Abstract
The Weiyuan (WY) and Changning (CN) fields are the largest shale gas fields in the Sichuan Basin. Though the shale gases in both fields are sourced from the Longmaxi Formation, this study found notable differences between them in molecular composition, carbon isotopic composition, and noble gas abundance and isotopic composition. CO2 (av. 0.52%) and N2 (av. 0.94%) were higher in Weiyuan than in Changning by an average of 0.45% and 0.70%, respectively. The δ13C1 (−26.9% to −29.7%) and δ13C2 (−32.0% to −34.9%) ratios in the Changning shale gases were about 8% and 6% heavier than those in Weiyuan, respectively. Both shale gases had similar 3He/4He ratios but different 40Ar/36Ar ratios. These geochemical differences indicated complex geological conditions and shed light on the evolution of the Lonmaxi shale gas in the Sichuan Basin. In this study, we highlight the possible impacts on the geochemical characteristics of gas due to tectonic activity, thermal evolution, and migration. By combining previous gas geochemical data and the geological background of these natural gas fields, we concluded that four factors account for the differences in the Longmaxi Formation shale gas in the Sichuan Basin: a) A different ratio of oil cracking gas and kerogen cracking gas mixed in the closed system at the high over-mature stage. b) The Longmaxi shales in WY and CN have had differential geothermal histories, especially in terms of the effects from the Emeishan Large Igneous Province (LIP), which have led to the discrepancy in evolution of the shales in the two areas. c) The heterogeneity of the Lower Silurian Longmaxi shales is another important factor, according to the noble gas data. d) Although shale gas is generated in closed systems, natural gas loss throughout geological history cannot be avoided, which also accounts for gas geochemical differences. This research offers some useful information regarding the theory of shale gas generation and evolution.
Highlights
Shale gas is produced from organic-rich black shale and self-generation and self-storage natural gas, and is continuously accumulated in nano-scale micropores in shale [1]
Formation shale gas in the Sichuan Basin: a) A different ratio of oil cracking gas and kerogen cracking gas mixed in the closed system at the high over-mature stage. b) The Longmaxi shales in WY and CN have had differential geothermal histories, especially in terms of the effects from the Emeishan
Large Igneous Province (LIP), which have led to the discrepancy in evolution of the shales in the two areas. c) The heterogeneity of the Lower Silurian Longmaxi shales is another important factor, according to the noble gas data. d) shale gas is generated in closed systems, natural gas loss throughout geological history cannot be avoided, which accounts for gas geochemical differences
Summary
Shale gas is produced from organic-rich black shale and self-generation and self-storage natural gas, and is continuously accumulated in nano-scale micropores in shale [1]. Shale gas geochemical irregularities include (1) the rollover of iso-alkane/normal alkane ratios [12]; (2) the rollover of ethane and propane isotopic compositions [13]; and (3) abnormally light ethane and propane δ13 C values and isotope reversals among methane, ethane, and propane [11,12,14,15,16] Together, these irregularities reflect the complicated history of shale gas generation and the isotopic fractionation associated with it, as well as the in situ “mixing and accumulation” of gases that are generated from different precursors at various thermal maturities [4]. We found that the gas geochemical characteristics of shale gases from the Longmaxi Formation, Sichuan Basin, China, show several apparent differences between the Weiyuan (WY) and Changning (CN) areas [14,15,16,17,21,22,23]. These results should increase our understanding of the generation and evolution of shale gas
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