Quaternary Evaporative Facies Interpretation by Seismic Sedimentology in Biogas-Bearing Sanhu Depression, Qaidam Basin, NW China

S-wave seismic data are unaffected by natural gas trapped in strata, making them a valuable tool for seismic sedimentology. In this study, S-wave seismic data were utilized to construct an isochronous framework and analyze evaporative facies in the Quaternary biogenic gas-bearing Taidong area, Sanhu Depression, Qaidam Basin, NW China, with calibration from wireline logs and modern analogs. Techniques of phase rotation, frequency decomposition, R (Red), G (Green), B (Blue) fusion, and stratal slices were integrated to reconstruct seismic geomorphological features within the isochronous framework. Linear and sub-circular morphologies, resembling those observed in modern saline pans such as Lake Chad, were identified. Observations from Upper Pleistocene outcrops of anhydrite and halite at Yanshan, east of the Taidong area, salinity measurements from Well TX1 to the north, and lithological and paleo-environmental records from boreholes SG-5, SG-1, and SG-1b to the northwest support the seismic findings. The RGB-fused slices generated from the S-wave seismic data in Taidong area indicate a progressive increase in the occurrence of evaporative features from the K2 standard zone upwards. The vertical occurrence of evaporative facies in the Taidong area mirrors the contemporary regional and global paleo-environmental changes. The interpretation of Quaternary stratal slices from seismic sedimentology—spanning K2, K1, and K0 standard zones—reveals a transition from a freshwater lake to brackish, saline, and finally, a dry saline pan, overlaid by silt. This analysis provides valuable insights into locating evaporites as cap rocks for biogenic gas accumulation and also into mining the evaporite resources in shallow layers of the Taidong area.