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Smectite Dehydration and Gas Production in Forbes Formation, Sacramento Valley, California

Craig A. Smith, Richard W. Berry

A mineralogical investigation was made of shale cuttings from three wells (Tenneco Poundstone 32-1, 30-1 and 24-1) in the Cretaceous Forbes Formation, Grimes field, Sacramento Valley, California. The -0.5 µm and the -4 µm size fractions were analyzed by x-ray diffraction. Clay mineral changes (smectite dehydration) begin near 7,000 ft and continue to the depth of the wells. Natural gas shows begin to occur several hundred feet into the smectite to illite conversion zone. The initiation temperature for smectite dehydration in these wells is near 80°C.

Quartz concentrations show a direct correlation with changes in relative permeability, determined from dual guard induction logs. Quartz increases correlate with relative permeability decreases. As smectite dehydration proceeds with depth, it yields silica as a reaction product. This silica can precipitate from pore fluids reducing permeability in finer grained beds. Zones of higher quartz concentrations exist stratigraphically adjacent to gas reservoirs, implying silica cementation while providing a hydrocarbon-trapping mechanism. Continued smectite dehydration, yielding water as a reaction product, likely contributed to overpressuring of the Forbes Formation.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.