DIAGENESIS AND POROSITY DEVELOPMENT IN THE MIDDLE TO LATE EOCENE POINT OF ROCKS SANDSTONE , MCKITTRICK OIL FIELD, CALIFORNIA

The Middle to Late Eocene Point of Rocks Sandstone serves as an important petroleum reservoir in at least a dozen oil fields on the west side of the San Joaquin basin. In the McKittrick oil field the Point of Rocks Sandstone consists of moderately sorted medium-to-coarse arkosic arenites deposited as turbidite sands interbedded with minor shales. Diagenesis included mechanical and chemical compaction, alteration of framework grains, precipitation of authigenic minerals, and dissolution of grains and cement. Authigenic minerals generally constitute about 14% of the sandstone and include clays, albite, laumontite, calcite, and quartz. Textural relationships between authigenic minerals and cements indicate the following paragenetic sequence for Point of Rocks Sandstone diagenesis: (1) syndepositional glauconite formation, (2) early diagenetic pyrite and siderite precipitation, (3) precipitation of poikilotopic calcite cement, (4) precipitation of pore-lining and pore-filling clays, (5) formation of quartz and feldspar overgrowths, (6) compaction of framework grains and generation of pseudomatrix, (7) fracture and deformation band formation, (8) albitization of detrital feldspars and contemporaneous formation of laumontite cement, (9) precipitation of late pore-filling clays and late calcite, (10) framework-grain dissolution, and (11) hydrocarbon migration; however, there was considerable overlap particularly during late-stage grain dissolution, alteration, and cementation. Compaction has had the greatest influence on reservoir quality. Porosity (ranging from 3 to 17% of the rock volume) and permeability were considerably decreased due to grain rearrangement, fracturing of brittle grains, and deformation of ductile grains to form pseudomatrix that flowed into adjacent pore spaces. Variability in porosity and permeability is largely due to the occurrence of scattered laumontite and clay cements, but the presence of deformation bands locally is a major factor. Most of the present porosity is secondary and the composition of the sands has been altered due to leaching of feldspars, with a notable decrease in K-feldspars with increasing depth. Mass-balance calculations indicate an overall export of aluminum out of the system.