Diagenetic History and Reservoir Quality of the Phacoides Sandstone, Temblor Formation, McKittrick Oil Field, California

Abstract

The McKittrick oil field is located near the western edge of the San Joaquin basin approximately 60 km west of Bakersfield. The oil field is currently in production with 480 wells producing from Eocene through Pleistocene strata including the Temblor Formation (Oligocene – Lower Miocene). Temblor Formation production is mainly from the Miocene Carneros and Oligocene Phacoides sandstones. The Phacoides reservoir is located in an anticlinal structure with approximately 300 m of closure. Reservoir temperature is 110°C and formation water contains 13,000 ppm total dissolved solids and 9,700 ppm dissolved NaCl. 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 the reservoir's properties. Thin sections were analyzed by quantitative optical petrography and scanning electron microscopy using backscattered-electron and cathodoluminescence imagery and energy dispersive X-ray spectrometry. The Phacoides sandstone consists of poorly to well-sorted, fine to very coarse, arkosic arenites and wackes. Major diagenetic processes included fracturing and dissolution of framework grains (especially plagioclase), albitization of K-feldspars and plagioclase, and precipitation of calcite, dolomite, and kaolinite cements as well as quartz and k-feldspar overgrowths. Significantly, no laumontite cement was observed. The average composition of albitized plagioclase indicates a maximum burial temperature approximately 30°C higher than the current reservoir temperature, corresponding to 1 km deeper burial. This is in good agreement with an estimate of the maximum burial temperature suggested by the lack of laumontite. Reservoir quality was controlled primarily by feldspar dissolution and precipitation of kaolinite. Secondary porosity accounts for at least half of the total porosity. Dissolution of plagioclase significantly altered the detrital composition of the sands as indicated by large shifts in QFRf, QFL, and QKP ratios. Mass-balance calculations suggest overall export of aluminum from the sands. Paragenetic relationships indicate that hydrocarbon emplacement was a very late event. Precipitation of pyrite framboids within pores containing both oil and clays suggests on-going diagenesis as sulfur liberated during continued hydrocarbon maturation reacted with ferrous ions in pore fluids.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016