Carbonate Slope Detrital Aprons and Fans: A Potential New Play Type in the Pennsylvanian Paradox Formation, Paradox Basin, Southeastern Utah and Southwestern Colorado, U.S.A.

Abstract

The Pennsylvanian (Desmoinesian) Paradox Formation is a major producer of oil in the Paradox Basin of southeastern Utah and southwestern Colorado. The basin hosts Utah’s largest field, Greater Aneth, which has produced over 483 MMBO since its discovery in 1956. Paradox deposition was cyclic consisting of clastic-carbonate intervals typically containing interbedded limestone, dolomite, and anhydrite, as well as black, organic-rich shale that serves as the source for hydrocarbons in the basin. Most fields are located in the Blanding sub-basin and Aneth platform in the central and southwest parts of the Paradox Basin, respectively. The principal reservoirs are phylloid-algal carbonate mounds or buildups and associated heterolithic oolitic, peloidal, and bioclastic deposits.

Recent coring along the western edge of Greater Aneth field, as well as re-evaluation of several cores off the Aneth platform, has revealed packages of very porous (mostly preserved interparticle porosity), chaotic, carbonate rubble and detritus. These discontinuous units of highly variable thickness were likely shed from “rimmed” parts of the Aneth platform as well as isolated buildups. Unlike the traditional phylloid-algal mound and oolitic shoal reservoirs linked to shallow platforms, the chaotic detrital packages are composed of coral/sponge/bryozoan blocks, poorly sorted skeletal calcarenites, and angular lithoclasts. As mounds and buildups grew during sea-level highstands, detrital skeletal material was eroded and deposited as dipping beds along the exterior flanks and within interior lagoons. During sea-level lowstands, the mounds and detrital aprons experienced considerable porosity modification including dolomitization. Examples of small detrital aprons and fans are also recognized in Paradox Formation outcrops along the San Juan River to the west of producing areas.

Detrital material shed downslope of carbonate buildups in the Paradox Formation could contain untested petroleum traps where reservoir-quality porosity and permeability has developed. If these types of deposits are in communication with buildup and oolitic reservoirs, they could also serve as conduits facilitating sweep efficiency in secondary/tertiary recovery projects.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018