The Chaco slope is the gently dipping, southern margin of the San Juan basin, bounded by the Zuni Mountains on the south, the Nacimiento Mountains on the east, the Defiance uplift on the west, and the "main gas producing trend" of the San Juan basin on the north. Approximately 6,800 sq mi fall within these arbitrary boundaries.

The Lower Cretaceous sandstone sequence, or Dakota Formation, for the purpose of this paper, is defined as that sandstone and shale unit occurring beneath the base of the Greenhorn Limestone and above the top of the Jurassic Morrison Formation. It includes, among the more important locally named units, the Graneros sandstone and shale interval, the Tres Hermanos sandstone, the Burro Canyon sandstone, and the Twowells sandstone. The Dakota of the Chaco slope is generally considered to be of early Late Cretaceous age based on paleontologic evidence.

Dakota deposition on the Chaco slope can be subdivided into four regional sandstone units and three regional shale units. The depositional environments of the sandstone units grade from fluvial in the west to deltaic and marine in the east. Accumulation of these sandstones occurred during stillstands of the northeast- to southwest-transgressing Dakota shorelines.

Oil and gas production has been established in the deltaic (pointbar) and/or marine facies at Hospah, Lone Pine, Stoney Butte, Snake Eyes, Red Mountain, Five Lakes, and Crosswise areas. The significant producing areas at the present time are attributed to a combination of structural and stratigraphic conditions. Recent photogeologic-geomorphic mapping indicates surface manifestation of these features. To date, approximately 250 Dakota wildcats have been drilled across the subject area.

The hydrocarbon potential of the Dakota Formation on the Chaco slope is analogous with the Muddy sandstone of the Powder River basin and the "D" and "J" sandstones of the Denver basin.

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