ABSTRACT

The deepwater sub-province of the Gulf of Mexico (GOM) has experienced significant exploratory activity in recent years. These exploration efforts have resulted in the discovery of numerous large fields having lowstand sandstone reservoirs. Although deepwater reservoirs have received considerable attention, systematic analyses of associated deepwater shale (seal) lithofacies are relatively scarce. This study characterizes Miocene mudstones into 10 microfacies and summarizes the sealing potential (determined by high-pressure mercury injection analysis) of each microfacies relative to sequence stratigraphic occurrence. The observed variations in seal character and seal capacity are attributable to systematic differences in shale texture and fabric. The average GOM Miocene shale contains 67% clay minerals and 23% detrital silt. Sealing capacity generally increases as the total content of clay increases and decreases as the content of detrital silt increases. The average GOM shale has a maximum sealing capacity (10% non-wetting saturation) of 4,645 psia (range 465 psia to 8,155 psia). Silt-poor shales in upper transgressive systems tracts have excellent to exceptional seal potential. Silt-rich highstand and lowstand shales have considerably lower sealing capacities. Increased percentages of detrital silt seem to reduce sealing capacity by inhibiting mechanical compaction and thereby allowing the preservation of relatively large-diameter pore throats. Diagenesis (cementation) and welldeveloped laminar fabrics appear to enhance seal capacity.