Geologic and Hydrologic Controls on the Occurrence and Producibility of Coalbed Methane: Fruitland Formation, San Juan Basin

Abstract

Coalbed methane resources in the Fruitland Formation in the San Juan Basin are estimated to be 43 to 49 trillion cubic feet at depths between 400 and 4,200 ft. The San Juan Basin leads the United States in coalbed methane production; in 1989, the basin produced approximately 65 billion cubic feet of coalbed methane. In the past 5 years, more than 1,000 coalbed methane wells have been drilled in the basin.

The thickest Fruitland coal seams occur in the northern part of the basin and trend northwestward, paralleling Pictured Cliffs barrier/strandplain sandstones; in the southern part of the basin, anomalously thick coal seams trend northeastward and occur between Fruitland fluvial sandstone complexes. Fruitland coal seams commonly are extensive, overriding sandstones. Coal beds are fractured reservoirs, and commonly, permeability is greatest in the direction of the dominant fracture set (face cleat). Face cleat strikes in Fruitland coal beds delineate two principal domains. In the southern part of the basin, face cleats strike northward or northeastward, whereas in the northern part of the basin, face cleat strike is predominantly northwestward.

Coal seams, major aquifers in the Fruitland Formation, are overpressured in the northern part of the basin due to artesian conditions. These overpressured coal seams commonly are water-productive. In the southern part of the basin, the Fruitland Formation is underpressured, and coalbed methane wells produce little or no water. The composition of Fruitland coalbed gas varies regionally and predictably; both dry and wet Fruitland gases are present, and carbon dioxide content ranges up to 13 percent. On the basis of geologic and hydrologic studies, the San Juan Basin was divided into regions in which Fruitland coal beds have similar reservoir characteristics.

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