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The mechanisms of kerogen carbonization in Gulf Coast Tertiary sediments have been studied. The principal phase of hydrocarbon generation was found to occur at carbonization levels of 75% carbon and greater, and the quantities generated were indicated to depend largely on the hydrogen content of the kerogen. Low-hydrogen kerogen, similar to coal, was suggested as a better source for gas than oil. High-hydrogen kerogen, similar to that of oil shales, was suggested as a better source for oil. The study has been extended to Upper Cretaceous sedimentary rocks of the Denver basin, where additional support for these conclusions was obtained.
In the Denver basin Upper Cretaceous, differences in the depositional environment affect the composition of the kerogen, its carbonization track, and the type of the hydrocarbons generated. The Pierre Shale generally contains low-hydrogen kerogen that is cellular or highly structured in appearance. These characteristics indicate a major contribution from terrestrial organic detritus. This type of kerogen has been correlated with gas generation, suggesting that the Pierre Shale is principally a gas generating facies.
The Niobrara-Graneros interval contains high-hydrogen kerogen that is amorphous in appearance. These characteristics indicate a major contribution from lipid-rich detritus of aquatic organisms. This type of kerogen has been correlated with oil generation, suggesting that the Niobrara-Graneros interval is principally an oil generating facies. Rock-extract to oil-correlation measurements indicate the Niobrara-Graneros interval may be a source of Cretaceous oil in the Denver basin.
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