Abstract
This paper focuses on Longmaxi shale gas geochemistry and carbon isotopic reversal in Changning and Fuling gas fields through comparative study of shale gas composition and carbon and hydrogen isotopes in North America and Changning and Fuling gas fields. Longmaxi shale gas in Changning and Fuling gas fields exhibits the features of dry gas. Specifically, the average methane (CH4) content is 98.72 and 98.17%, respectively. The humidity is less than 0.5%. Nonhydrocarbon gases include a small amount of CO2 and N2. Extremely heavy δ13C1 value, average δ13C2 value of −33.3 and −34.6‰ for Changning and Fuling, and sapropelic organic matter indicate the properties of petroliferous dry gas. Carbon isotopic reversal, i.e. δ13C1>δ13C2>δ13C3, may be caused by combined secondary effects at high maturity and high geotemperature. The reversal may also be related to ethane Rayleigh fractionation and late methane generation by water and transition metals reaction. Geologic setting in these two gas fields may have an impact on carbon isotopes distribution.
Highlights
Shale gas exploration and development in China concentrate in the Sichuan Basin, where some shale gas fields including Changning and Fuling have been discovered
Shale gas deliverability in Changning, Weiyuan, and Zhaotong reached 26 Â 108 m3/a by April 2015 (Dong et al, 2014; Zhou and Jiang, 2015). Shale gas in both gas fields is produced from high-quality marine shales in the Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation, which are promising shale beds with large area, constant thickness, and high content of organic matter and brittle minerals (Dai et al, 2014a; Dong et al, 2014; Wang et al, 2012)
Data collected include the latest test data published for the Fuling gas field (Dai et al, 2014b, 2016c; Liu, 2015) and geochemical parameters of typical shale gas inside and outside China
Summary
The d13C1 value of Fuling shale gas ranges between À28.4 and À32.3%, with an average of À30.3%, which is close to that (À31.2%) of the Horn River shale (Tilley and Muehlenbachs, 2013), heavier than that (À38.2%) of the high- to postmature Fayetteville shale (Zumberge et al, 2012), and remarkably heavier than that (À41.6%) of the mature Barnett shale (Hill et al, 2007; Rodriguez and Philp, 2010; Zumberge et al, 2012) and that (À52.6%) of the low-mature New Albany shale (Strapocet al., 2010). The Longmaxi shale gas in the Changning and Fuling gas fields exhibits some abnormal carbon and hydrogen isotopic compositions.
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