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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 68 (1984)

Issue: 4. (April)

First Page: 498

Last Page: 499

Title: Geologic Characteristics of Low-Permeability Gas Reservoirs in Greater Green River Basin of Wyoming, Colorado, and Utah: ABSTRACT

Author(s): Ben E. Law

Abstract:

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 8,000-11,500 ft (2,440-3,500 m). The reservoirs are typically lenticular nonmarine and marginal marine sandstones. In situ permeabilities to gas are generally less than 0.1 md, and porosities range from 3-12%. Secondary porosity, after dissolution of framework grains and cements, is the dominant type of porosity. Gas accumulations are characterized by the presence of water updip and little or no recoverable water downdip. The seal of these overpressured gas-bearing reservoirs cuts across structural and stratigraphic boundaries and is not ass ciated with any particular lithologic unit. The trapping mechanism is

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a water block similar to that described by J. A. Masters in the Deep Basin of Canada and the San Juan basin of New Mexico and Colorado. Recent work suggests that the formation of the water block in the Greater Green River basin is related to a dewatering process associated with the thermal generation of gas.

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 interbedded source rocks. Consequently, the temporal relationships of hydrocarbon generation and migration with respect to the development of structural and stratigraphic traps are not as important in these unconventional reservoirs as in more conventional ones. The more important factors related to gas generation and occurrence are source-rock quantity and quality, organic maturation, thermal history, formation pressure, porosity and permeability variations, and the nature of formation-water occurrence.

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