BERMAN, BRIGITTE H., THUMS Long Beach Co., Long Beach, CA, and California State University, Long Beach, CA, and JOHN C. PINKERTON, California State University, Long Beach, CA

ABSTRACT: Carbonate Rocks within the Siliciclastic Sequences of the Wilmington Oil Field, Los Angeles County, California

Authigenic carbonate beds averaging 3 ft thick are prominent features within the sand/shale turbiditic sequences of the Long Beach unit portion of the Wilmington oil field. These laterally continuous carbonate zones act as fluid barriers and are distinctive in porosity and resistivity logging suites, making them useful in the correlation of flow units.

X-ray diffraction spectrometry and thin-section point-count analyses of 28 carbonate samples representing a composite stratigraphic section reveal differences in composition ranging from calcite, Mg calcite, to dolomite. Shales within the UP-Ford and Ranger zones are characterized by matrix dolomitization, whereas sands have been cemented by calcite. In the Terminal zone, Mg calcite is the main constituent in both shales and sandstones. Effective porosities in all cases remain below 3% of the total bulk rock volume, with carbonate content ranging from 35 to 100%.

Calcite is precipitated as a cement in a framework of quartz, feldspar, and plagioclase and as framework grain replacement. Because of these differences in occurrence, calcite is used for estimating the timing of cementation relative to burial. Calcite preserves original intergranular porosities that equal up to 40% of the total bulk rock volume. Dolomite occurs as a replacement of matrix in fine-grained rocks in association with minor amounts of quartz, feldspar, and illite.

Variations in the enrichment of 13C in dolomite correspond to changes in the rate of sedimentation (i.e., burial rate). Likewise, 18O variations correspond to changes in temperature at the time of formation.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.