The Oil and Gas Reservoirs of the San Juan Basin, New Mexico and Colorado
James E. Fassett
U.S. Geological Survey, Scientist Emeritus/Independent Geologist, Santa Fe, NM
The San Juan Basin of northwest New Mexico and southwest Colorado is the second largest gas field in the conterminous U.S., second only to the giant Hugoton field of Texas, Oklahoma, and Kansas. The relatively recent (since the 1970’s) development of the Fruitland coal-bed methane (CBM) reservoir has doubled previous estimates of ultimate recoverable gas reserves for the basin from 25 TCFG (for the fractured sandstone reservoirs) to more than 50 TCFG for all gas reservoirs in the basin. To date the basin has produced in excess of 31 TCFG; CBM from Fruitland coals accounts for 9 TCFG. Cumulative oil production is more than 300 million barrels. The basin contains more than 300 oil and gas fields that produce from Paleozoic, Jurassic, Cretaceous, and Tertiary reservoirs, but more than 93% of the oil and 99% of the gas in the basin has come from Upper Cretaceous rocks. Nearly all of the non-CBM gas in the basin has been produced from three, fractured-sandstone reservoirs in the Dakota Sandstone, Mesaverde Group, and Pictured Cliffs Sandstone. These reservoirs have produced nearly 21 TCFG; 10.6 TCFG from the Mesaverde Group. Down-spacing and infill drilling has occurred in the fractured-sandstone reservoirs resulting in the production of “new” gas and reversing the decline in gas production from these reservoirs. The origin, extent, and geometry of the fracture systems in these reservoirs has been poorly understood in the past, but recent studies have begun to shed light on those mysteries.
AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming