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AAPG Bulletin


AAPG Bulletin, V. 86, No. 11 (November 2002), P. 1853-1890.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Coalbed gas systems, resources, and production and a review of contrasting cases from the San Juan and Powder River basins

Walter B. Ayers Jr.1

1Harold Vance Department of Petroleum Engineering, Texas A&M University, TAMU 3116, College Station, Texas, 77843-3116; email: [email protected]


Walter B. Ayers Jr. is visiting professor in the Harold Vance Department of Petroleum Engineering and adjunct professor in the Department of Geology and Geophysics at Texas A&M University. His teaching and research interests include petroleum geology, integrated reservoir studies, clastic depositional systems, and unconventional hydrocarbon reservoirs. He holds B.S. and M.S. degrees in geology from West Virginia University and a Ph.D. in geology from the University of Texas at Austin.


The Gas Research Institute (now the Gas Technology Institute) funded the San Juan basin research at the University of Texas at Austin, Bureau of Economic Geology. This work led to early petroleum systems models of coalbed gas occurrence and producibility. Collaborators in that research were W. R. Kaiser, S. E. Laubach, W. A. Ambrose, R. W. Baumgardner Jr., A. R. Scott, R. Tyler, J. S. Yeh, G. J. Hawkins, T. E. Swartz, D. D. Schultz-Ela, and S. D. Zellers at the Bureau of Economic Geology; C. M. Tremain of the Colorado Geological Survey; and N. H. Whitehead III at the New Mexico Bureau of Mines and Mineral Resources. Colleagues in operating companies, research organizations, and consulting groups shared insights on coalbed gas systems, reservoir behavior, and operations. Their diverse viewpoints have added to my understanding of coalbed gas reservoirs, and they are gratefully acknowledged. This article benefited from reviews by B. S. Kelso and J. C. Pashin.


Coalbed gas has been produced commercially from the northern Appalachian basin since the 1930s and from the San Juan basin since the early 1950s. However, the magnitude and economic sig nificance of coalbed gas resources were realized only in the 1970s and early 1980s when the U.S. Bureau of Mines, U.S. Department of Energy, the Gas Research Institute, and oil and gas operators made a concerted effort to demonstrate commercial production of coalbed gas from vertical wells. Exploration and development ex panded in the late 1980s and early 1990s, due partly to an uncon ventional fuels tax credit. By 2000, coalbed gas accounted for 8.8% of the reserves (15.7 tcf [0.44 Tm3]) and 9.2% of the annual pro duction (1.38 tcf [40 Gm3]) of dry gas in the United States. From 1989 through 2000, cumulative United States coalbed gas produc tion was 9.63 tcf (272 Gm3). Today, coalbed gas development has spread to about a dozen basins in the United States, and exploration is progressing worldwide.

Coal beds are self-sourcing reservoirs that can contain ther mogenic, migrated thermogenic, biogenic, or mixed gas. Coalbed gas is stored primarily within micropores of the coal matrix in an adsorbed state and secondarily in micropores and fractures as free gas or solution gas in water. The key parameters that control gas resources and producibility are thermal maturity, maceral compo sition, gas content, coal thickness, fracture density, in-situ stress, permeability, burial history, and hydrologic setting. These param eters vary greatly in the producing fields of the United States and the world.

In 2000, the San Juan basin accounted for more than 80% of the United States coalbed gas production. This basin contains a gi ant coalbed gas play, the Fruitland fairway, which has produced more than 7 tcf (0.2 Tm3) of gas. The Fruitland coalbed gas system and its key elements contrast with the Fort Union coalbed gas play in the Powder River basin. The Fort Union coalbed play is one of the fastest developing gas plays in the United States. Its production escalated from 14 bcf (0.4 Gm3) in 1997 to 147.3 bcf (4.1 Gm3) in 2000, when it accounted for 10.7% of the United States coalbed gas production. By 2001, annual production was 244.7 bcf (6.9 Gm3).

Differences between the Fruitland and Fort Union petroleum systems make them ideal for elucidating the key elements of contrasting coalbed gas petroleum systems.

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