THE HIGH-PERMEABILITY FRUITLAND FORMATION COAL-BED METHANE FAIRWAY, SAN JUAN BASIN, NEW MEXICO AND COLORADO: A PRODUCT OF THE BASIN'S STRUCTURAL EVOLUTION?

The San Juan Basin of northwest New Mexico and southwest Colorado contains the largest known coal-bed methane deposit in the world. The coal-bed methane in this field is contained in hundreds, if not thousands, of discrete coal beds in the Fruitland Formation; these beds are distributed unevenly, in space and time, across the basin. (The geometry and distribution of these coals is described in detail in USGS Professional Paper 1625-B, Chapter Q, published in 2000.) The net-coal thickness for Fruitland coal beds in the basin ranges from zero to 100 ft and a band of thick net coal (in excess of 40 ft) trends northwest across the north-central part of the basin. Cumulative coal-bed methane production from the San Juan field is more than 9 TCFG (trillion cubic feet of gas) with about 80% of this production having come from the “Fruitland Fairway”. The fairway trends northwest within, but not exactly coincident with, the thickest-net-Fruitland-coal band that also trends northwest across the basin. Rates of coal-bed methane production within the fairway are much higher than anywhere else in the basin – from 1 to more than 10 MCFGD (million cubic feet of gas/day). The Fruitland coal beds within the fairway have much higher permeability than elsewhere in the basin and the fairway is an over-pressured zone within what is generally an under-pressured basin.

A map of vitrinite reflectance values for Fruitland coals in the San Juan Basin (fig. 37, USGS Professional Paper 1625-B) shows that the highest values (1.6 R_m) are present 12-15 miles north of the present structural axis of the basin. It is suggested that the Fruitland-coal vitrinite-reflectance map of the San Juan Basin is a good proxy for a paleo-structure map of the basin 30-40 Ma and that the basin axis has shifted southward since that time reaching its present position about 30 Ma. It is further suggested that the process of basin-axis-shifting resulted in the fracturing of the Fruitland coal beds in the Fruitland coal-bed methane fairway.