Geochemical characteristics of Lower Silurian shale gas in the Changning-Zhaotong exploration blocks, southern periphery of the Sichuan Basin.

Abstract Recent significant breakthroughs have been made of the Lower Silurian shale gas in the Changning-Zhaotong blocks recently, the key exploration area of China. Through comparative research of the gas geochemistry, Longmaxi Formation shale gas is mainly composed of methane (98.8%), with extremely low wetness coefficient (0.49%), and possesses negligible quantities of ethane and propane. The Non-hydrocarbon gases are mainly composed of small-scale quantities of N 2 and CO 2 of mixed origin. Enriched δ13C 1 (from −29.4‰ to −26.7‰) and δ13C 2 (from −34.5‰ to −31.6‰) values, together with sapropelic organic matter, indicate a typical overmature oil-type gas. Abnormal geochemical characteristics of shale gas (δ13C 1 > δ13C 2 > δ13C 3) result from merged secondary impacts in the high-maturity and well-confined shale reservoirs, like the mixing of secondary cracking gases, Rayleigh fractionation of C 2 when transition metals and water undergo redox reactions. Notable geographic variation can also be observed, as the δ13C 1 and δ13C 2 values of the Longmaxi Formation become more positive toward the thrust-faulted folds developed in the southern CZ blocks. Highlights • Study presents the systematic analysis of the shale gas geochemistry in the CZ blocks. • Isotopic reversals resulted from secondary effects at high-maturity and are restricted to closed-system shale. • Details of geographic variation of carbon isotope ratios have been discussed. [ABSTRACT FROM AUTHOR]