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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 3. (March)

First Page: 569

Last Page: 569

Title: Overpressured, Low-Permeability Gas Reservoirs in Green River, Washakie, and Great Divide Basins, Southwestern Wyoming: ABSTRACT

Author(s): Charles W. Spencer, Ben E. Law

Article Type: Meeting abstract


Upper Cretaceous and lower Tertiary "tight" (< 0.1 md) gas-bearing sandstone reservoirs are being investigated in deep basins in the Rocky Mountain region. The reservoir rocks were deposited in fluvial, deltaic, and marginal marine environments. The marginal marine sandstones are generally better sorted and higher in quartz content, and generally have slightly better porosity (> 10%) and permeability than fluvial and fluviodeltaic reservoirs. The greatest thickness (> 10,000 ft, > 3,048 m) of gas-bearing intervals is in the fluvial sequences in the Upper Cretaceous Mesaverde Group and Lance Formation and the overlying lower Tertiary Fort Union Formation.

Normal reservoir pressures are in the range of ±0.43 psi/ft (± 9.73 kPa/m). However, broad areas of the deeper Washakie, Great Divide, and Green River basins have reservoir pressures in excess of 0.5 psi/ft (11.31 kPa/m) and, locally, pressures higher than 0.8 psi/ft (18.10 kPa/m) have been recorded. The thermal maturation level of organic matter in cores and cuttings from six deep wells drilled in the Washakie and Green River basins has been determined. In addition, present-day subsurface temperatures and pressures have been compiled for many other wells. This work indicates that the overpressuring occurs in sequences of tight sandstones enclosed in humic-rich shales and/or in association with coals having present-day temperatures greater than 190°F (88°C). The 19 °F (88°C) isotherm surface ranges in depth from about 9,000 to 11,500 ft (2,743 to 3,505 m). We interpret the over-pressuring to be due mainly to pressure maintenance caused by present and recently active generation of natural gas in organic-rich sequences subjected to temperature levels greater than 190°F (> 88°C).

Operators have had difficulty in completing wells in zones deeper than 12,000 ft (3,568 m) in the study area. Successful completion of deep wells will require innovative drilling and completion techniques that address the problems of proppant crushing, formation damage, high-formation temperatures, and well-log analysis. A substantial geologic and engineering research effort is needed to exploit this potential major gas resource.

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