Organic geochemical characteristics and origin of natural gas in the Sinian-Lower Paleozoic reservoirs, Sichuan Basin

The composition and isotopic values of natural gas in the Sinian Dengying Fm and Cambrian Qongzhusi Formation, Longwangmiao Formation, Xixiangchi Fm in the Sichuan Basin exhibit unique features in different types of structures. The understandings of its origin and sources are multiple which is unfavorable for the decision making. For this purpose, the comparative study in geochemistry on natural gases composition, the isotope values and light hydrocarbon composition have been developed based on the previous research results. The natural gas is typical dry gas as a whole, which is dominated by hydrocarbon gas. The value of methane is 74.85-97.35%, ranging from 83.0-96.0% mostly. The contents of non-hydrocarbon gas show some difference in the respects of N2 and He. The contents of N2 and He are relatively high in Weiyuan and Ziyang areas, while they are relatively low in the regions of Gaoshiti, Moxi. The reason of this is related to clay contents in source rocks. The carbon isotope values (δ13C1 and δ13C2) of natural gases in different regions differ greatly. The Ziyang Fm from the Cambrian system is the lightest with the δ13C1 from -38.0 to -35.5‰. The number of δ13C1 in other regions is from -33.9 to -32.0‰. According to the research work, the divergence of δ13C1 reflects the different staged captures of natural gases which captured from early stage are relatively light. The carbon isotope values (δ13C2) from Sinian and Cambrian system in Weiyuan is from -36.5 to -32.7‰; those from Longwangmiao Fm in Gaoshiti and Moxi is from -33.6 to -31.8‰. But it is -29.1--26.8‰ in the value of δ13C2 from Dengying Fm in Gaoshiti-Moxi presenting the dramatic differences of the δ13C2 from above areas which can be resulted from source rocks. Light hydrocarbon composition of C6-C7 mainly consists of cyclanes and isomerization alkanes which is characterized by crude oil cracking gas.