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

James E. Fassett¹ and Bradford C. Boyce²

Search and Discovery Article #10093 (2005)

Posted November 27, 2005

*Oral Presentation at Rocky Mountain Section AAPG Annual Meeting, Jackson, Wyoming, September 24-26, 2005. Appreciation is expressed to Lyn George, Technical Program Chair, and Don French, for encouraging the authors to submit this presentation.

Click to view presentation in PDF format.

¹Independent Geologist, 552 Los Nidos Drive, Santa Fe, NM 87501, phone: 505-983-6011 ([email protected])

²Oso Energy Resources, Inc, 900 Main Avenue, Suite D, Durango, CO 81301

Abstract

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, 2004, 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” (Roche, 2004).

In reality, the three major gas reservoirs in the San Juan Basin; in the Upper 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.