Marine shales are top seals for approximately seventy percent of documented hydrocarbon accumulations, but they represent one of the least studied elements of petroleum systems. Sealing capacity is determined from laboratory measurements of high-pressure mercury injection capillary pressure (MICP), 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, organic enrichment) and textural (fissility, silt content, degree of compaction) aspects of shales. Mineralogical 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 total organic carbon (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 140 Tertiary shale samples we analyzed were deposited in middle neritic to outer neritic/bathyal paleoenvironments. MICP data indicate that maximum top seal capacity ranges from 735 ft to 2,305 fl (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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