Microfacies and Facies Differentiation of the Late Ediacaran Dengying Formation on the Northern Slope of the Central Sichuan Paleo-Uplift, Southwest China

The Late Ediacaran Dengying Formation in the Sichuan Basin is a key target for deep-gas exploration, yet the controls on facies differentiation in the Penglai area, on the northern slope of the Central Sichuan Paleo-Uplift, remain debated. This study integrates 3D seismic data, well logs, cores, and thin sections from 20 wells to characterize the microfacies, microfacies associations, and sedimentary architecture of the Dengying Formation. Nine microfacies types are recognized and grouped into three microfacies associations: tidal flat, mound–shoal complex, and inter-mound. The same nine microfacies types occur in both members, but the mound–shoal complex association in the third and fourth members is more grain-rich, more strongly overprinted by recrystallization, and makes up meter-scale shallowing-upward cycles. The lateral variations in microfacies types across the study area are minor; facies differentiation is instead expressed through variations in microfacies associations and their cumulative thicknesses. Facies distribution analysis and 3D seismic paleogeomorphology demonstrate that the Penglai area developed a broad, gently NW-dipping carbonate ramp without distinct slope breaks, in contrast with the previously proposed rimmed platform model of the Gaoshiti–Moxi area. Within this ramp, mound–shoal complexes record large-scale lateral migration driven by high-frequency relative sea-level fluctuations, but their along-strike continuity is constrained by NE-trending basement faults oriented sub-perpendicular to the NW-trending rift axis. A hierarchical three-level control on facies differentiation is proposed: regional paleogeomorphology provides the first-order NW-dipping ramp framework; syn-sedimentary basement faults impose a second-order segmentation; and high-frequency sea-level fluctuations drive the third-order stacking patterns. This hierarchical mechanism refines the sedimentary model of the Dengying Formation and provides a predictive basis for delineating high-quality reservoir targets along the northern slope of the Central Sichuan Paleo-Uplift.