Compound Clinoform Deltas with Tidal Influence in Mixed Siliciclastic–Carbonate Systems in the Cisco Group (Pennsylvanian), Eastern Shelf of the Permian Basin, U.S.A.

Deltas in the Cisco Group contain significant volumes of potential reservoir sandstones in the mixed siliciclastic–carbonate succession in the Eastern Shelf of the Permian Basin. We aim to discern the primary processes shaping these deltas and their influence on sandstone geometry. Through analysis of 290 m [952 ft] of core and correlation of +500 well-logs, we identify six (6) facies associations related to tide-influenced deltaic systems in highstand intervals of the proximal Eastern Shelf: lower-delta-plain/embayed tidal flat, distributary channel and distributary mouth bar deposits on the subaerial delta; subaqueous delta platform, subaqueous delta front, and prodelta deposits on the subaqueous delta. Moreover, two (2) additional facies associations, lag deposits and subtidal shallow-water carbonates, record continuous transgressions. Well-log cross sections reveal repetitive stacking of deltas, typical of icehouse systems, and sandstone mapping suggests elongated tidal bars forming the interpreted subaqueous delta platforms. Results suggest a compound clinoform morphology of the Cisco Group deltas akin to modern tide-influenced deltas such as Southeast Asia. The interaction of carbonates with the deltaic system entails reciprocal sedimentation and lateral facies variation. This compound clinoform morphology has implications for the elongate geometry and isolation of sandstone reservoirs, impacting fluid migration and storage. Moreover, these insights extend beyond the Cisco Group to other Pennsylvanian and Permian levels on the Eastern Shelf, as well as to other deltas in mixed siliciclastic–carbonate systems in icehouse conditions.