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Sealing Potential and Sequence Stratigraphy of Miocene Shales: Deepwater Gulf of Mexico

By

DAWSON, W.C., and ALMON, W.R.

Chevron/Texaco, Bellaire, TX

 

The deepwater sub-province of the Gulf of Mexico (GOM) has experienced significant exploratory activity in recent years. These exploratory efforts have resulted in the discovery of numerous large fields having lowstand sandstone reservoirs. Deepwater reservoirs have received considerable attention, but analyses of associated deepwater shale (seal) lithofacies are scarce. This study characterizes Miocene mudstones into ten microfacies and summarizes the sealing potential (determined by high-pressure mercury injection analysis) of each microfacies relative to sequence stratigraphic occurrence. 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.

Sealing capacity can be enhanced by diagenesis (e.g., carbonate cementation). The average GOM shale has a maximum sealing capacity (10% Hg-saturation) of 4,645 psia (range 465 psia to 8,155 psia). This average value equates to a potential oil column of 625 feet (40°API oil with a GOR of 900 and an estimated interfacial tension of 21 dynes). Maximum potential sealing capacity commonly exceeds documented column heights by a factor of 2 to 5 indicating that top seal is not a critical risk in four-way closures.

Results of case history studies show that shales in upper transgressive systems tracts have excellent to exceptional seal potential. Highstand and lowstand shales have considerably lower seal capacities. Some condensed shales and maximum flooding shales are good to excellent seals, but their overall petrophysical character is highly variable.