The depositional environments and reservoir characteristics of the upper Miocene Etchegoin and Chanac formations in Kern Front oil field, eastern San Joaquin basin, California, were interpreted in cores from three wells. Sedimentary facies were correlated to the log responses and then mapped throughout the reservoirs. The Etchegoin Formation is a shallow-marine unit consisting of a basal transgressive and overlying deltaic units. Facies include shoreface, river-mouth bar, prodelta, paralic, and marine and distributary channel deposits. The shoreface deposits trend north-south, and are interbedded with, and overlain by, bioturbated marine units. They are gradational upward into river-mouth bars and marine channels, and are incised locally by distributary channels. The Chanac Formation underlies the Etchegoin Formation and contains meandering stream sequences deposited on a low-relief, mud-rich coastal plain. West-trending channels are recognized in the Chanac Formation.
Log-derived data, combined with core porosity and permeability measurements, indicate that the upper Miocene reservoir sandstones have an average porosity 36.5% and average permeability 2,200 md. The Etchegoin sandstones are poorly indurated, arkosic arenites and wackes composed of subangular, medium- to coarse-grained, poorly-sorted detritus. Detrital modes for the sandstones are Q33 F47 L20 and Qm38 P48 K14. Authigenic minerals are rare and include calcite, illite, and Ca-zeolite. The best reservoir sandstones in the field are the Etchegoin deltaic shoreface and channel facies and the Chanac channel facies. The two factors controlling reservoir quality are grain size and sorting. The proportion of detrital matrix ranges from 11 to 12% in the better reservoir facies to 22 to 45% in the poorer reservoirs; the proportion of the coarsest-grained fraction (sand/gravel) ranges from 54% in the poorer reservoirs to 83% in the better reservoir facies.