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Secondary Porosity and Hydrocarbon Reservoirs in Lower-Middle Miocene Sandstones, Southern San Joaquin Basin, California

R. A. Horton, Jr., R. J. Menzie, Jr.

Immature lower to middle Miocene marine sandstones constitute important reservoir rocks in many southern San Joaquin basin oil fields. Surface samples from the east and west margins of the basin and subsurface samples from Round Mountain, Belridge, and Coalinga fields were examined. These localities have undergone recurrent uplift since middle Tertiary time and maximum burial probably did not exceed 2,500-3,000 m. Diagenetic features common to east- and west-side sandstones include phosphatization, early calcite cementation, pressure solution and replacement of silicate grains by calcite cement, framework grain dissolution and creation of secondary porosity, and replacement of biotite and hornblende by chlorite. Differences include recrystallization and dolomitization of arly calcite on the west side, and massive carbonate dissolution followed by extensive crushing and pressure solution of silicate grains and late replacement of plagioclase by calcite and calcite by hematite on the east side. Replacement of biotite by chlorite occurred only in the deepest samples on either side of the basin. Basinwide differences in diagenesis reflect different tectonic evolutions between east and west sides of the basin. Local variations in diagenetic patterns are pronounced in all areas and are controlled by initial sediment composition. For example, in one core from Coalinga early calcite cement, recrystallized calcite cement, and dolomitized calcite cement are interbedded over the 60-m interval sampled. Hydrocarbons in all samples reside mainly in secondary pores cre ted by cement and framework-grain dissolution, underscoring the importance of diagenesis in creating reservoirs in this basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.