Organic geochemistry of Ordovician ultra-deep natural gas in the north Shuntuoguole area, Tarim Basin, NW China: Insights into genetic types, maturity, and sources

As a gas-rich region in the Tarim Basin, the northern Shuntuoguole area (also known as the Shunbei area) is an attractive prospect. Non-etheless, the debate about the origins of these natural gas continues. The analysis on the geological context, natural gas components, and the carbon and hydrogen isotope ratios prove that methane is the predominate component of alkane gases. Alkane gases’ carbon isotope fractionation (δ 13 C 2 < −28‰ and δ 13 C 3 < −25‰) shows that they are oil-associated gas, and their parent material type is I kerogen. Natural gas can be broken down further into three subgroups—Type I 1 , Type I 2 , and Type I 3 . Based on the link between the carbon number 1/n and δ 13 Cn of the gas. Modified plots of lnC 1 /C 2 vs. lnC 2 /C 3 reveal that kerogen cracking is the primary source of natural gas in the Shunbei area, and that this gas is combined with the contribution of oil cracking gas. Petroleum exploration and development in the Shunbei area can be justified on the basis that natural gas in the area originate primarily from Ordovician source rocks, as shown by carbon isotopic compositions.