The San Juan Basin is NOT a Model for "Basin-Centered Gas"

James E. Fassett, Independent Geologist, 552 Los Nidos Drive, Santa Fe, NM 87501, phone: 505-983-6011, [email protected] and Bradford C. Boyce, Oso Energy Resources, Inc, 900 Main Aveenue, Suite D, Durango, CO 81301.

In 1979, John Masters of Canadian Hunter Exploration described a radically new trapping mechanism for natural gas in Western Interior basins. He characterized these traps as being: “low porosity-low permeability Cretaceous sandstone, in downdip structural locations, with porous water-filled reservoir rock updip.” In his paper, Masters used the San Juan Basin of New Mexico and Colorado as a model for this kind of trap. Over the last 25 years, other authors have also suggested that the San Juan Basin of New Mexico is a good example of a ‘basin-centered” gas deposit. Most recently, Masters stated in the December issue of the RMAG Outcrop that the San Juan Basin is “an almost perfect basin-centered accumulation . . . the basin syncline [is] rimmed all the way around by water. The water holds that gas in.”

In reality, the three major gas reservoirs in the San Juan Basin; in the Late Cretaceous Dakota Sandstone, Mesaverde Group, and Pictured Cliffs Sandstone are all stratigraphic traps. These rocks units are all tightly cemented, fractured-sandstone reservoirs with a cumulative production of 22 Tcfg. The Dakota Sandstone is the most complex consisting of interbedded and discontinuous marine and continental rocks. The Mesaverde Group consists of a basal regressive shoreface sandstone, a middle continental sandstone and mudstone complex, and an upper transgressive shoreface sandstone. The Pictured Cliffs Sandstone is a regressive shoreface sandstone. Stratigraphic permeability barriers create the traps in all of these rock units.