--> Abstract: The Randolph Oil Zone, Semitropic Field, Kern County, California: A Highly Bioturbated, High-Pressure, Low-permeability Reservoir in the Lower Pliocene Etchegoin Formation, by Scott T. Hector, Michael Johns, and Brian Cunningham; #90076 (2008)

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The Randolph Oil Zone, Semitropic Field, Kern County, California: A Highly Bioturbated, High-Pressure, Low-permeability Reservoir in the Lower Pliocene Etchegoin Formation

Scott T. Hector1, Michael Johns2, and Brian Cunningham2
1Hobby Energy, 2500 Airport Drive, Rio Vista, CA 94571
2Bonanza Creek Oil Company, 4900 California Avenue #350B, Bakersfield, CA 93309

The Randolph oil zone at Semitropic field in Kern County, California, has produced just over 3 million barrels of relatively high gravity (28-31 API) oil since production began in the 1950's. The oil accumulation occurs at drill depths of 7400' to 7700' in shaley sandstones of lower Pliocene age. The reservoir occurs approximately 1200' above the base of the Etchegoin formation. More than 60 oil wells have been drilled in a productive area of 3000 acres. Most of these wells were drilled in the 1970's by G.E.C. and Gary Drilling Company. Carneros Energy drilled two wells to the Randolph zone in late 2004 and early 2005, in order to determine if the zone had been drained by the existing wells or if the field could be "down-sized" from 40-acre spacing to 20-acre spacing. Ninety (90') feet of core was taken in the first well, the Carneros Energy "Bradford B" #17. The resulting core data gave invaluable information on the nature of the reservoir. Cores showed a very highly bioturbated section of probable paludal (i.e., lagoonal) origin, with "splotchy" oil saturation separated by tightly calcareously cemented "non-reservoir" rock. The reservoir rock, measured with a micro-permeameter, averaged just over 10 millidarcies in permeability, with the highest values just over 200 millidarcies. Porosity ranged from below 20% to the mid 20's. Pressure data showed that the B-17 well had nearly original reservoir pressures of over 5000 psi. The well was drilled as a 20 acre offset to some of the best wells in the field (cum's of up to 140,000 bbls.) and yet showed that these wells had not drained the area that the new well was drilled into. These results confirmed the premise for drilling the well: that downspacing from the 40-acre to 20-acre spacing could results in high pressure, commercial wells. The results of the new drilling by Carneros Energy in this "old" field should bode well for future field development.

 

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California