Molecular and carbon isotopic variation in 3.5 years shale gas production from Longmaxi Formation in Sichuan Basin, China

Abstract

A time series of produced shale gas samples from Lower Silurian Longmaxi Formation (Fm.) were collected from six production wells during 3.5 years production period from the Weiyuan and Changning shale gas pilots in Sichuan basin, China. The molecular compositions and carbon isotopes of CH4, C2H6, C3H8 and CO2, and the hydrogen isotope of methane were analyzed for these samples by an integrated method of gas chromatography combined with mass spectrometry. The results showed that the produced shale gases are dominated by CH4 (90.1–99.3%) with minor C2H6 (0.3–0.8%) and non-hydrocarbon gases, and have low wetness (0.3–0.8%). The non-hydrocarbon gases are mainly CO2 and N2, and trace amount of He. The Longmaxi shale gases display completed reversal pattern of carbon isotopic compositions with carbon number (i.e., δ13C1 > δ13C2 > δ13C3) except for Wells W201 and W201-H1 with normal patterns between C2H6 and C3H8 (i.e., δ13C2 < δ13C3), and it is a mixture of oil cracking gas and kerogen cracking gas at high-over level of thermal maturity. Some CO2 (δ13CCO2 ranging from −2.5 to −6.0‰) is mainly derived from the decomposition of carbonate minerals.CH4 concentration and the carbon isotopes of methane and ethane during 3.5 years shale gas production from year 2012 to 2016 displayed small fluctuations, which possibly suggested that gas supply from shale matrix to fracture network after hydraulic fracturing is abundant. The Weiyuan shale gases show larger decrease of CH4 content than the Changning shale gases, indicated more adequate supply in the Changning area. The difference in δ13C1 values between horizontal and vertical wells on the same drilling site in the Weiyuan pilot indicated that the vertical heterogeneity of Lower Silurian Longmaxi shale plays an important role to shale gas production. Interestingly, the shale gases in the Weiyuan pilot display much lighter δ13C1 (−34.46 to −36.82‰) and δ13C2 (−37.59 to −41.89‰) values than those (δ13C1 = −27.18–−27.27‰, δ13C2 = −33.70–−34.14‰) in the Changning pilot. The difference in carbon isotopic compositions in Weiyuan and Changning pilots mainly attributes to mixing between varied proportion of oil cracking gas and kerogen cracking gas because of the regional heterogeneity of the Longmaxi marine shale, in particular amount of residual oil retained in organic matter rich shale. The isotopic difference between Weiyuan and Changning shale gases doesn't disappear in the period about 3.5 years of shale gas development, which further suggests the regional heterogeneity of retained residual oil exists in the Longmaxi Fm. marine shale.