Pacific Section AAPG Convention

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Lateral Variation Of Siliceous Sedimentary Lithofacies In The Upper Monterey Formation, South Belridge-Lost Hills Fields, Kern County, California

Abstract

The character of diatomaceous rocks in the Monterey Formation is significantly related to the compositional ratio of biogenic silica-to-detrital material, which has significant effects on primary and diagenetic rock and reservoir properties. Limited published studies on the Midway-Sunset, Buena Vista, Elk Hills, Belridge and Lost Hills, fields show a gradation of lithofacies from more highly siliceous sediments at the top of anticlinal paleobathymetric highs to the surrounding lows where biogenic or diagenetic silica is diluted with detritus. In the southwest (Midway-Sunset, Buena Vista and Elk Hills), these lows were primarily sand-filled submarine channels, but farther to the north (Belridge and Lost Hills) are sand-poor. This study will be a detailed examination of the fine-grained, highly siliceous portion of the Upper Monterey Formation to better understand the lateral changes in composition from structural highs to structural lows, as well as, identify the depositional and transportation mechanisms that cause these variations. Several recent wells have been drilled across South Belridge to Lost Hills anticlines and into the surrounding Buttonwillow sub-basin which contain complete, modern well log suites as well as substantial core through much of the upper McDonald to the lower Antelope Shale – the interval of interest. Well logs will be used to calculate variations in porosity and clay content, cores and cuttings used to identify and quantify lateral variation in composition and sedimentary features, and well logs and cuttings from infilling wells used to provide better spatial resolution. Core and cuttings samples will be analyzed for mineralogy by XRD and geochemically by XRF or ICP-OES/MS and integrated with petrophysical findings. Preliminary results will be presented.