Abstract

The Chaco Slope is defined as the gently dipping, southern margin of the San Juan Basin, bounded by the Zuni Mountains on the south, by the Nacimiento Mountains on the east, by the Defiance uplift on the west, and by the “main gas producing trend” of the San Juan Basin on the north. Approximately 6,800 square miles fall within the arbitrary boundaries.

The Lower Cretaceous sandstone sequence or Dakota Formation, for the purpose of this paper, is defined as that unit of sandstone and shale 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 lower Upper Cretaceous age based on paleontological 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 to 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 indicate 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.