Dudley D. Rice, 1983. "Application of Organic Geochemistry to Hydrocarbon Occurrence", Patterns of Sedimentation, Diagenesis, and Hydrocarbon Accumulation in Cretaceous Rocks of the Rocky Mountains, Dudley D. Rice, Donald L. Gautier
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Hydrocarbon generation takes place during three main stages: immature, mature, and postmature. These stages can be related to patterns of digenesis. During the immature stage, methane-rich biogenic gas is generated by microbiological activity. With increasing temperature and geologic time, the full range of hydrocarbons may be generated during the mature stage by thermal cracking processes. At high temperatures (postmature stage), methane is the main hydrocarbon product, which results from the cracking of existing hydrocarbons. Production, accumulation, and preservation of organic matter in sediments are the first important steps leading to the accumulation of hydrocarbons. In the Cretaceous of the Western Interior, three main environments were favorable for these steps: (1) coal swamps where major source beds for gas were formed; (2) progradational marine environments where thick fine-grained sequences with moderate amounts of organic matter, that will generate both oil and gas, were deposited; and (3) transgressive marine settings where organic-rich muds or carbonates, that are oil-prone, were deposited. Organic matter is converted to kerogen, which is the precursor of most hydrocarbons. Three important kerogen parameters should be evaluated in determining source-rock potential: concentration, type, and maturity.
An understanding of the processes that lead to the accuniulation of organic matter, and generation, expulsion, migration, and accumulation of hydrocarbons is necessary for effective exploration of Cretaceous rocks in the Rocky Mountain area. Although most of these short course notes is devoted to the characterization of reservoir rocks, we believe that an understanding of the basic geochemical and geological processes resulting in the