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Geology of the Belridge Diatomite, Northern South Belridge Field, Kern County, California
The South Belridge field is located in western Kern County, California. One reservoir in the field, accounting for about 20% of the daily field oil production, is the Belridge diatomite member of the Reef Ridge Shale. The field is developed on a elongated anticline. Within the field, normal faulting appears in three episodes: early Pliocene large offset lystric faults, the middle Pliocene large offset Middle Belridge fault, which geologically separates the northern portion of South Belridge field from the southern portion, and late Pliocene small offset vertical faults. The small offset faults, along with the natural fracture system, are the primary permeability conduits in the Belridge diatomite reservoir. In northern South Belridge field the natural fracture system trends N45°E and is near vertical.
Around section 12, T28S, R20E, MDB&M, four intervals within the Belridge diatomite are productive: the 12 Zone, upper diatomite, middle diatomite, and the lower diatomite; all of the lower Belridge diatomite north of the Middle Belridge fault lies in the opal-CT facies and is a distinct hydrocarbon reservoir. Lithologies of the three opal-A intervals vary from diatomaceous mudstone to silty diatomite. Productive zone thicknesses range from an average of 100 ft to 775 ft. Average effective porosity is 36.7% to 55.4%, average helium permeability is 1.86 md to 103 md (0.11 md to 0.56 md to brine), and average free water saturation is 55.4% to 69.0%. Formation waters vary from 25,085 ppm TDS to 40,000 ppm TDS with resistivities of 0.35 ohms to 0.17 ohms at 25° C. Oil gravities range from 22.2° API gravity to 32° API gravity.
Diatom floras from wells in South Belridge field show the middle and lower diatomite to be late Mohnian to middle Delmontian and the upper diatomite and 12 Zone to be late Delmontian. The section is probably correlative to the Bitterwater Creek Shale of the southern-most Temblor Range and is time-correlative to the Sisquoc Formation diatomites of coastal California.
The diagenetic transition of Belridge diatomite from opal-A to opal-CT occurs between 2000 ft and 2300 ft and is coincident with the top of the lower diatomite interval. The transition from opal-CT to quartz occurs at about 3700 ft. Formation temperatures within the Belridge Diatomite suggest that there has been 925 ft of post-opal-CT/quartz transition uplift and 300 ft of post opal-A/opal-CT transition uplift in South Belridge field. The coincidence of the opal-A/opal-CT transition with the top of the lower diatomite suggests that diagenesis preceded structural deformation. Using average compacted sediment deposition rates, the estimated time to deposit the Belridge diatomite was 1,643,400 years. Comparison of the lower Belridge diatomite sections at the north and south ends of South Belridge field suggests that the relative basinal subsidence during deposition was -0.48 ft/1000 years.
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