Geochemistry of the extremely high thermal maturity Longmaxi shale gas, southern Sichuan Basin

Abstract

The marine shale of the Silurian Longmaxi Formation in southern Sichuan Basin, China is characterized by its huge thickness (100–600m), high TOC values (0.35–18.4%), favorable types of organic matter (I and II1), high Ro values (1.8–4.2%), abundant brittle minerals (average 56.3%) and strong gas generation intensity. Particularly, its bottom section, which has TOC>2% and thickness of 20–70m, has recently been a main target for shale gas exploration and development. Geochemical parameters of 13 gas samples from Longmaxi shale from 10 wells were analyzed in this study. Wetness of the gas (molar fraction of ethane through pentanes in total gaseous hydrocarbon) is only 0.24–0.70%, making the Longmaxi shale gas among the driest in the world. Carbon isotopic compositions of the alkane gases from Longmaxi shale are characterized by δ13C1>δ13C2>δ13C3, with the most positive δ13C1 values in the world; a positive relationship was observed between its δ13C1 and δ13C2 values reflecting the extremely high thermal maturity of the source rock. δ2HCH4 values range from −140‰ to −151‰ and the δ2HC2H6 values range from −130‰ to −173‰; the hydrogen isotopic compositions were mainly characterized by δ2HCH4<δ2HC2H6. The helium isotopic composition shows R/Ra values range from 0.01–0.03, indicating a crustal origin from uranium and thorium in the source rock. CO2 was most likely derived from the high temperature decomposition of carbonate sediments, but with some organic contribution, since its δ13C values were mainly 5.4‰ to −2.2‰.