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ABSTRACT: The Floating Oil Pool--A Unique Trapping Mechanism at Kern River Field

Edward J. Kodl, J. C. Eacman

Geologic reevaluation of the giant Kern River field, a 4 billion bbl oil accumulation, suggests a nontrapping oil emplacement mechanism not previously described in the literature. The field produces 12-14° API oil from a shallow, thick sequence of stacked fluvial sands situated across a homocline that dips 4-5° southwest. Migration of hydrocarbons generated in deeply buried Miocene shales ended in the Kern River field when the oil reached the top of the regional groundwater table, a surface that truncates the dipping beds in the subsurface. The oil floats on top of the groundwater and is confined by an updip seal consisting of simply gravity, or barometric pressure. Significant vertical changes in the groundwater support system in the geologic and historic past ave probably been caused by climatic fluctuations or changes in the volume or location of the nearby Kern River. A recharged groundwater support system would have hydraulically lifted the floating oil into shallower sands; a depleted support system would have allowed the oil to flow downdip, leaving behind undersaturated dry oil sands. Lateral propagation of this floating oil pool is limited by the inability of the unpressured oil to overcome capillary resistances and opposing groundwater movements. Stratigraphic complexities and faulting occasionally influence oil distribution, but the upper limits of producible oil are determined by the configuration of the regional groundwater table. Other shallow, heavySan Joaquin Valley oil fields that lack definitive conventional trapping mechanism probably can also be reclassified as floating oil pools.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990