Noble gas isotopic variations and geological implication of Longmaxi shale gas in Sichuan Basin, China

The large difference in carbon isotopic compositions of shale gases from the Lower Silurian Longmaxi Formation marine organic-rich shales between Changning and Weiyuan areas in Sichuan Basin, China is a still debate issue. In this study, 30 produced shale gas samples were collected during 3 years production period and analyzed for noble gas (He, Ne and Ar) abundances and isotopic compositions to reveal the origin and variation causes of shale gases. The results show that the Longmaxi shale gases show low 3He/4He and 20Ne/22Ne, and varied but high 21Ne/22Ne and 40Ar/36Ar ratios. The Weiyuan shale gases are characterized by low 3He/4He, high 40Ar/36Ar ratios and varied 4He and 40Ar abundances, and a large increase in 40Ar contents and a slight decrease in 40Ar/36Ar ratios display during 3 years shale gas production period from Jan. 2014 to Jun. 2016. Whereas the Changning shale gases are characterized by low and varied 3He/4He ratios and high 4He and 40Ar abundances, and small fluctuations in 40Ar contents and 40Ar/36Ar ratios exhibit during 3 years production period. In addition, the shale gases produced from horizontal wells in the Weiyuan area possessed higher 40Ar and 40Ar/36Ar ratio than that those from vertical wells at same well site. The noble gas isotopic compositions and variation of Longmaxi shale gases suggested that they are originated mainly from crust, air saturated fluid (ASF) and radiogenic accumulation. The variation of 40Ar and 4He contents during shale gas production mainly reflects the degree of shale gas source supply from mineral matrix to fracture network, and the greater increases in 40Ar and 4He contents with production time show less abundant shale gas supply. Therefore, the increase trend of 40Ar contents with shale gas production time in the Weiyuan area during 3 years production period possibly suggests less adequate shale gas supply. In contrast, the uniform or nearly constant of 40Ar content through production time in the Changing area indicates abundant supply of shale gas sources from mineral matrix, resulting in less production decline. Variation in 4He and 40Ar contents through shale gas production time could be sensitive indicators of abundant or depleted shale gas source supply.