Geochemical Characteristics and Significance of Residual Gases in Shale and Granite

Shale and granite are the most potential helium source rocks; however, the distinctions in geochemical characteristics and the origin of residual gas remain unclear, which restricts the exploration and development of helium resources. A total of 11 rock samples (six shales and five granites) were collected from the Yangtze region for the analysis of the chemical composition, noble gas component, and isotope ratio of rock residual gas, as well as the mineral composition, major and trace elements of rock samples. The total absolute content of residual gas in shale is much higher than that in granite. The residual gas is mainly composed of N₂, CH₄, and CO₂ in shale and granite. The noble gas constitution of the residual gas within shale and granite is ⁴⁰Ar > ⁴He > ²⁰Ne. The helium present in the residual gas of shale and granite is predominantly of crustal and radiogenic origin. The discrepancy between the measured neon isotope ratio and the corresponding air value is likely to be primarily attributed to mass fractionation and the addition of radioactive ²¹Ne. The ⁴⁰Ar/³⁶Ar ratio significantly diverges from the corresponding ratio of the atmosphere, with the superfluous ⁴⁰Ar stemming from the radioactive disintegration of K. ⁴He and ²⁰Ne are concurrently displaced and accumulated in conjunction with groundwater within the formation. ⁴He and ⁴⁰Ar* are generated in greater quantities; however, a relatively smaller proportion is currently retained in rocks. The retention coefficients P-(⁴He) and P-(⁴⁰Ar*) of shale are both lower than those of granite, which may mainly be related to the fact that the porosity of granite is lower than that of shale. The P-(⁴⁰Ar*) in shale and granite is higher than P-(⁴He), which also proves that the sealing temperature of Ar is higher than that of He. The emission of helium and argon is primarily dictated by temperature.