--> The San Juan Basin is NOT a Model for "Basin-Centered Gas", by James E. Fassett and Bradford C. Boyce,#10093 (2005)

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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



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.


Structure map of San Juan Basin, on Huerfanito Bentonite Bed (in Upper Cretaceous Lewis Shale).


Structural profiles (top Pictured Cliffs Sandstone [P.C.] and Huerfanito Bentonite Bed [H.B.B.]), San Juan Basin.



Roche, Pat, 2004, Are tight gas resources overstated?: Outcrop (Rocky Mountain Association of Geologists), v. 53, no. 12, p. 1, 6-10.