Geologic Overview, Coal, and Coalbed Methane Resources of the Wind River Basin—Wyoming

Abstract
The Wind River Basin is an east-west-trending intermontane asymmetrical syncline covering an estimated 8,100 sq mi in central Wyoming. The basin contains comparatively minor structural deformation in the form of folding and faulting. Structurally, the surrounding area is more complex, with major thrust fault zones traversing northwest to southeast over the region. Major coal-bearing strata of the Wind River Basin are contained in the Upper Cretaceous Mesaverde Formation. The area is divided into seven coal fields and regions: Muddy Creek coal field; Pilot Butte coal field; Hudson (Lander) coal field; Beaver Creek coal field; Big Sand Draw coal field; Alkali Butte coal field; and Arminto (Powder River) coal field. Their major coal deposits are found in the Mesaverde and Meeteetse Formations, along the basin perimeter. The Alkali Butte coal field contains three coalbeds greater than 4 ft thick, the most important being the Signor, Beaver, and Shipton coals, with a rank of subbituminous C. The Hudson coal field contains the Signor coalbed or zone, which has been traced some 22 mi from the Alkali Butte coal field to the east. Coal contained within this zone is ranked as subbituminous. The remaining five coal fields have not been extensively developed. Thin and split coalbeds are prevalent in a few of these fields, and in many instances top-quality coals are scattered, making it difficult to justify development. Little is known about the basin’s coal resource at depth. The data collected suggest that a large part of the Wind River Basin may have some potential for coalbed methane production. However, a primary target area of about 1,500 sq mi, located in the northeastern part of the basin (Fremont County and western Natrona County) has been designated as having the highest potential for coalbed methane production. This target was determined by evaluating desorption data obtained from U.S. Geological Survey coal cores from the Wind River Basin Indian Reservation and integrating the data with coalbed geology. On the basis of this information, an estimated range of 5.2 to 2,225 billion cubic feet of gas may be contained in the Wind River Basin. Ranges for the expected inplace methane resource have been established for the coal fields of the Wind River Basin, based on available data. Methane has been observed in coalbeds since underground mining of that resource began. This gas, however, has only recently been recognized as an economically producible resource. Coal underlies approximately 360,000 sq mi of the conterminous United States but because of the paucity of data on coals deeper than 3,000 ft the size of the deep coal and therefore deep gas resource is not well known. Coal responds to increased temperature over time by increasing in rank or thermal maturity. During this maturation process increased volumes of methane are generated. Coal is identified as a humic, Type III, kerogen and as such yields methane as its primary hydrocarbon product and water, carbon dioxide, and nitrogen as nonhydrocarbon products. It is estimated that more than 7,000 cubic feet of methane is generated for each ton of coal during coalification from lignite to anthracite rank. Methane is found in coals either adsorbed on the coal surfaces, as free gas in fractures and large pores or dissolved in ground water in coalbeds. The amount of gas stored in the coals is influenced by depth of burial and its related pressure, rank of coal and its related porosity distribution, and a time-maturity relationship. Adsorption isotherm determinations show the maximum amount of methane that can be adsorbed on coals of various ranks. Desorption analyses of coal core samples indicate that for high rank coals adsorbed gas may exceed that predicted by isotherm analyses. Methane has been produced from coals since the early 1900s. Producibility is influenced by depth, rank, permeability, water saturation, and other hydrogeologic characteristics. Current commercial gas production from coalbeds has been documented in the Warrior Basin, Northern Appalachian Basin, and the San Juan Basin. One well in the San Juan Basin is producing at a rate in excess of 1.5 MMcfd from a 17-ft coalbed. Possible constraints to coalbed gas production involve completion technology and legal and institutional inconsistencies. Research is currently underway to enhance gas production from deep coals. Legal and regulatory constraints are being addressed through appropriate state and federal channels to resolve questions of gas ownership and price. Data indicate that the coalbed methane resource is producible at currently economic rates.

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