Carbon isotopic composition of shale gas in the Silurian Longmaxi Formation of the Changning area, Sichuan Basin

Abstract

A comprehensive analysis was carried out on the geochemical characteristics of 15 shale gas samples from the Changning area to study the carbon isotopic composition features of shale gas and the reversal cause of carbon isotopic composition in post-mature shale gas in the Silurian Longmaxi Formation of the Changning area, Sichuan Basin. Through the analysis of alkane gas component and carbon isotopic composition, combining with the research on carbon isotopic composition from the Longmaxi Formation of the Fuling and Weiyuan areas in Sichuan Basin, the methane from the Longmaxi Formation shale gas accounts for 97.11% to 99.45%, the average gas wetness is 0.49% representing typical dry gas, abnormal average δ13C1 value as −28.2‰ and the average of δ13C2 values is −33.2‰, in view of sapropel-type kerogen, the Longmaxi Formation shale gas belongs to the typical oil-associated gas. With the increasing degree of thermal evolution, the wetness of shale gas decreases gradually, and carbon isotopic composition of methane becomes heavier, and the carbon isotopic composition of ethane and propane will reverse, but the carbon isotopic composition of ethane and propane in the post-mature shale gas of the Changning area stays in the post-stage of reverse and will not get continuously heavier. The abnormal heavy carbon isotopic composition of methane and the reversal phenomenon of carbon isotopic (δ13C1>δ13C2>δ13C3) mainly generate from the secondary cracking effect in the post-mature stage and reactions between ethane with ferrous metals and water under Reileigh fractionation situation. Furthermore, the high temperature is also one of the important influence factors. Key words: Sichuan Basin, Changning area, Silurian Longmaxi Formation, shale gas, carbon isotope