Through Permian time eastern Colorado was geologically in the northwestern part of the Anadarko basin. The Denver basin and the Las Animas arch are the major geologic features in this area, and both of these attained their present structural configuration during the Laramide uplift (Late Cretaceous through early Eocene). All during Pennsylvanian and Permian times the Las Animas arch, as reflected on the top of the pre-Pennsylvanian unconformable surface, was a south-trending nose in the northwestern part of the Anadarko basin.

Early Mississippian (Osagean) trapping mechanisms are primarily structural and are along the most obvious Early Pennsylvanian regional growth feature. Late Mississippian (Meramec) reservoirs also are structural traps but indicate definite stratigraphic influence. All series of the Pennsylvanian produce, and all of the production is from stratigraphic traps except 2 minor accumulations. Permian production is limited to the west flank of the Denver basin and primarily is trapped structurally.

Before 1965, the pre-Cretaceous well density in most of eastern Colorado was extremely sparse. 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 enabled many previous seismic, structural, and stratigraphic problems to be solved. As the result of the new interpretations of seismic and subsurface control, numerous prospects are being defined and no doubt will result in many pre-Cretaceous discoveries in the near future.

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