--> Abstract: South Belridge Field, Borderland Basin, U.S., San Joaquin Valley, by D. D. Miller and J. G. McPherson; #91004 (1991)

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South Belridge Field, Borderland Basin, U.S., San Joaquin Valley

MILLER, DONALD D., Mobil Exploration and Producing U.S., Inc., Denver, CO, and JOHN G. MCPHERSON,* Mobil Research and Development Corp., Dallas, TX

South Belridge is a giant field in the west San Joaquin Valley, Kern County. Cumulative field production is approximately 700 MMBO and 220 BCFG, with remaining recoverable reserves of approximately 500 MMBO. The daily production is nearly 180 MBO from over 6100 active wells. The focus of current field development and production is the shallow Tulare reservoir. Additional probable diatomite reserves have been conservatively estimated at 550 MMBO and 550 BCFG.South Belridge field has two principal reservoir horizons; the Mio-Pliocene Belridge diatomite of the upper Monterey Formation, and the overlying Plio-Pleistocene Tulare Formation. The field lies on the crest of a large southeast-plunging anticline, sub-parallel to the nearby San Andreas fault system. The reservoir trap in both the Tulare and diatomite reservoir horizons is a combination of structure, stratigraphic factors, and tar seals; the presumed source for the oil is the deeper Monterey Formation.

The diatomite reservoir produces light oil (20-32 degrees API gravity) from deep-marine diatomite and diatomaceous shales with extremely high porosity (average 60%) and low permeability (average 1 md). Depth to the top of the diatomite pay zone averages 1000 ft (305 m), and the gross pay zone is typically 800 to 1200 ft (244-366 m) thick. Cycles of clastic and biogenic-rich sediments, attributed largely to distal submarine-fan depositional events, are an important control on reservoir quality. Diatomite porosity is principally intergranular and intragranular, with a very small fracture volume; the natural fracture system and related faulting forms the primary permeability system.

In contrast, the shallow (<1000 ft [305 m] deep) overlying Tulare reservoir produces heavy oil (13-14 degrees API gravity) from unconsolidated, arkosic, fluviodeltaic sands of high porosity (average 35%) and permeability (average 3000 md). The depositional model is that of a generally prograding fluviodeltaic system sourced in the nearby basin-margin highlands. The lower Tulare sands are mostly of delta-front and delta-plain origin; the upper Tulare sands display a variety of fluvial styles, including high sinuosity meandering, low-sinuosity meandering, and braided systems. More than 6000 closely spaced, shallow wells are the key to steamflood production from hundreds of layered and laterally discontinuous reservoir sands which create laterally and vertically discontinuous reservoi flow units.

 

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