Characteristics and Genetic Mechanism of High-Sulfur Crude Oil in the Jinxian Sag: Insights from High-Resolution Mass Spectrometry

High-sulfur crude oils are widely developed in the saline lacustrine facies of the Jinxian Sag, yet their genetic mechanisms remain a subject of long-standing debate. Investigating these mechanisms is crucial for advancing petroleum genesis theories and guiding hydrocarbon exploration strategies. This study employs chromatography–mass spectrometry (GC–MS) in combination with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to analyze high-sulfur crude oil in the Jinxian Sag at the molecular level. The analysis identified heteroatom compounds, including N₁S₁, O₁S₁, O₂S₁, S₁, S₂, and S₃, with S₁ class compounds being the predominant species. Organic sulfur compounds (OSCs) in crude oil have significant geochemical implications for assessing crude oil maturity. It is proposed that the ratios of DBE_5–10/DBE_1–4-S₁ and C_11–20/C_21–45-DBE₃-S₁ can serve as maturity indicators for crude oil. These parameters reflect the aromatization and side-chain cracking processes of OSCs during thermal evolution, providing a potential method for accurately delineating crude oil maturity and assessing highly mature oils. Unlike typical high-sulfur oils derived from thermochemical sulfate reduction, the studied samples lack a predominance of dibenzothiophenes (DBE₉-S₁). Instead, they are enriched in sulfur-containing compounds with biomarker-like carbon skeletons (C_26–30-DBE₅-S₁), which exhibit structural affinities to sterols formed via sulfate assimilation under the microbial reaction and subsequently incorporated into sediments. These compounds show a strong positive correlation with the sulfur content of the crude oils. In reducing environment, a large amount of inorganic sulfur undergoes intramolecular sulfurization with organic matter, primarily composed of sterol-like compounds, forming OSCs that retain the original carbon skeleton of the precursor compounds. These OSCs are subsequently incorporated into the kerogen, which is the primary mechanism for the formation of high-sulfur crude oil in the Jinxian Sag.