Subtle Stratigraphic Traps in Paleozoic Rocks of Paradox Basin

Abstract

Significant quantities of petroleum occur in stratigraphic traps of Devonian, Mississippian, and Pennsylvanian ages in the Paradox basin. Devonian reservoirs are isolated marine sandstone bodies, but the Mississippian and Pennsylvanian traps are biohermal carbonates. Exploration has proven the reservoirs to be elusive and relatively unpredictable, but new realization that these subtle traps are localized on paleostructures has simplified exploration and led to several recent discoveries. The tectonic framework of the Paradox basin was set by the end of the Precambrian, but rejuvenation through the Paleozoic caused repeated vertical movements along basement fractures, sufficient to alter sedimentary facies during Cambrian, Devonian, and Mississippian times. Subsurface data indicate that both major and minor structures at the surface today existed in Middle Pennsylvanian time, and grew throughout Permian time. These elements served to localize Paleozoic reservoir facies by creating shoaling conditions on the paleoseafloor that produced Devonian offshore sandbars, Mississippian crinoid banks, and Middle Pennsylvanian algal bioherms. Erosion and salt movement affected much of the potential for continuous reservoir facies, and today's reservoirs are considered to be randomly distributed stratigraphic traps. However, there is ample evidence to indicate that the sedimentary growth of bioherms was localized over gentle, basement-controlled paleostructures. Whereas the reservoirs are too thin for modern seismic detection, isopach thins of underlying evaporites are evident, indicating paleostructure and the development of algal mounds along the crests and flanks of positive structures. Thus, the mapping of paleostructure, not Laramide structure, is the key to further exploration in this basin.