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The purpose of this study was to reconstruct the structural and thermal history of the Piceance Creek basin to try to predict the occurrences of hydrocarbons in the Upper Cretaceous Mesaverde Group. A vitrinite reflectance map of basin-wide coal zone and several coal rank cross sections using vitrinite data was constructed. Isopach maps were used to reconstruct the burial history. In general, the Mesaverde Group can be divided into two parts: a lower mixed marine and nonmarine part, and an upper, largely nonmarine section. Vitrinite reflectance values range from Ro .50 to Ro 2.1, and indicate that both the nonmarine and marine Mesaverde are within the range of thermal gas generation throughout the basin, with the possible exception of the upper part f the nonmarine Mesaverde along the extreme west and
southwest margins of the basin. The occurrence of gas correlates reasonably well with this finding. Both the marine and nonmarine Mesaverde are within the window of oil generation for most of the basin, except in the deeper parts where the upper limit of oil stability has been exceeded. Oil, however, is seldom encountered in the basin, probably because of a lack of abundant source beds with oil-generating capabilities.
The vitrinite values are much too high to have formed under the present geothermal gradient, which averages about 1.7°F/100 ft (3°C/100 m), and appear to reflect a paleothermal gradient of between 2.2 and 3.5°F/100 ft (4 and 6.3°C/100 m), with the highest gradients in the southern part of the basin. It is suggested that this high gradient occurred during Oligocene time when the southeastern part of the basin was extensively intruded by magmas of intermediate composition.
Overpressuring has thus far only locally been encountered in the basin. The lack of a well-defined overpressured area may be a combination of: (1) a decrease in the geothermal gradient since Oligocene time, and (2) uplift and removal of overburden during the last 10 million years. As much as 5,000 ft (1,500 m) of overburden has been removed in some parts of the Colorado River drainage.
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