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AAPG Bulletin


AAPG Bulletin, V. 87, No. 1 (January 2003),P. 1-13.

Copyright ©2003. The American Association of Petroleum Geologists. All rights reserved.

E & P Notes

Coalbed methane, Cook Inlet, south-central Alaska: A potential giant gas resource

Scott L. Montgomery,1 Charles E. Barker2

1Petroleum Consultant, 1511 18th Avenue East, Seattle, Washington, 98112; email: [email protected]
2U.S. Geological Survey, Box 25046, MS 977, Denver Federal Center, Denver, Colorado 80225; email: [email protected]


Scott L. Montgomery is a petroleum consultant and author residing in Seattle, Washington. He holds a B.A. degree in English from Knox College (1973) and an M.S. degree in geological sciences from Cornell University (1978). He is the lead author for the E&P Notes series in the AAPG Bulletin (since 1996) and has published widely on many topics in the petroleum industry.

Charles E. Barker has been a senior research geologist on the USGS Energy Resources Team for 24 years. His publications have focused on geothermometry, basin modeling, and coalbed methane. He has prior experience with Tenneco and Union Oil Companies. He has been active in several societies: 1999-2000 chairman of the Coal Geology Division of the Geological Society of America; 1998-1999 president of the Society for Organic Petrology; 1992 president of the Society for Luminescent Microscopy and Spectroscopy. Charley was educated at San Diego State University (B.S. degree, 1974), University of California, Riverside (M.S. degree, 1979) and the University of Adelaide (Ph.D., 1995).


Geology in this article is by Charles E. Barker, U.S. Geological Survey; Dan Seamount, Alaska Oil and Gas Commission; Todd A. Dallegge, U.S. Geological Survey and University of Alaska; and Robert F. Swenson, Philips Petroleum, Alaska. Data for this article are courtesy of UNOCAL (Erik Graven), Ocean Energy (Brett Jameson), Forest Oil ([formerly Forcenergy] Richard Nelson), Evergreen Resources (John Tanigawa), Lapp Resources (David Lappi), the Alaska Division of Geological and Geophysical Surveys (Jim Clough), and the U.S. Geological Survey.


Cook Inlet Basin of south-central Alaska is a forearc basin containing voluminous Tertiary coal deposits with sufficient methane content to suggest a major coalbed gas resource. Coals ranging in thickness from 2 to 50 ft (0.6 to 15 m) and in gas content from 50 to 250 scf/ton (1.6 to 7.8 cm2/g) occur in Miocene-Oligocene fluvial deposits of the Kenai Group. These coals have been identified as the probable source of more than 8 tcf gas that has been produced from conventional sandstone reservoirs in the basin. Cook Inlet coals can be divided into two main groups: (1) those of bituminous rank in the Tyonek Formation that contain mainly thermogenic methane and are confined to the northeastern part of the basin (Matanuska Valley) and to deep levels elsewhere; and (2) subbituminous coals at shallow depths (<5000 ft [1524 m]) in the Tyonek and overlying Beluga formations, which contain mainly biogenic methane and cover most of the central and southern basin. Based on core and corrected cuttings-desorption analyses, gas contents average 230 scf/ton (7.2 cm2/g) for bituminous coals and 80 scf/ton (2.5 cm2/g) for subbituminous coals. Isotherms constructed for samples of both coal ranks suggest that bituminous coals are saturated with respect to methane, whereas subbituminous coals at shallow depths along the eroded west-central basin margin are locally unsaturated. A preliminary estimate of 140 tcf gas in place is derived for the basin.

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