the Smackover reservoir into the overlying lower Haynesville Formation sandstones.[4] Where the Buckner anhydrite is continuous, there is no Haynesville play.

See also


  1. 1.0 1.1 Shea, W. T., J. R. Schwalbach, and D. M. Allard, 1993, Integrated rock-log evaluation of fluvio-lacustrine seals, in J. Ebanks, J. Kaldi, and C. Vavra, eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg Research conference, unpublished abstract.
  2. Allard, D. M., 1993, Fault leak controlled trap fill, rift basin examples (abs.), in J. Ebanks, J. Kaldi, and C. Vavra, eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg conference, Crested Butte, Colorado, June 21–23.
  3. Zieglar, D. M., 1992, Hydrocarbon columns, buoyancy pressures, and seal efficiency: comparisons of oil and gas accumulations in California and the Rocky Mountain area: AAPG Bulletin, vol. 76, no. 4, p. 501–508.
  4. Stephenson, M., J. Cox, and P. Jones-Fuentes, 1992, How 3D seismic-CAEX combination affected development of N. Frisco City field in Alabama: Oil & Gas Journal, vol. 90, no. 43, p. 127–130.

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Fault-dependent leak points, continuity, and charge
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