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After the Fire Is Out: A Post In-Situ Combustion Audit, Upper Miocene Deepwater Sands, San Joaquin Valley, California

EAGAN, JAMES M., and MARY L. BARRETT, Mobil Exploration & Producing U.S., Bakersfield, CA, and PAUL G. SOUSTEK, Mobil Exploration & Producing U.S., Denver, CO

An audit of small-scale, air in-situ combustion projects developed in the upper Miocene Monarch and Webster unconsolidated, arkosic sand reservoirs, Midway Sunset field, Kern County, California, demonstrates minor rock diagenesis. Burn distribution and progression is controlled by reservoir continuity, layering, and original permeability variations.

Air in-situ combustion projects were operated between 1962 and 1976. Injected air drives a burning oil (coke) front through a reservoir reaching maximum temperatures of 650 degrees C. Dense new well control including 3000 ft of core is part of a large steamdrive development. Fireflood-induced diagenesis was clearly visible in core. Altered zones include sands with reduced oil saturations, burn zones with remaining coke, and reddish (oxidized) zones with no hydrocarbons. Wireline log responses in these zones have been highly modified. Detailed mapping by subzone using pre- and post-burn logs permits the determination of three-dimensional burn and reduced saturation geometries.

Little rock alteration occurred in these sands. The only diagenesis of the sand fraction was to calcite grains, where oil/calcite reactions produced calcium sulfate rims and CO(2) gas. X-ray diffraction of finer "matrix" reveals no recrystallization of opal-CT, no irreversible collapse of smectite, and only minor removal of kaolinite. Partial dissolution of opal and zeolites was visible in SEM. This nonequilibrium mineral suite probably reflects kinetic control by grain size, protective grain coatings, and alteration time.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)