Controls on facies variability and distribution during the Pennsylvanian glacial period from the lower Strawn Group, Fort Worth basin, Texas

The lower Strawn Group (Desmoinesian) in Jack and Wise Counties of the Fort Worth basin, Texas, comprises laterally and vertically heterogeneous deltaic to shelfal deposits that include sandstone, siltstone, and shale, with thin, discontinuous carbonate and coal seams that reflect variable icehouse condition controls on sedimentation and facies distribution. This study highlights the relationships between component facies of the lower Strawn Group by defining five distinct depositional environments in core, develops a high-resolution stratigraphic framework consisting of 11 regressive-transgressive cycles, and identifies depositional con-trols on stratal characteristics to improve reservoir predictability. Depositional environments of the lower Strawn Group in-clude prodelta, medial delta front, interdistributary bays, chan-nel-mouth bars, and distributary channels. These environments, their wire-line log patterns, and facies relationships indicate dominantly fluvial-driven deltaic depositional systems. Wire-line log correlations using regionally extensive maximum flooding surfaces provide the basis of a stratigraphic framework identify-ing 11 regressive-transgressive, fluvio-deltaic sequences each between 90 and 240 ft (27 and 73 m) thick, with an overall thickness ranging between 1000 and 2700 ft (305 and 823 m), and progradation of the deltas to the south-southwest. The depocenters of these sequences are similar in size and geome-tries to modern analogues that include select parts of the fluvio-deltaic systems of the Mississippi and Yukon Rivers, with primary depocenters ranging from 8 to 25 mi (13-40 km) in lateral extent. This study provides improved predictability for targeting hydrocarbons in the Strawn Group and analogous systems in the Fort Worth basin through high-resolution map-ping and identification of elevated porosity trends in homoge-neous channel-mouth bar and distributary channel deposits.