THE DESMOINESIAN STRATIGRAPHY OF THE SOUTHERN DENVER BASIN BASED ON RECENT DRILLING

Historically, the Desmoinesian strata in the Denver basin have generally been ignored by the petroleum industry and academia. The Desmoinesian strata of Pennsylvanian age in the southern Denver Basin contain numerous thin organically rich mudstones (one to eight feet thick) that are interbedded with carbonates or non-organic shales. These mudstones are generating hydrocarbons and at least within the Cherokee are charging adjacent thin carbonate reservoirs.

The Atoka mudstones average 10% TOC, predominantly terrestrial in origin, and yield a high pour point 33^o to 38^oAPI gravity crude. These mudstones have been identified in literature as Atokan in age but recently palynological evidence suggests they are of Cherokee age.

The Cherokee mudstones are marine in origin, average 11% TOC, and yield a low pour point 35^o to 41^oAPI gravity crude. The thin limestone and dolomites in the Cherokee are known as “A”, “B” and “C” zones are porous and highly fractured and are being charged with petroleum from the adjacent mudstones.

The Desmoinesian strata interfingers with the Fountain Formation to the west which represents alluvial fans shed off the ancestral Rockies. The Atoka sediments represent a lacustrine, closed restricted environment. Coaly material is occasionally seen in association with these mudstones. The Cherokee strata represents open marine. All of the mudstones can be mapped across the southern part of the basin. The Desmoinesian represents a series of environments that have not been well studied. This presentation will present new data that better defines these strata in context of deposition during Pennsylvanian time in the Denver Basin.