Throughout Permian time, eastern Colorado, like western Kansas, was on the shelf area of the Anadarko basin; and the Las Animas arch, as reflected on top of the pre-Pennsylvanian unconformity, was a south-trending nose in the north part of the Anadarko basin. This relation was very similar to that of the Central Kansas uplift, from which over 2 billion bbl of oil has been produced from pre-Cretaceous rocks. Many other geologic similarities can be discerned between the Las Animas arch and the Central Kansas uplift. The two most dissimilar factors are (1) that the pre-Pennsylvanian unconformable surface on the Central Kansas uplift is underlain chiefly by Cambro-Ordovician rocks, whereas on the Las Animas arch, Mississippian rocks underlie most of the area; and (2) that the dip of the pre-Permian beds in northeastern Colorado was reversed during the Laramide orogeny.

The entire pre-Pennsylvanian sequence in eastern Colorado and western Kansas demonstrates a very subnormal but continuous pressure system. Over most of eastern Colorado, the Mississippian is the porous and permeable stratigraphic unit of this very low-pressure system. Fluids will migrate from the higher energy Pennsylvanian system to the lower energy Mississippian. Thus, the Mississippian is capable of deriving hydrocarbons transversely as well as laterally.

Since 1965, exploratory interest has been primarily in Mississippian reservoirs, although some Pennsylvanian oil and gas fields also have been found. Since 1965, 20 Mississippian and Pennsylvanian oil and gas fields have been found and over 200 wildcat wells have been drilled. The data from these wells have resulted in new interpretations of hydrocarbon accumulations. Early Pennsylvanian growth strongly influenced Mississippian accumulations, and recent refinements in seismic techniques have allowed definition of these growth features. By incorporating recent subsurface interpretations with the extensive seismic control available, numerous prospects are being defined and doubtless will result in additional pre-Cretaceous discoveries.

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