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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 [[Calculating charge volume|charge]].

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

==Fault-dependent leak points==

==Fault-dependent leak points==

A comparison of top seal capacity determined from [[displacement pressure]]s 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.<ref name=ch10r68>Shea, W., T., Schwalbach, J., R., Allard, D., M., 1993, Integrated rock-log evaluation of fluvio-lacustrine seals, in Ebanks, J., Kaldi, J., Vavra, C., eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg Research conference, unpublished abstract.</ref> Oil column heights in this basin are controlled instead by fault-dependent leak points.<ref name=ch10r3>Allard, D., M., 1993, Fault leak controlled trap fill, rift basin examples (abs.), in Ebanks, J., Kaldi, J., Vavra, C., eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg conference, Crested Butte, Colorado, June 21–23.</ref><ref name=ch10r68 />

A comparison of top seal capacity determined from [[displacement pressure]]s 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.<ref name=ch10r68>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Ebanks, J. Kaldi, and C. Vavra, eds., Seals and Traps: A Multidisciplinary Approach: AAPG Hedberg Research conference, unpublished abstract.</ref> Oil column heights in this basin are controlled instead by fault-dependent leak points.<ref name=ch10r3>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.</ref><ref name=ch10r68 />

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<ref name=ch10r98>Zieglar, D., M., 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0004/0000/0501.htm 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.</ref> The remaining 90–95% are controlled by some other factor, including [[Calculating charge volume|charge]], faults, and [[Syncline|synclinal]] spill points.

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<ref name=ch10r98>Zieglar, D. M., 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0004/0000/0501.htm 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.</ref> The remaining 90–95% are controlled by some other factor, including [[Calculating charge volume|charge]], faults, and [[Syncline|synclinal]] spill points.

==Seal continuity==

==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.<ref name=ch10r81>Stephenson, M., Cox, J., Jones-Fuentes, P., 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.</ref> Where the Buckner anhydrite is continuous, there is no Haynesville play.

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.<ref name=ch10r81>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.</ref> Where the Buckner [[anhydrite]] is continuous, there is no Haynesville play.

==See also==

==See also==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Evaluating top and fault seal]]

[[Category:Evaluating top and fault seal]]