The Petrology and Diagenetic History of the Phacoides Sandstone, Temblor Formation at the McKittrick Oil Field, California

Abstract:

The McKittrick oil field is located near the western edge of the San Joaquin Basin, California. The oil field is currently in production with 480 wells producing from the Tulare, San Joaquin, Reef Ridge, Monterey, Temblor, Tumey, and Kreyenhagen formations. Within the Temblor Formation production is mainly from the Miocene Carneros and the Phacoides sandstones. Eighty-two samples from the Phacoides sandstone (2403 – 3045 m below surface) were obtained from the California Well Sample Repository to characterize and understand the diagenetic history and its influence on its reservoir properties. Petrographic thin sections were analyzed by quantitative optical petrography, energy dispersive X-ray spectrometry, and imaging with back-scatter electron and cathodoluminescence. The Phacoides sandstone consists of fine to very coarse, poorly to well-sorted, arkosic arenites, and wackes with detrital framework grains including sub-angular quartz, K-feldspar (microcline and orthoclase), plagioclase, and lithic fragments. Ba-free, Ba-rich, and perthitic K-feldspars are present. Accessory minerals include glauconite, biotite, muscovite, magnetite, titanomagnetite, sphene, zircon, apatite, corundum, and rutile. Diagenetic alteration includes: (1) compaction, (2) mineral dissolution, (3) albitization of feldspars, alteration of biotite to pyrite and chlorite, replacement of framework grains by calcite, (4) alteration of volcanic rock fragments, (5) cementation by kaolinite, calcite and dolomite, and (6) precipitation of K-feldspar and quartz overgrowths. Early-formed fractures were healed by authigenic quartz, albite, and K-feldspars. Precipitation of carbonates and clays, rearranging of broken grains, and formation of pseudomatrix reduced primary porosity. Secondary porosity is common and formed initially by the dissolution of plagioclase (excluding albite) and volcanic fragments, and later by dissolution of calcite, dolomite, and detrital K-feldspars. Hydrocarbon emplacement was followed by precipitation of late pyrite framboids in pores containing both oil and clays; suggesting that maturation of hydrocarbons supplied S that reacted with ferrous ions in pore fluids trapped within clays.