Geochemical characteristics and origin of Sinian-Lower Paleozoic natural gas in the Sichuan Basin

Abstract

The composition and isotopic values of natural gas in the Sinian Dengying Fm, Cambrian Qiongzhusi Fm, Longwangmiao Fm and Xixiangchi Fm in the Sichuan Basin exhibit different features in different tectonic position, causing controversy over its origin and sources. The geochemical features of the natural gases in this area were compared based on previous research results and large amounts of drilling data of new exploration wells. The results show: (1) The natural gas is typical dry gas on the whole, with hydrocarbon gas in dominance, and a methane content of 74.85%–97.35%, mostly 83.0%–96.0%; due to different clay contents in source rocks, the difference in non-hydrocarbon gas mainly lies in N2 and He content, with gas in Weiyuan and Ziyang areas having high contents of N2 and He, while gas in Gaoshiti and Moxi areas having low N2 and He. (2) Natural gases in different regions differ greatly in carbon isotope value (δ13C1 and δ13C2). The δ13C1 of natural gas in Cambrian system of Ziyang area is the lightest, from −38.0‰ to −35.5‰, and that in other regions is from −33.9‰ to −32.0‰, indicating different capture stages of natural gases. For example, gas captured in early stage is relatively light in carbon isotope; the carbon isotope value (δ13C2) of gas from Sinian and Cambrian system in Weiyuan is from −36.5‰ to −32.7‰; that from the Longwangmiao Fm in Gaoshiti and Moxi is from −33.6‰ to −31.8‰, but that in the Dengying Fm in Gaoshiti-Moxi area is from −29.1‰ to −26.8‰, quite different from the above natural gases, which mainly shows the differences in kerogen types. (3) Light hydrocarbons of C6–C7 mainly consist of cyclanes and isomerization alkanes, which is the feature of oil cracking gas.