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Most hydrocarbon production from low-permeability Upper Cretaceous and lower Tertiary reservoirs in the Greater Green River basin of Wyoming, Colorado, and Utah is gas. The most likely sources of the gas are the interbedded coal beds and other carbonaceous lithologies. A source-rock evaluation of these rocks indicates predominantly humic, type III organic matter capable of generating mainly gas.
The relatively closed nature of these low-permeability rocks facilitates examination of the geologic processes involved in gas generation and occurrence. All gas accumulations are associated with overpressuring. Thermal generation of gas is the main cause of overpressuring and is directly related to organic richness, level of organic maturation, and temperature. Distances of gas migration, in most areas, do not exceed a few hundred feet. Consequently, the temporal relationships of gas generation and migration with respect to the development of structural and stratigraphic traps are not as important as in more conventional reservoirs. On the basis of the premise of minimal gas migration, the initiation, or threshold of significantly large volumes of thermogenic gas occurs at a temperat re of about 190°-200°F (88°-93°C) and a vitrinite reflectance of about 0.80 Ro.
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