## 6 Conclusions  
1. The fourth member is dominated by shore-shallow lake subfacies deposits, with locally developed delta-front subfacies. In the early stage, under the control of the northwestern provenance, small-scale delta depositional systems developed, characterized by subaqueous distributary channel sands. Frequent lake-level fluctuations facilitated the reworking of sands by lake waves, forming stable sand bars nearly parallel to the lake shoreline. The planar distribution of these sand bars was jointly controlled by paleotectonic patterns and lake-level fluctuations, exhibiting strip-shaped or lenticular geometries. In the late stage, as lake levels declined, the delta deposition expanded, with subaqueous distributary channels extending southward and south-eastward. Concurrently, new delta deposits emerged in the central-eastern part of the study area.
2. The formation of sand bodies in shore-shallow lake-delta facies is closely related to hydrodynamic conditions at the lake basin margin. Deltaic sand bodies are prone to reworking by lake waves and litteral currents, forming strip-shaped sand bar deposits distributed along the lake shoreline. This depositional model not only reveals the influence mechanisms of physical processes on sediment distribution in shore-shallow lake environments but also provides crucial reference cases for understanding sand body formation and distribution in analogous delta-lactustrine systems along foreland basin margins worldwide, particularly in environmental settings subjected to multiple lake transgression and regression cycles.  
Early-stage sand bars along the shoreline were formed through reworking and redistribution by lake waves, exhibiting well-sorted grains and high porosity and permeability. Late-stage subaqueous distributary channel sands in the delta front experienced stacked development, resulting in greater thickness. Sand bodies pinch out into mudstones, forming lithologic traps that provide favorable conditions for hydrocarbon accumulation.
4. The NE-SW-trending major faults in the study area connect with Paleozoic hydrocarbon source rocks, forming efficient migration pathways. Combined with lithologic traps formed by sand pinch-outs, these features establish a \"fault-controlled lithologic composite\" hydrocarbon accumulation model, demonstrating significant exploration potential. However, the pervasive NE-SW-oriented secondary short-offset faults within the study area compartmentalize sand bodies, leading to complex internal reservoir architecture. Future research should prioritize unraveling the internal structure and heterogeneity of the fourth-member reservoirs, as these factors are critical for optimizing hydrocarbon exploration strategies.  
Conceptualization, Z.L. and Y.Y.; methodology, L.W.; formal analysis, H.W.; investigation, Z.L.; resources, Y.Y.; data curation, Q.L.; writing--original draft preparation, Z.L.; writing--review and editing, Y.Y.; supervision, L.W.; project administration, H.W.; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Hunan Provincial Natural Science Foundation of China, grant number 2023J30239, and the Scientific Research Fund of Hunan Provincial Education Department, grant number 23B0467.  
Not applicable.  
All data and materials are available on request from the corresponding author. The data are not publicly available due to ongoing research using a part of the data.  
The Northwest China Petroleum Bureau, SINOPEC, is thanked for providing access to their geological data and permitting the publication of the results.  
The authors declare no conflicts of interest.