GEOCHEMISTRY OF THE DEEPST SHALE GAS FROM SICHUAN BASIN, CHINA

Shale gas is a kind of unconventional gas, for which shale is both source and reservoir. Shale gas production has increased rapidly in the last decade. In 2018, the annual shale gas production was 6245×10⁸m³ in the United State, which accounted for 64.72% of the total annual gas production. In China, great success has been made for the shale gas exploration. In 2018, the annual shale gas production reached 108×10⁸m³, and 898 wells have been drilled.

In the last several years, abnormal geochemical characters have been found for the shale gas, such as the carbon isotope rollover of ethane and propane, or even carbon isotopic reversal. A number of studies have focused on the cause of the carbon isotopic reversal, however, the mechanism is still poorly understood.

Due to the remarkable differences of the geological conditions of shale basins between China and USA, shale gas in China demonstrates its unique characteristics. Shale gas from the Silurian Longmaxi Formation in the Sichuan Basin show extremely high thermal maturity, carbon isotopic reversal between ethane and propane, but normal carbon isotopic composition of ethane and propane toward wetness.

In order to further investigate the geochemical characters of shale gas in Sichuan Basin, we collected shale gas samples with burial depth over 5000 m from the Longmaxi Formation in the Sichuan Basin, China, which have been the deepest burial depth of shale gas. The results show, these shale gases are dominated by methane, with minor amount of C₂₊ hydrocarbons and the gas dryness coefficient is generally >99%, indicating the shale gas belongs to dry gas with high thermal maturity. The carbon isotope is around −35‰ for methane and it is characterized by a carbon isotopic reversal between ethane and methane, i.e., δ¹³C₁>δ¹³C₂. Compared to the shale gas from the same Longmaxi Formation but with much shallower burial depth (mostly 2000~3000 m), and shale gas from Barnett, Fayettville and Western Canadian basins, it is concluded that, shale gas from the Longmaxi Formation with burial depth over 5000 m is similar to those with shallower burial depth from the same formation. However, the deep shale gas has increasing δ¹³C of ethane with increasing gas wetness, which is opposite to the shallow one. The cause for the abnormal geochemical characteristics is further investigated, which may provide a hint for the future shale gas research and exploration.