DAWSON, WILLIAM C., and WILLIAM R. ALMON, Texaco Inc., Houston, TX

Abstract: Seal Character and Sequence Stratigraphy of Marine Shales in Gulf Coast-Style Basins

Marine shales are top seals for approximately 70% of documented hydrocarbon accumulations, but remain the least studied element of petroleum systems. The sealing capacity is derived from laboratory measurements (high-pressure mercury-injection) used to determine hydrocarbon column height. Specifically, these petrophysical measurements indicate that the largest interconnected pore throats delimit 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 rock samples. Petrographic and X-ray diffraction analyses show that the average composition of these top seals is: 65% total clay; 18% quartz; 3% feldspars, 7% pyrite; and 7% accessory constituents (authigenic carbonates, glauconite, bioclasts, and carbonaceous debris). These seals record deposition in shallow to middle neritic marine paleoenvironments and are interstratified with sandstones representing deltaic to submarine fan lithofacies. Mercury injection capillary pressure data indicate that average sealing capacity ranges from 175 to 985 ft (oil column) and 300 to 1920 ft (gas column) with hydrocarbon saturations in the seal between 5% and 10%. Seal capacity typically exceeds average hydrocarbon column heights of known accumulations by a factor of 5 to 20. Hence, seal capacity is not a significant risk in structures having four-way closure. The interpretation of seal data within a sequence stratigraphic context reveals that marine shales occurring within the upper part of second- and third-order transgressive systems tracts are good to excellent top seals. Shales occurring at parasequence boundaries act as internal baffles and thereby contribute to reservoir heterogeneity. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana