Characteristics and Genetic Mechanisms of High-Sulfur Crude Oil in the Linhe Depression, Hetao Basin

Typical lacustrine high-sulfur crude oils are widespread in the Linhe Depression of the Hetao Basin, yet their genetic mechanisms remain poorly constrained. We applied molecular geochemistry analyses to characterize these oils and elucidate their genesis. The oils are shallowly buried, generally medium to heavy, and contain elevated proportions of nonhydrocarbons and asphaltenes that correlate positively with total sulfur content. GC-MS data indicate low to moderate thermal maturity and deposition in a strongly reducing hypersaline lacustrine setting dominated by aquatic algae with minor terrestrial input. The oils are enriched in aromatic compounds such as methyltrimethyltridecylchromans (MTTCs), dibenzothiophenes (DBTs), and triaromatic steroids (TAS), consistent with hypersaline lacustrine high-sulfur oils. A low geothermal gradient and shallow burial in the Linhe Depression preclude temperatures required for thermochemical sulfate reduction (TSR), and no evidence of biodegradation is present; therefore, secondary alteration can be discounted. The precursor source rocks of high-sulfur crude oils were deposited in a strongly reductive, hypersaline-lacustrine environment under an arid and hot climate. Characterized by the development of gypsum-bearing mudstones, these source rocks exhibit a high reduced sulfur content and widespread framboidal pyrite, signifying an anoxic-euxinic water column during the depositional period. The sulfur content of the crude oil is primarily controlled by the highly saline, reducing depositional environment and significant input of algal organic matter. Steroidal compounds, largely derived from eukaryotes such as algae, underwent sulfurization during the early diagenetic stage within the anoxic hypersaline-lacustrine environment. The abundant algal biomass provided ample precursors for the sulfurization of sterols. While the strongly reducing, sulfur-rich environment not only facilitated the preservation of organic matter but also promoted the incorporation of inorganic sulfur into organic matter, forming organic sulfur compounds that were incorporated into kerogen. Subsequent thermal evolution of this sulfur-rich kerogen led to the generation of primary high-sulfur crude oils.