The sources of soil gases in the Tangshan fault zone, North China, were discussed, based on the soil gas compositions and isotopic ratios obtained by measurement in the field and sample analysis in the laboratory. Soil gas compositions and isotopic ratios indicate that air (A) end-member, limestone (L) end-member, and sediment (S) end-member are the major end-member components contributing to the soil gas in our study area, with fractional contributions in the range of 2-15 vol.%, 23-36 vol.%,

more »

... nd 62-65 vol.%, respectively, to CO2 from the gas wells. According to the relationship among the 3He/4He, average CO2 concentration, and He concentration of soil gas, the deepest depth the fault cut downward and the most developed fractures in the segment where the Heibeiligong (HBLG) well located were inferred, and the shallower depth the fault cut downward and the more developed fractures in the fault segments where the Weifengshan (WFS), Siwangzhuang (SWZ), Tianjingyice (TJYC), and Douhetai (DHT) wells located were inferred. Significant variations in CO2 concentration were observed in soil gases sampled in DHT, HBLG, and WFS soil gas wells in concomitance with a local seismic sequence by 2018 confirming for the first time a possible source of carbon dioxide generated in underlying limestones.