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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 49 (1999), Pages 190-197

Top Seal Character and Sequence Stratigraphy of Selected Marine Shales in Gulf Coast Style Basins

William C. Dawson and William R. Almon

Texaco, Inc., 3901 Briarpark Dr., Houston, TX 77042

ABSTRACT

Marine shales are top seals for approximately seventy percent of documented hydrocarbon accumulations, but represent one of the least studied elements of petroleum systems. Sealing capacity is determined from laboratory measurements (high-pressure mercury injection capillary pressure), which are used to calculate hydrocarbon column heights. The largest interconnected pore throats control seal capacity. Pore throat size is influenced by numerous compositional (mineralogy, total percentage of clay, and organic enrichment) and textural (fissility, silt content, and degree of compaction) aspects of shales. Mineralogic analyses indicate an average composition of: 65% clay; 19% quartz; 4% feldspars; 5% pyrite; and 7% accessory constituents (authigenic carbonates, glauconite, bioclasts, and carbonaceous debris). The TOC of these shales ranges from 0.79 to 4.00 (wt %). In general, seal capacity increases with increasing clay and organic content and decreases with increasing detrital silt content. The analyzed Tertiary shales (140 samples) record middle neritic to outer neritic/bathyal paleoenvironments of deposition.

Mercury injection capillary pressure (MICP) data indicate that maximum top seal capacity ranges from 735 ft to 2,305 ft (oil column) with hydrocarbon saturations in the seal of 10 percent. Calculated seal capacity typically exceeds hydrocarbon column heights by a factor of 5 to 20 times. Consequently, top seal capacity is not a significant risk in structures having four-way closure. The interpretation of seal analysis data within a sequence stratigraphic context reveals a systematic trend in top seal capacity. Shales occurring in the uppermost transgressive systems tracts and maximum horizons are excellent top seals. Shale samples from the lower part of transgressive systems tracts are good to excellent seals, whereas shales within highstand systems tracts have moderate to good sealing capacity. Some condensed intervals contain exceptional top seals.


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