Pennsylvanian Evaporite-Carbonate Cycles and Their Relation to Petroleum Occurrence, Southern Rocky Mountains

Abstract

The Paradox and Eagle basins of southeastern Utah and western Colorado contain a thick series of cyclic evaporite deposits of Pennsylvanian (Desmoinesian) age. In response to marked salinity gradients, halite and potash were deposited in the deeper parts of these basins, whereas on the basin margins, or shelves, carbonates were the predominant facies. Thus each evaporite cycle of the inner basin has a shelf counterpart in the carbonate facies. Each cycle has its own particular facies pattern, which is a reflection of biologic and chemical response to changes in sea level. Within the shelf carbonate facies some cycles contain lens-shaped buildups of biogenic carbonates. These carbonate mounds form the productive reservoir rocks on the southwest shelf of the Paradox basin. imilar, but as yet unproductive and relatively untested, carbonate mounds are present in the Eagle basin. Most of the petroleum production in the Paradox basin is from porosity development associated with moundlike buildups of algal and leached oolite limestones. Aneth, the major field on the southwest shelf of the basin, contains an estimated 300 million bbl of oil. Some production comes from the inner-basin evaporite facies from thin beds of intensely fractured black shale and dolomite. The Paradox basin is only one of the many areas of the world where petroleum resources are associated with evaporites. A close appraisal suggests that the evaporite environment plays a direct role in generation and accumulation of petroleum. Barred basins, which probably have contributed the greatest volume of marine evaporites, not only concentrate oceanic salts but also form an efficient trap for organic matter. In addition, much of this organic matter is in solution, which may facilitate its conversion to petroleum. Evaporation losses in a large basin create a strong basinward flow of nutrient-rich water. If this water moves across a shallow shelf, it may stimulate the activities of the shelf's biologic community and accelerate production of organic matter. Anoxic conditions within the evaporite basin, caused by low solubility of oxygen in high-salinity brines, combined with high levels of H2S produced by biogenic sulfate reduction, retard the decay process and allow preservation of most of the organic matter swept in from the shelf.