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ABSTRACT: Hydrocarbon Columns, Buoyancy and Seals: Comparisons of California and Colorado-Montana-Wyoming Accumulations

Donald L. Zieglar

Hydrocarbon columns from 443 oil and gas accumulations in California are compared with 364 accumulations in the Colorado-Montana-Wyoming area. Maximum columns for oil are 4500 and 5500 ft, respectively, and for gas are 2200 and 1800 ft. Seals for these accumulations range in age from Ordovician to Pliocene-Pleistocene, with evaporitic units most common in Paleozoic beds and mudstone or shale characterizing the Mesozoic and Tertiary. In both areas accumulations influenced by faults are found to have different frequency distributions of pay zones compared to nonfaulted accumulations. On cumulative frequency curves, faulted accumulations also are found to have longer columns at a given percentile. Frequency distributions of length/width ratios also differ between faulted and nonfaulted accumulations. World class accumulations have columns that fall within the range of those observed, but their median is 2-5 times larger.

Buoyancy pressures calculated from hydrocarbon columns and fluid properties provide qualitative information that can be used to evaluate top seals. The higher pressures generated by the longer columns are held by seals as thin as 100-200 ft. Using published pore sizes for mudstone or shale, calculated displacement pressures for such top seals are in accord with the buoyancy pressures generated by only the longest 5-10% of the hydrocarbon columns. Shorter column heights with their lower pressures are probably not controlled by their top seal but rather by their structural or stratigraphic spill points.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990