Oil and gas resources of the San Juan Basin, New Mexico and Colorado
— James E. Fassett

The San Juan Basin of northwestern New Mexico and southwestern Colorado is the second-largest gas basin in the conterminous United States, second in total estimated gas reserves to the Hugoton field of Texas, Oklahoma, and Kansas. The basin is in the Four Corners area, near the common Corners of New Mexico, Arizona, Utah, and Colorado. The major tectonic element of the basin is the monocline bounding the central basin on the east, north, and west sides. The central basin has no southern structural boundary; its southern limit for purposes of this report is drawn roughly along the outcrop of the Pictured Cliffs Sandstone. Outside the monocline, rocks generally dip less steeply toward the basin’s structural axis. The San Juan Basin comprises three elements: the central basin, Chaco slope, and Four Corners platform. All of the oil and gas fields discussed herein are within the San Juan Basin and the production statistics for the San Juan Basin are for fields within this area.

The structural axis, or deepest part of the central basin is arcuate and generally trends northwest in the northern part of the basin. Except along the monoclinal rim of the central basin, dips are gentle and range from less than one degree to commonly less than two degrees. Precambrian rocks crop out north of the San Juan Basin on the San Juan uplift, to the east on the Nacimiento uplift, to the south on the Zuni uplift, and to the southwest on the Defiance uplift.

Oil and gas production in the San Juan Basin through 2009 has been from more than 300 fields or reservoirs in New Mexico and Colorado. Most production has come from Upper Cretaceous rocks. Most of the basin’s historical gas production has come from stratigraphic traps in Cretaceous fractured-sandstone reservoirs but starting in the late 1970s, Fruitland Formation coalbed methane production has grown enormously. Cumulative gas production from all San Juan Basin fields (through late 2009) is 42.6 trillion cubic feet of gas (TCFG) with 26 TCFG coming from fractured sandstone reservoirs, 16 TCFG from Fruitland coal beds, and the remainder from smaller oil and gas fields. Cumulative oil production is 381 million barrels of oil (MBO) with 175 MBO coming from Tocito Sandstone fields. This “oil” production includes nearly 100 million barrels of condensate from the basin’s three fractured-sandstone, gas-producing reservoirs. Nearly all the fields in the central basin area produce from stratigraphic traps whereas the relatively small oil and gas fields on the Four Corners Platform produce from structural traps. The Fruitland Formation’s coal-bed methane is trapped by adsorption of the gas in the coal, and thus is in a category of trap all its own.