Origin and carbon isotope reversal of shale gas in Wufeng-Longmaxi formations, Sichuan Basin: implication from pyrolysis experiments

Abstract

The Wufeng-Longmaxi shale in the Sichuan Basin is the only stratum in China to realize commercial development of shale gas. There are some differences in shale gas generation and a lack of direct experimental evidence of the reasons for these differences. Hydrocarbon generation experiments using pyrolysis in a gold tube with low and medium maturity shale and graptolite of O3-S1 were carried out. Gas from remaining oil produced by hydrogen-rich and lipid-rich organic matter such as planktonic algae is the main contributor of shale gas in the Wufeng-Longmaxi formations. The oil generation ability of graptolite is poor, and it can produce gas during the high-maturity and over-mature stages, up to about 20% of the hydrogen-rich and lipid-rich organic matter such as algae. The lower layer of the Wufeng-Longmaxi shale in the Sichuan Basin is superior to the upper layer, which is mainly composed of graptolite, because of abundant hydrogen-rich and lipid-rich organic matter such as phytoplankton. Pure thermal evolution fractionation and the mixture of primary (from kerogen) and secondary (from oil) gas do not cause hydrocarbon gas isotopes to "reverse". Formation uplift, formation water, minerals, metals and other late transformation of hydrocarbons in shale gas may be an important cause of shale gas isotopic "inversion". Carbon isotope "inversion" may be more of a reflection of the later shale gas preservation process.

Keywords

• shale gas genesis
• isotope reversal
• wufeng-longmaxi formations
• pyrolysis experiment
• sichuan basin