Organic metamorphism in the California petroleum basins; Chapter A, Rock-Eval and vitrinite reflectance

Bulletin 2174-A

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The results of ROCK-EVAL and vitrinite reflectance analyses of a large sample base from more than 70 wells located in three oil-rich California petroleum basins are reported. The cores from these wells have a wide range of present-day burial temperatures (40 ? to 220 ? C). The rocks in these basins were deposited under highly variable conditions, sometimes resulting in substantially different organic matter (OM) types in rocks tens of meters vertically apart from each other in one well. The kinetic response of these different OM types to equivalent wellknown burial histories is a pivotal point of this study. In the Los Angeles and Ventura Basins, rock organic-richness significantly increased with depth, as did kerogen hydrogen content, and the percentage of fine-grained versus coarsegrained rocks. The shales in these basins are perceived as containing primarily hydrogen-rich amorphous OM. In actuality, the shallowest 2,000 to 3,000 m of rocks in the basins, and at least the upper 6,000 m of rocks in parts of the Los Angeles Basin central syncline, are dominated by type III/IV OM. In the Los Angeles Basin, mainstage hydrocarbon (HC) generation commences in the type III/IV OM at present-day burial temperatures of 85 ? to 110 ? C, most likely around 100 ? C, and is largely complete by 220 ? C. In the Southern San Joaquin Valley Basin, mainstage HC generation commences in type III/IV OM at 150 ? C and is also largely complete by 220 ? C. In the Ventura Basin, mainstage HC generation commences above 140 ? C in type III/IV OM. The apparent lower temperatures for commencement of HC generation in the Los Angeles Basin are attributed to the fact that parts of the basin were cooled from maximal burial temperatures by increased meteoric water flows during the last glaciations. All aspects of organic metamorphism, including mainstage HC generation, are strongly suppressed in rocks with hydrogenrich OM in these basins. For example, ROCK-EVAL data suggest that mainstage HC generation has not commenced in rocks with hydrogen-rich OM at present-day temperatures of 198?C. This observation is attributed to much stronger bonds in hydrogen- rich OM compared to types III and IV OM and, therefore, significantly higher burial temperatures are required to break these bonds. This difference in OM kinetics has profound ramifications for petroleum-geochemical exploration models. Organic-matter characteristics inherited from original depositional conditions were overlaid on, and at times confused interpretation of, characteristics from organic metamorphism in all study areas. In all the basins examined in this study, immature fine-grained rocks occasionally had high to very high carbon-normalized concentrations of pre-generation indigenous bitumen. This unusual characteristic may be due to unique depositional conditions in these basins.

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Organic metamorphism in the California petroleum basins; Chapter A, Rock-Eval and vitrinite reflectance
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Version 1.0
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U.S. Department of the interior, U.S. Geological Survey,
34 p.