Sedimentation patterns and main controlling factors of the Weizhou Formation beach-bar deposits on the northern steep slope zone of the Haizhong Depression

Addressing the mismatch between the traditional delta model and new well evidence from the northern steep slope of the Haizhong Depression, this study centers on Wells H3X and H301 and integrates core observations, well logs, seismic facies, and grain-size data, with additional constraints from provenance, paleogeomorphology, and paleowater depth, to reconstruct a beach bar-dominated system and evaluate its controls. Results show that the Weizhou Formation commonly develops interval sand–mud thin interbeds, with thick box/bell-shaped channel sands absent; log responses are dominated by high-frequency finger- and funnel-shaped motifs; seismic facies are characterized by moderate-low amplitudes, short-axis discontinuity, and chaotic/hummock-like reflections—diagnostic of beach bar deposits. Combining diagnostic sedimentary structures and grain-size evidence, the system is subdivided into alongshore, nearshore, and storm-influenced offshore beach bars. Provenance derives mainly from wave-reworked, far-traveled sands of the braided-delta front in the Weizhou Oilfield to the north, supplemented by basement-derived detritus from the Weixinan low uplift. During deposition, the Hai-3 and Hai-4 structures formed subaqueous low-relief highs; although Fault 3 segmented the north-south depositional pattern, cross-fault sediment supply persisted, and these lows acted as sand unload/accumulation sites. Joint paleodepth inversion (paleontology, wave-ripple parameters, and facies-thickness stacking) indicates alongshore beach bars at ～0–3 m water depth, nearshore beach bars at ～3–8 m, and offshore beach bars at ～8–11 m. We propose a “source-paleohigh-paleowater depth” co-control model for beach bar deposition, providing a basis for beach bar-type reservoir identification, key-control analysis, and map-scale prediction in the Haizhong Depression, with applicability to analogous lacustrine rift basins.