Stratigraphic framework and incised-valley systems (lower Hope sandstone) of the Upper Pennsylvanian lower Cisco Group, southern Eastern Shelf of the Permian Basin, West Texas
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The lower Cisco Group records the latest Pennsylvanian episode of progradational siliciclastic and aggradational carbonate deposition on the Eastern Shelf of the Permian Basin. This detailed study documents the spatial distribution of depositional systems in four of the six high-frequency cycles that constitute the lower Cisco Group. A dataset of 4,860 wells in an area about 4,900 mi² (12,690 km²) is the basis for detailed correlations and subsurface maps for each cycle. Challenges in the definition and construction of a chronostratigraphic framework based on well-logs have been revisited. The use of the allostratigraphic approach proved to be an important tool for the construction of this stratigraphic framework. Three main depositional systems were the focus of this study: shelf-carbonate, HST delta, and incised-valley systems. Moreover, shelf-edge delta systems have been studied in a general way. Regional cross-sections show varied progradational or aggradational styles for contemporaneous depositional systems. Isochore maps enable the delineation of depositional systems. Furthermore, the recognition of four regional sediment pathways is possible from these maps. Multiple incised-valleys record variation in relative sea-level during the Late Pennsylvanian. This study identifies three fifth-order incised-valley systems, delineates their regional extent, and links them spatially to shelf-edge deltas. These incised-valley systems have the generic name of lower Hope sandstone; however, they have different ages and depositional styles. Four cores and seven thin sections enable characterization of the vertical lithologic succession, mineralogy, and diagenetic framework in these systems. This stratigraphic framework documents the locations of several depositional units of the lower Cisco Group on the Eastern Shelf. Their distribution is not uniform and depends on the amount of sediments supplied through the sediment pathways. Knowledge of these complicated patterns may be applied in the extension of under-developed oilfields and identification of new exploration targets.