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Abstract

The reconstructed structural and thermal history of the Piceance Creek basin of western Colorado defines the geologic and geochemical conditions for the occurrence of gas in the Upper Cretaceous Mesaverde Group. In general the Mesaverde consists of two parts: a lower marginal marine section with mostly blanket reservoirs and an upper nonmarine part with mostly lenticular reservoirs. Most of the gas produced has been from reservoirs in the marginal marine section; however, because of its great thickness and abundance of gas shows, the nonmarine section is thought to contain more gas in place. Reservoirs in the nonmarine rocks usually have low permeability and are unconventional.

Attempts to explain coal ranks using recent coal metamorphism models were unsuccessful, primarily because of the margin of uncertainty in present–day formation temperature readings in the basin. Coals around the basin margins were uplifted to their present high stratigraphic position in the basin during the final stages of the Laramide orogeny during the late Eocene and appear to be frozen at roughly pre–uplift coal ranks. The relatively high rank of some of these coals suggests that coal ranks throughout the basin attained close to their present–day rank from burial heating prior to the end of the Eocene. The effects of later thermal events on coal rank, such as Oligocene–age plutonism in the southern part of the basin, appear to have been minimal. If this is correct, then peak hydrocarbon generation in the basin peaked during the Eocene. Closed anticlines, which have produced much of the gas in the basin, appear to be Laramide growth structures; hence, peak gas generation occurred while the anticlines were growing.

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