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

The marine shale of the Silurian Longmaxi Formation in southern Sichuan Basin, China is characterized by its huge thickness (100-600 m), high TOC values (0.35-18.4%), favorable types of organic matter (I and II1), high R-o 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 20-70 m, 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 delta C-13(1) > delta C-13(2) > delta C-13(3), with the most positive delta C-13(1) values in the world; a positive relationship was observed between its delta C-13(1) and delta C-13(2) values reflecting the extremely high thermal maturity of the source rock. delta H-2(CH4) values range from -40% to -151% and the delta H-2(C2H6) values range from -130% to -173%; the hydrogen isotopic compositions were mainly characterized by delta H-2(CH4) < delta H-2(C2H6). 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 delta C-13 values were mainly 5.4% to -2.2%. (C) 2014 Elsevier Ltd. All rights reserved.