Geologic Characterization of Low-Permeability Gas Reservoirs in Selected Wells, Greater Green River Basin, Wyoming, Colorado, and Utah
Published:January 01, 1986
Ben E. Law, Richard M. Pollastro, C. W. Keighin, 1986. "Geologic Characterization of Low-Permeability Gas Reservoirs in Selected Wells, Greater Green River Basin, Wyoming, Colorado, and Utah", Geology of Tight Gas Reservoirs, Charles W. Spencer, Richard E. Mast
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Large gas resources occur in low-permeability Upper Cretaceous and lower Tertiary reservoirs in the Greater Green River basin of Wyoming, Colorado, and Utah. Most of the gas- bearing reservoirs are overpressured, beginning at depths of 8,000 to 11,500 ft (2,440 to 3,500 m). The reservoirs are typically lenticular nonmarine and marginal marine sandstones. In situ permeabilities to gas are generally less than 0.1 millidarcy (md) and porosity ranges from 3 to 12%. Secondary porosity, after dissolution of framework grains and cements, is the dominant type of porosity. Gas accumulations are characterized by the presence of updip water-bearing reservoirs and downdip gas-bearing reservoirs. The top of these overpressured gas-bearing reservoirs cuts across structural and stratigraphic boundaries and is not associated with any particular lithologic unit. These overpressured accumulations are the result of gas accumulating at rates greater than it is depleted.
Data from reference wells indicate that in the deeper parts of the basin the relatively closed nature of this system imposes severe restrictions on the ability of gas to migrate appreciable distances from the inter bedded source rocks. Consequently, the temporal relationships of hydrocarbon generation and migration with respect to the development of structural and stratigraphic traps is not as important in these unconventional reservoirs as in more conventional reservoirs. The more important factors related to gas generation and occurrence are source rock (quantity and quality), organic maturation, thermal history, formation pressure, and porosity and permeability variations.
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Geology of Tight Gas Reservoirs
Tight gas reservoirs occur in low-permeability, gas-bearing formations that are present to some extent in all gas-producing basins worldwide. This is the first volume to bring together data on tight reservoirs for a variety of basins and different geologic settings. The papers in this volume discuss characteristics of some of the most significant tight gas areas in the United States; however, these data are equally applicable to many other recognized and unrecognized tight gas provinces in other nations. In general, tight reservoirs in the United States are grouped into tight gas sandstones and eastern Devonian shales. The Devonian shale sequences are dominantly marine shale but include some siltstone and sandstone. Tight gas sandstone formations of other than Devonian age are present throughout the United States and consist primarily of fluvial and marine sandstones and siltstones. In addition, gas also occurs in low-permeability marine carbonate reservoirs. The 14 papers in this volume cover such topics as: coal-bed methane and tight gas sands interrelationships; gas-bearing shales in the Appalachian basin; exploration and development of hydrocarbons from low-permeability chalks; and geologic characterization of low-permeability gas reservoirs.