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

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 CO2 of mixed origin. Enriched delta C-13(1) (from - 29.4 parts per thousand to - 26.7 parts per thousand) and delta C-13(2) (from - 34.5 parts per thousand to -31.6 parts per thousand) values, together with sapropelic organic matter, indicate a typical overmature oil-type gas. Abnormal geochemical characteristics of shale gas (delta C-13(1) > delta C-13(2) > delta C-13(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 delta C-13(1) and delta C-13(2) values of the Longmaxi Formation become more positive toward the thrust-faulted folds developed in the southern CZ blocks.