Paper No. 126-9
Presentation Time: 10:00 AM-10:15 AM
FASSETT, James E., U. S. Geol Survey, Emeritus, 552 Los Nidos Drive, Santa Fe, NM 87501,

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 Rm) 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.

2002 Denver Annual Meeting (October 27-30, 2002)
Session No. 126
Coal Resource and Utilization Issues
Colorado Convention Center: A101/103
8:00 AM-12:00 PM, Tuesday, October 29, 2002

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