Geologic Evaluation of Critical Production Parameters for Coalbed Methane Resources: Part 1, San Juan Basin


The Fruitland Formation in the San Juan Basin is the major producer of coalbed methane in the Western U.S. Forty-three to forty-nine Tcf of methane occur in 245 billion short tons of Fruitland coal at depths between 400 and 4,200 ft. Thickest Fruitland coal seams trend northwest and occur in the northern part of the basin, northeast of a syndepositional, structural hinge line; they occur in coastal plain facies southwest of Pictured Cliffs barrier/strandplain sandstones. South of the hinge line, northeast-trending coal seams occur in floodplain facies between northeast-trending Fruitland fluvial systems. Face cleat trends in Fruitland coal seams are predominantly northeast in the southern two-thirds of the basin and northwest but variable in the northern third. Suggested targets for enhanced coalbed permeability are tectonic fractures and fractures associated with subtle folds. Fruitland Formation waters are evolved meteoric waters; water composition reflects hydrologic setting. Waters in the north-central San Juan Basin have high alkalinity and low chlorinity; waters in the southern part are Na-Cl type. Distribution of low-chloride ground water in the Fruitland Formation in the north-central basin coincides with the overpressured area and with flow patterns inferred from the head map. The Fruitland Formation acts regionally as a single hydrologic unit or homogeneous aquifer, but large pressure gradients locally indicate that Fruitland strata may be hydraulically disconnected and behave at the field scale as compartmentalized aquifers. Hydrologic studies defined reservoir characteristics and permeability boundaries in the Fruitland Formation. Geologic and hydrologic parameters were used to divide the San Juan Basin into areas in which coal beds have similar reservoir characteristics. Coalbed wells have negative declines early in their production history followed by exponential decline rates at less than 5 percent/year. Sandstone wells that exhibit coal-decline behavior probably are producing coalbed methane indirectly from coal seams.


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