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In the westernmost Los Angeles basin, the "nodular shale," a distinctive, richly organic bituminous and phosphatic mudstone, occurs just above the unconformable base of the upper Miocene Modelo Formation. This 11.5 m.y. old bathyal oil-source rock is present in wells to burial depths > 3,810 m (12,500 ft), and is inferred to be present within the central syncline of the basin beneath about 9 km (5.5 mi) of late Miocene to Holocene clastic cover.
Forty-four subsurface samples of the nodular shale were collected from 14 selected wells located mostly between the Playa del Rey and Crescent Heights oil fields. Sites were selected to give the widest available range of sample depth and temperature where present burial depths are maximal, and where geothermal gradients are firmly established. Median random reflectance (%Ro) of first-cycle vitrinite is least in the shallowest samples, clusters about 0.24% in the deeper samples, and exceeds 0.30% only in the deepest and hottest samples. Extremes in the range of measured median %Ro are tabulated below with corresponding extremes of sample temperatures, depths, Time-Temperature Indices (TTI), and calculated %Ro equivalents of the TTI values.
All measured values of Ro are significantly depressed compared to other maturity criteria. Significantly, second-cycle and oxidized vitrinite from these same samples show normally elevated reflectance.
Eight of the samples processed for reflectance measurements were analyzed for total organic carbon content, which ranges from 2.21 to 9.41%. Most of the organic detritus is amorphous degraded algal material; less than 10% is structured vitrinite. Thermal alteration index values for the amorphous material range from 2 to 2 ½, corresponding with hypothetical conversion Ro values between 0.45 and 0.75%, again notably higher than the measured values. The ratios of extractable hydrocarbons to TOC in the 8 samples suggest "mature" levels of thermal evolution, as do carbon preference indices of 0.93 and 1.14 from extracts of 2 samples.
Striking similar patterns of vitrinite reflectance values have been described from alginites in some Australian coalfields and oil shales. The data suggest to us that hydrogen-rich organic matter matures at lower temperatures and at a substantially faster rate (and lower TTI values) than detritus dominated by structured organic matter of lower hydrogen content. The depressed Ro measurements evidently reflect the hydrogen-rich nature of the dominant detritus and thus are not reliable indicators of either paleotemperature or thermal maturity in the most oilprone source rocks. In fact, depressed Ro values may be indicators of ultra-rich source rocks when normalized for other influences.
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