Cyclic evaporite deposits of Desmoinesian Pennsylvanian age are associated with carbonate-mound petroleum reservoirs in the Paradox and Eagle basins of southeastern Utah and western Colorado. In both basins there are basin-type evaporites associated with starved-basin trough sediments that cover a complete range of evaporitic deposits including hypersaline potash beds, halite, anhydrite, very fine-grained dolomite, and black sapropelic muds. A broad facies belt of petroleum-bearing biogenic carbonates and fine clastics occurs in cyclic association on the mildly tectonic basin shelves. Coarse arkosic facies and narrow belts of biogenic carbonates occupy the tectonically active basin edges adjacent to the Uncompahgre and Front Range highlands of the "Ancestral Rockies." Fiv major juxtaposed evaporite-carbonate mound cycles are mappable in the Paradox basin. Each major cycle contains several subcycles showing different degrees of completeness; individual salt, anhydrite, and black shale beds demonstrate a well-defined microcyclic sedimentation pattern, perhaps related to annual or seasonal climatic changes. The major cycles and most subcycles can be correlated throughout the basin by use of mechanical logs. Microstratigraphic studies of evaporite cores indicate that individual microcycles can be traced for relatively large distances where sufficient cores are available.

The evaporite environment is believed to be directly responsible for the presence of petroleum deposits in the Paradox basin because of its role in the genesis of organic-rich basinwide potential petroleum source beds, improvement of reservoir quality by dolomitization in some places, and the indirect influence of the sapropelic shale-bed geometry in determining size and positioning of carbonate-mound reservoir belts. Conversely, some detrimental effect of the evaporite phase is found in the sealing of some original porosity by later anhydrite infilling.

Most of the petroleum deposits occur in shelf carbonate reservoirs along the southwest flank of the basin. However, probably even greater potential reserves are locked in sapropelic fractured shale beds in the central basin salt anticline area, not now recoverable under the limitations of present exploitation technology.

Evaporite deposits equivalent in age to those of the Paradox basin are present in the Eagle basin of northwestern Colorado. Although thinner than the Paradox evaporites, and more closely interrelated with clastic sediments, these beds occur also in a well-developed cyclic evaporite-black shale depositional sequence. Significant petroleum deposits in facies association with the evaporite section have not yet been found in the Eagle basin, although in most of the basin area drilling density is sparse.

End_of_Article - Last_Page 544------------