William C. Dawson¹, W. R. Almon¹

(1) Texaco, Inc, Bellaire, TX

ABSTRACT: Seal Character of Marine Shales Interpreted in a Sequence Stratigraphic Framework

Controls on sealing characteristics of marine shales are among the least understood aspects of petroleum systems. Capillary properties, determined from laboratory measurements (high-pressure mercury-injection analysis) are used to estimate seal capacity. Specifically, petrophysical measurements indicate that the largest interconnected pore throats delimit seal capacity. Pore throat diameter is influenced by numerous factors: composition (mineralogy, total percentage of clay, and organic enrichment); fabric and texture (fissility, silt content and bioturbation); and diagenesis (compaction and cementation). In general, seal capacity increases as clay content increases and decreases as the content of detrital silt increases. Disruption of depositional fabrics by bioturbation tends to decrease seal effectiveness. Although shale fabric appears to exert primary control on seal character, cementation may significantly enhance seal capacity The analyzed shales record deposition in shallow to deep (middle to outer neritic) marine paleoenvironments and are interstratified with sandstones representing deltaic to submarine fan lithofacies. Capillary pressure data show that highstand shales have initial Hg-saturation pressures ranging form 550 to 1,325 psia. Transgressive shales have initial Hg-saturation pressures ranging form 1,250 to 6,340 psia. Lowstand and condensed shales exhibit distinctly bimodal Hg-saturation pressures: 4,000 to 10,000 psia and 13,000 to 20,000 psia. The higher injection pressures correspond to shales containing significant percentages of authigenic carbonates. Marine shales occurring within the upper part of third-order transgressive systems tracts and some condensed shales are excellent top seals. The sealing capacity of transgressive shales commonly exceeds that of condensed and maximum flooding shales. Highstand shales generally have less sealing capacity than transgressive shales.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado