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This updated geochemical-geologic appraisal of Powder River basin resources appreciably increases earlier estimates of generated and expelled Cretaceous oil and gas. However, volumetric estimates of trapped and preserved petroleum were not significantly revised.
Both hydrocarbon expulsion phases were evaluated in detail as follows. (1) Oil-expulsion phase--volumes of oil generated and expelled from each source rock, then trapped and preserved; also volumes of gas expelled with oil, then trapped and preserved, including gas generated with oil and minor quantities that resulted from incipient thermal cracking of oil. (2) Gas-expulsion phase--volumes of thermal gas generated, expelled, trapped and preserved after oil expulsions ceased, along with some gas formed from thermal cracking of unexpelled oil in effective gas source rocks which are fewer in number and areally much smaller than effective oil source rocks.
Gas derived from thermal cracking of oil in deep reservoirs, bacterial gas, and gas and oil generated beyond the geochemically defined limits of each effective source rock unit were not included in this appraisal. Volumes of biogenic gas and of gas formed by thermal cracking in reservoirs are minor, relative to gas expelled from source beds.
Expulsion efficiency appears to have averaged about 7% of generated oil. Most of the generated oil was retained in source beds, some of which was then expelled as gas. Although a high percentage of thermal gas was expelled, only a small quantity of expelled gas was preserved in traps.
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