Genetic type and maturity of Lower Paleozoic marine hydrocarbon gases in the eastern Tarim Basin

Hydrocarbon compositions and their carbon isotopic data for 94 natural gas samples were combined with the results of thermal simulation experiments to complete a comprehensive study of maturity, genetic types and sources of Lower Paleozoic marine hydrocarbon gases from the eastern Tarim Basin. All of the Lower Paleozoic gases are relatively dry (dryness coefficient >0.9) and belong to the marine sapropelic type of cracked gases. These gases are characterized by low δ13C1 and low Δ(δ13C2−δ13C1) values due to a lack of terrestrial source input. Thermal simulation experiments on Upper Proterozoic to Lower Paleozoic source rocks resulted in a δ13C1–Ro regression equation that was used to discriminate the maturities and sources of the natural gases. These natural gases are divided into four genetic types: (1) humic, coal-derived pyrolytic gas; (2) lacustrine sapropelic oil-cracking gas; (3) the marine sapropelic, highly mature oil-cracking gas; and (4) overmature oil-cracking gas. Types (1) and (2) were mainly generated from the Upper Triassic-Middle and Lower Jurassic; types (3) and (4) are mainly generated from the Lower Paleozoic. The data can be used to describe multiple sources and multiple generating stages of natural gas.