Molecular hydrogen isotopic compositions of petroleum n-alkanes and its geological significance in the Mahu Sag, Junggar Basin, NW China

• Mahu Sag source rocks are classified into P 1 f (saline lacustrine) and P 2 w (freshwater to brackish lacustrine). • δ 13 C and δ 2 H can effectively classify oil type and determine oil sources. • The crude oils are classified into three groups: P 1 f -derived, P 2 w -derived, and mixed oils. • Depositional environment is the primary factor controlling the δ 2 H composition of n -alkanes in oils. • High maturity of the crude oil can affect the hydrogen isotopes of n -alkanes in the oil. The Mahu Sag is the primary petroleum enrichment area in the Junggar Basin, northwestern China, contains two major petroleum source rocks intervals: saline lacustrine deposits of the Lower Permian Fengcheng Formation (P 1 f ) and freshwater to brackish lacustrine deposits of the Middle Permian Lower Wuerhe Formation (P 2 w ). Organic matter from these source rocks exhibits similar carbon isotope and biomarker compositions creating long-standing controversy over oil sources and the relative contribution of each rock. Building on previous studies, we conducted compound-specific isotope of hydrogen and carbon of source rock extracts and crude oils to resolve this uncertainty. Results reveal significant differences in n -alkanes isotope compositions between the two sets of source rock. P 1 f source rock extracts display heavier carbon and hydrogen isotopes of n -alkanes, and the variations of the carbon and hydrogen isotopes of the n -alkanes with different carbon numbers are relatively small. Whereas P 2 w source rock extracts show lighter isotopic compositions of the n -alkanes, and the carbon and hydrogen isotopes of different carbon number alkane molecules vary relatively greatly. Based on hydrogen and carbon isotopic compositions of n -alkanes and isoprenoid abundance, crude oils in the Mahu Sag are classified into three groups. Group I crude oil has heavy hydrogen and carbon isotopes and high isoprenoid abundance, and is derived from the saline lacustrine source rocks of the Fengcheng Formation; Group II oil has light hydrogen and carbon isotopes and low isoprenoid abundance, and is originated from the freshwater-brackish source rocks of the Lower Wuerhe Formation; and Group III oil is the mixture of the Group I oil and the Group II oil. In-depth comparative analysis demonstrates that depositional environment primarily controls n -alkanes hydrogen isotopes composition in organic matter, with saline environments producing heavier values and freshwater-brackish environments producing lighter values. The type of organic matter exerts relatively small influence, while thermal maturity significantly affects the carbon and hydrogen isotopies of n -alkanes only at highly mature and overmature stages. This study provides a robust geochemical method for oil source correlation in geologically complex petroleum systems.