Haian Depression is one of the key areas for oil and gas resource replacement in Jiangsu Oilfield. Since the 13th cycle of the Five Year Plan, with the continuous improvement in the exploration level of the Taizhou Formation (K₂t), the difficulty of tapping potential has gradually increased. It is urgent to change our thinking and expand new exploration layers. From the perspective of oil and gas display frequency in different layers of the Haian Depression, except for K₂t, the oil and gas systems with the Fusan Member (E₁f₃) as the main reservoir have good oil and gas display frequency, demonstrating great exploration potential. This study of sedimentary characteristics is the basis of analyzing the sedimentary environment and lithofacies paleogeographic conditions and is of great significance for determining the distribution range of subtle oil and gas reservoirs. Based on this understanding, this study was specially established to systematically analyze the logging curves of forty-three wells in the research area, combined with core observations of eighteen coring wells and the analysis of eight seismic profiles. The results show that the low slope, warm and humid climate, sufficient provenance, and frequent lake level rise and fall cycles during the deposition period of the E₁f₃ member of the Haian Sag provide a favorable depositional background for the development of shallow water delta in the study area. There are many gullies in the research area, mainly consisting of U-shaped gullies and W-shaped gullies. Slope breaks are mainly affected by structural factors leading to fractures, and the types are mostly fault terrbreakslope breaks. In the study area, the shallow water delta deposits during the deposition period of the four key sand groups in the Fu3 Formation are dominated by the shallow water delta front and shallow water prodelta. The shallow water delta plain subfacies are not significantly developed because of erosion. The sand bodies are mainly distributed in the Sunjiawa Subdepression, and the Fuan Subdepression in the north of the depression, and the sand bodies in the plane show the filling characteristics of the strip. Based on the above research, a sedimentary model of shallow water delta during the E₁f₃ section of the Haian Depression was established, providing a geological basis for the design of exploration and development plans for hidden oil and gas reservoirs in the next step.

Shallow-water deltas are not only a hot spot for sedimentological research but also a key target for oil and gas exploration. In this paper, taking the third member (E₁f₃) of the Funing Formation in the Upper Jurassic as an example, based on observations made from core samples, well logging, cathode luminescence characteristics, and analytical assays, the development conditions, sedimentary characteristics, and sedimentary models of shallow-water deltas are summarized. These shallow-water deltas were deposited in conditions with the following characteristics: a gentle terrain platform, a subtropical climate with ample rainfall, an abundant source supply, strong hydrodynamic forces, shallow water bodies, and a frequently eustatic lake level. Shallow-water deltas are characterized by sediment deposition from traction currents, numerous underwater distributary channel scour structures, overlapping scouring structures, sand body distribution with planar features, underwater distributary channels as skeletal sand bodies, and undeveloped mouth bars. Based on these, it is believed that during the deposition period of E₁f₃, the Gaoyou Sag in the Subei Basin had favorable geological conditions for the development of shallow-water delta deposition. The shallow-water delta deposition that occurred during the sedimentary periods of the five major sand units in the Funing Formation is characterized by front subfacies, with underwater distributary channels as the framework for sand bodies, and multiple intermittent positive rhythms overlapping vertically with the Jianhu Uplift as the source of material supply. In this paper, a depositional model for shallow-water delta deposition during the E₁f₃ deposition period in the Gaoyou Sag is established, expanding the scope of oil reservoir exploration in the north slope region of the Gaoyou Sag and providing important geological evidence for the selection of favorable subtle zones.