During the past 2 years, a detailed analysis of the southern flank of the San Juan basin, northwestern New Mexico, has been made. This area probably has been one of the most neglected and yet one of the most promising basin areas remaining in the Rocky Mountain region for the discovery of Cretaceous and major Pennsylvanian reserves at reasonable depths.

The author believes that the "Gallup" production within the study area is related primarily to an offshore marine facies and a possible unconformity above the lower massive Gallup equivalents. Several of the productive sandstone bodies appear to be channel fills on this unconformity. The lower massive Gallup sandstone beds southwest of the study area are believed to be the source for the sandstone in the younger Hospah-type channel fills.

As shown by discrete mapping of Graneros-Dakota reservoirs, this section is a highly prospective interval for petroleum accumulation. Many of the sandstone developments and pinch-outs mapped have had significant oil or gas shows in downdip wells. The updip evaluation of these shows has not been accomplished to date by the industry. Even within the Dakota-Graneros productive areas at the north, development and extension have not been realized fully.

Production from the Pennsylvanian in the Four Corners area is associated with shelf, shelf-edge, or a high-energy-zone type of deposition. Similar environmental conditions were present along the southern margin of the San Juan basin, and conditions favorable to the development of bioherm reefs, biostromes, and massive beach sand bodies, all potential major petroleum reservoirs, existed. The significant factors indicative of environmental conditions along the southern margin of this basin are: (1) presence of a continental facies barrier in wells on the North Chaco slope; the section consists chiefly of red, maroon, and variegated shale with tight red and gray, thin-bedded limestone; (2) the presence of a shelf-carbonate facies, potentially prospective for biostromal development, sands one pinch-outs, and truncation traps; these facies have been found in the few basinward wells tion of a high-energy shelf-carbonate zone, indicated in the West Cuba area, where an unusually thick, porous dolomite section may be associated with possible reefing; and (4) the presence of a deep-water marine downdip from the continental facies; (3) the sugges-facies consisting of hard, dense, tight limestone, with dark gray to black shale, and very fine-grained sandstone and siltstone penetrated in the deeper part of the basin.

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