Subtle Stratigraphic Traps in Paleozoic Rocks of Paradox Basin: ABSTRACT

Abstract

Significant quantities of petroleum occur in stratigraphic traps of Devonian, Mississippian, and Pennsylvanian age in the Paradox basin. Devonian reservoirs are isolated marine sandstone bodies; the Mississippian and Pennsylvanian traps are biohermal carbonates. Exploration in the past has proven the reservoirs to be elusive and relatively unpredictable, but the realization that the subtle traps are localized on paleostructures simplifies exploration and has led to several recent discoveries. The tectonic framework of the Paradox basin, which includes a northwesterly series of major basement rifts and a subordinate series of northeast-trending fractures, was already set by late Precambrian. The basin was repeatedly rejuvenated throughout the Paleozoic. Vertical movements along the basement fractures were sufficient to alter sedimentary facies during Cambrian, Late Devonian, and Mississippian throughout the basin. These Paleozoic elements served to localize reservoir facies by creating shoaling conditions that produced Devonian offshore sandbars, Mississippian crinoid banks, and Middle Pennsylvanian algal bioherms. Algal bioherms grew over subtle paleostructures along the southern and western margins of the Paradox basin in Middle End_Page 892------------------------------ Pennsylvanian, providing reservoirs in numerous oil fields such as Aneth and Cache-Ismay. Although these are generally considered to be randomly distributed stratigraphic traps, there is ample evidence that sedimentary growth of the bioherms was localized on very gentle, basement-controlled paleostructures. Although the reservoirs are generally too thin for recognition on present-day seismic sections, seismic mapping of paleostructure, not Laramide structure, is the key to further exploration of the lucrative basin. End_of_Article - Last_Page 893------------