GEOLOGIC CONTROLS ON STRATAL ARCHITECTURE AND REGIONAL SEDIMENT DISTRIBUTION IN THE CISCO GROUP, EASTERN SHELF OF THE PERMIAN BASIN

Coastal-plain, incised-valley, shelf, and shelf-edge depositional facies in the Virgilian-Wolfcampian Cisco Group in a ~12,000 mi² (~31,000 km²) area in the southern part of the Eastern Shelf of the Permian Basin record a significantly increased sediment supply coupled with rejuvenation of the Ouachita Fold Belt. As a result, extensive lower Cisco (Virgilian) fluvial-deltaic systems built out westward across the Eastern Shelf and the Canyon carbonate deposits, producing a progradational succession of regionally interstratified and cyclic delta and carbonate shelf systems deposited throughout the Virgilian and Wolfcampian Series.

The Cisco section consists of a cyclic series of thirteen mudrock, limestone, and sandstone facies (top of the Home Creek to top Coleman Junction Limestone). It forms a progradational succession from the Eastern Shelf (Bunger Limestone) to the deeper part of the Midland Basin (Coleman Junction Limestone). The top of the Home Creek Limestone coincides with a regional downlap surface for progradational lower Cisco shelf strata.

The Cisco Group, which produces oil gas in Lake Trammel South field in Nolan County, forms a 1,000- to 1,700-ft (300- to 520-m) clinoformal succession of slope-channel deposits that lap onto and overtop aggradational carbonate-reef and mound deposits of the underlying Canyon Group. Reservoir sandstone bodies in this slope succession occur in narrow (commonly 500 to 3,000 ft [150 to 900 m] across), sinuous and anastomosing channel trends. Distribution of successive slope-channel trends in the field are dominantly controlled by autocyclicity. Progressive upward decrease in height of shelf-margin clinoforms indicates that accommodation decreased in the upper Cisco Group.

Our investigation shows that concurrent accelerated subsidence of the Midland Basin produced as much as 1,800 to 2,000 ft (~550 to 610 m) of depositional relief between Cisco shelf edges and equivalent basin-floor deposits of the Midland Basin. During deposition of the uppermost Cisco Group (upper Wolfcampian), sediment supply from the Ouachita fold belt again decreased, and thick, low-relief limestone shelf-edge banks became increasingly prominent.