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Combined geochemical and geologic information from this structural basin accurately delimited areas and stratigraphic sequences prospective for crude oil and thermal hydrocarbon gases. Using volumetric and performance data for each effective source sequence, quantities of expelled oil and gas were calculated which readily account for in-place oil reserves of more than 6 billion bbl and minor amounts of associated gas.
Oils expelled from Lower and Upper Cretaceous source beds are similar. The Mowry siliceous shale and Niobrara calcareous shale and marl expelled most of the oil indigenous to the basin. A second major oil type is correlated to the remote Permian Phosphoria source area centered in southeastern Idaho. Oil migration paths have been mapped, gathering areas identified, and time of migration determined. Three of five giant oil fields--Salt Creek, Lance Creek, and Bell Creek--are located on separate gathering areas around the basin periphery. Hilight and Hartzog Draw fields are stratigraphic traps paralleling structural strike on the basin's eastern flank, oriented to receive maximum flow of migrating oil.
An Early Jurassic regional migration emplaced Phosphoria oil in upper Paleozoic reservoirs before the basin formed. Expulsion from deepest Cretaceous source rocks began in Eocene time and probably continued into Pliocene time as the expulsion front moved upsection and updip. Laramide structure controlled migration of Cretaceous oil.
Recharge water affected oil preservation. Consequently, temperature and salinity anomalies are commonly associated with accumulations in recharge areas, where two types of bacterial alteration are recognized.
More than 20 mutually supporting chemical and physical parameters from rocks and fluids proved useful in defining prospective areas.
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