Sedimentation and controlling factors on beach-bar facies of Weizhou Formation in northern steep slope zone of Haizhong Sag

To address the mismatch between the traditional delta model and new well evidence from the northern steep slope of the Haizhong Sag in Weizhou Formation, Wells H3X and H301 were analyzed by combining core observation, well logging, seismographic facies, and grain-size distribution, with additional constraints from provenance, paleogeomorphology, and paleowater depth, based on which a beach bar-dominated system was reconstructed and its controlling factors were evaluated. Results show that the Weizhou Formation develops vast interval sand-mud thin interbeds with no thick box/bell-shaped channel sands. Log responses are dominated by high-frequency finger- and funnel-shaped superimposition motifs. Seismic facies are characterized by moderate-low amplitudes, short-axis discontinuity, and chaotic/hummock-like reflections. All above-mentioned descriptions are diagnostic of beach bar deposits. By combining diagnostic sedimentary structures and grain-size distribution patterns, we subdivided the system into alongshore, nearshore, and storm-influenced offshore beach bars. In addition, we found that material sources were derived mainly from wave-reworked and far-traveled sands of the braided-delta front in the Weizhou Oilfield to the north, and supplemented by basement-derived detritus from the Weixinan low uplift. During deposition time, the Hai-3 and Hai-4 structures formed subaqueous low-relief highs. Although Fault #3 separated the north-south depositional realm, cross-fault sediment supply persisted, in which low uplifts became sand accumulation sites. Meanwhile, joint paleodepth inversion based on paleontology, wave-ripple parameters, and facies-thickness stacking indicated that alongshore beach bars were formed at ~0–3 m water depth, nearshore beach bars at ~3–8 m, and offshore beach bars at ~8–11 m. Therefore, we proposed a “source–paleo-uplift–paleodepth” co-control model for beach bar deposition, provided a basis for beach bar-type reservoir identification, key-controller identification, and horizontal prediction in the Haizhong Sag, and promoted the applicability to similar lacustrine rift basins.