Depositional Environments and Geometry of Coal-bed Natural Gas Reservoirs in the Raton Basin, Colorado and New Mexico

CLOSE, JAY C., and RUSSELL R. DUTCHER, Southern Illinois University, Carbondale, IL

Recognition of the depositional environments, occurrence, and geometry of Raton basin coals and coal-bearing rocks is essential to the reliable estimation of potential gas-in-place reserves and profitability, optimum placement and operation of production wells, and effective completion design for all Raton basin coalbed degasification projects. This knowledge is needed for two reasons: (1) coals of the Vermejo and Raton Formations, coalbed gas reservoirs in the Raton basin, are laterally discontinuous and separated by significant innerburden as a function of peat accumulation in a fluvial-dominated deltaic system that prograded northeastward during the last regression of the latest Cretaceous-earliest Tertiary seaway from the United States Western Interior and (2) Vermejo and Raton Fo mation coals have been naturally coked or replaced by middle Tertiary igneous sills and dikes in many areas of the basin.

Diagnostic criteria for facies recognition indicate the major environments of peat deposition included generally northeast-trending Vermejo interdistributary bays, northwest-trending Vermejo distributary-mouth bars, and northeast-trending Raton alluvial plains. New stratigraphic, sedimentologic, and coal petrographic data indicate Raton basin coals are vitrinite rich, composed of discrete coal and shale interbeds 2-10 ft thick at drilling depths of up to 2500 ft, present in a stratigraphic interval at least 1,000 ft thick, that collectively form net thicknesses of 15 to 70 ft and dramatically thicken, thin, pinch out, or abut against sills or dikes in distances of less than 5000 ft. The data base for these conclusions is comprised of petrographic observations for approximately 200 coa crushed particle pellets, 54 measured sections, 294 paleocurrent measurements at 45 stations, geophysical logs for 50 wells, and observations of field depositional relationships at over 200 outcrops.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)