Geochemical characteristics of ultra-deep natural gas in the Sichuan Basin, SW China

• Published in: Petroleum exploration and development Vol. 45; no. 4; pp. 619 - 628
• Main Authors: DAI, Jinxing, NI, Yunyan, QIN, Shengfei, HUANG, Shipeng, PENG, Weilong, HAN, Wenxue
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-08-2018; KeAi Communications Co., Ltd
• Subjects: coal-derived gas; geochemistry; H2S; origin; Sichuan Basin; ultra-deep gas; Sichuan Basin; H2S; ultra-deep gas; geochemistry; coal-derived gas; origin
• ISSN: 1876-3804; 1876-3804
• DOI: 10.1016/S1876-3804(18)30067-3

The natural gas components and geochemistry of 38 ultra-deep gas wells (burial depth greater than 6 000 m) in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The ultra-deep natural gas components of the basin have the following characteristics: Methane has an absolute advantage, which can be up to 99.56% with an average of 86.6%; ethane is low, with an average of 0.13%; there is nearly no propane and butane. So it is dry gas at over-mature thermal stage. The content of H2S can be up to 25.21%, with an average of 5.45%. The alkane gas isotopes are: the carbon isotope varies from −32.3‰ to −26.7‰ for methane and from −32.9‰ to −22.1‰ for ethane. There is nearly no carbon isotopic reversal among methane and its homologues. Hydrogen isotope varies from −156‰ to −114‰ for methane, and from −103‰ to −89‰ for some ethane. The carbon isotope of CO2 varies from −17.2‰ to 1.9‰ and most of them fall within the range of 0±3‰. According to the δ13C1-δ13C2-δ13C3 plot, except some wells, all other ultra-deep gas wells are dominated by coal-derived gas. Based on the CO2 origin distinguishing plot and δ13CCO2, except some individual wells, most of the ultra-deep CO2 are of carbonate metamorphic origin. H2S in the ultra-deep layer of Longgang and Yuanba gas fields belongs to thermochemical sulfate reduction (TSR), while H2S from Well Shuangtan belongs to thermal Geochemical characteristics of ultra-deep natural gas in the decomposition of sulfides (TDS).