Objective and Method  The Xujiaweizi Fault Depression in the Songliao Basin contains multiple helium-bearing gas fields. However, the assessment methods for helium resources and helium accumulation patterns in the fault depression are yet to be determined. Using 3D seismic data and logs from 24 wells, this study optimized the helium resource assessment method based on natural gamma-ray spectral logging. Using the optimized method, this study quantitatively assessed the helium-generating potential and theoretical helium production of crust-derived helium in deep strata in the Xujiaweizi Fault Depression. By combining the geochemical characteristics of the helium-bearing gas reservoirs and the simulation results of crust-derived helium, this study explored the charging histories and accumulation patterns of gas reservoirs derived from the crust and from a mixed crust-mantle source in the study area.

Results and Conclusions  The results indicate that the Lower Cretaceous Yingcheng and Shahezi formations and the Jurassic Huoshiling Formation in the study area have helium-generating potential of 1.22×10⁻⁴ cm³/g, 1.08×10⁻⁴ cm³/g, and 0.90×10⁻⁴ cm³/g, respectively. The deep strata in the study area exhibit average helium-generating potential of 0.96× 10⁻⁴ cm³/g and total helium production of 7.4 km³, suggesting a moderate to slightly low helium-generating capacity. Crust-derived helium, generated by the radioactive decay of uranium and thorium in deep strata of the study area, migrated with hydrocarbon gases as carriers. From 95 Ma, such gas migrated on a large scale and then rapidly charged the volcanic and clastic reservoirs in the overlying Yingcheng Formation via fracture systems, contributing to the formation of in situ or proximal helium reservoirs. In contrast, mantle-derived helium migrated with mantle-sourced inorganic gases such as carbon dioxide as carriers. Such gas migrated upward via deep-seated faults, volcanic conduits, and unconformities. These two types of helium finally accumulated in traps, leading to the formation of helium-bearing gas reservoirs. The crust-derived helium reservoirs are governed by the quality of helium source rocks and the hydrocarbon-helium coupling, while helium reservoirs with a mixed crust-mantle source are controlled by mantle-derived fluid activity and deep-seated faults. It is recommended that exploration in basins in eastern China should highlight the charging of mantle-derived helium. Additionally, the future helium exploration in the Xujiaweizi Fault Depression should focus on the supplementation of helium reservoirs with mantle-derived helium.