FRUITLAND FORMATION COAL-BED-GAS SEEPS, NORTHERN SAN JUAN BASIN, COLORADO

Significant natural-gas seeps are present in five areas on the Upper Cretaceous Fruitland Formation outcrop on the northern rim of the San Juan Basin, New Mexico and Colorado. All five gas seeps are in La Plata County, Colorado and occur where the steeply dipping Hogback Monocline has been breached by stream cuts; the seeps are thus in relatively low topographic areas. The Fruitland Formation is the major coal-bearing rock unit in the San Juan Basin and contains in excess of 230 billion tons of coal throughout the basin; the Fruitland crops out around most of the margin of the basin. Fruitland coal is strip mined in three large mines in northwestern New Mexico. Fruitland coal has also been mined, mostly underground, in many small workings around the north and northwest rim of the basin; nearly all of those mines are now abandoned. A relatively small Fruitland coal strip-mining operation in the northeast part of the basin, the Chimney Rock mine, is also now abandoned. Over the past three decades, the northern San Juan Basin has experienced a gas-drilling boom targeting coal-bed methane in the Fruitland Formation. As a result, more than four thousand coal-bed-gas wells have produced more than 9 TCFG (trillion cubic feet of gas) from Fruitland coal beds. The USGS has recently estimated that another 24 TCFG of undiscovered Fruitland coal-bed methane is present in the San Juan Basin (this estimate may be too low by a factor of two or more). Some Fruitland coal-bed methane wells are located within a mile or two, down dip, from Fruitland gas seeps, raising the question of whether these gas wells have caused or contributed to the gas seeps. Detailed geologic studies by the USGS have shown that for at least one seep: the Pine River gas seep; gas production from Fruitland coal-bed methane wells down-dip from that seep is unrelated to the seep. Geologic evidence of the relationship of gas production from nearby gas wells to Fruitland gas seeps in other areas is less clear cut.