THE EAGLE BASIN: A COMPARTMENTALIZED EVAPORITE BASIN AND A HYDROCARBON EXPLORATION FRONTIER

The Eagle Basin, a Pennsylvanian (predominantly Desmoinesian) aged evaporite basin located in northwestern Colorado, contains carbonate rocks, which are exploration targets for oil and gas reserves. During Pennsylvanian time the Eagle basin was part of the Central Colorado Trough, which was bounded by ancestral mountain-range uplifts. Our primary area of investigation focuses on Moffat and Rio Blanco Counties.

The thick sequence of Desmoinesian, cyclically deposited evaporites, carbonates, and black shales, commonly referred to as the Minturn Formation and its equivalents, bears a striking resemblance to the coeval Paradox Formation of the Paradox basin. Normal marine and penesaline facies are found flanking hypersaline facies in both basins.

Geologists have assumed that the northwest portion of the Central Colorado Trough is one evaporite depocenter. Using gamma-ray and electric log signatures and subsurface rock samples, we subdivided the Minturn into individual rock packages. The Eagle basin is here interpreted to be a series of four or five smaller basins, each basin having served as a center for halite accumulation. We suggest that halite depocenters might have been structurally controlled by a tensional style of faulting, and that halite depocenters were located in down dropped grabens. Later, during the Laramide Orogeny, some preexisting normal faults were reactivated by compressional forces into thrust faults. Lithologically, halite subbasin margins comprise attractive hydrocarbon exploration targets. Fault related fracturing enhances reservoir permeability.