Hydrocarbon generation potential and organic carbon enrichment in evaporitic lacustrine source rocks from the Qianjiang Formation, Jianghan Basin (Central China): An integrated geochemical-petrological investigation

Abstract

Continental hypersaline lacustrine basins constitute critical repositories of unconventional shale oil resources, often hosting source rocks with primary exploration value. The hydrocarbon generation potential and organic matter accumulation within the Qianjiang Formation of Jianghan Basin, however, remain elucidated inadequately. This study targets the third member of the Qianjiang Formation (Eq³) through systematic analysis of dolomitic shale sequences from the QYP1 directional well in Qianjiang Depression. A comprehensive analytical approach is employed, integrating programmed open-system pyrolysis, major and trace element geochemistry, biomarker analysis, and organic petrology. The multidisciplinary framework is utilized to evaluate sediment provenance, organic matter supply, paleoenvironmental reconstruction (including paleoclimate, redox condition, and paleosalinity) during the Eq³ depositional period, and the thermal maturity of organic matter. Based on these influencing factors, the hydrocarbon generation potential and organic matter enrichment model for source rocks of the Eq³ interval are assessed.

Results demonstrate that the organic-rich sediments of the Eq³ shales in the Qianjiang Depression exhibit exceptional hydrocarbon potential, qualifying as prime targets for shale oil exploration. Key characteristics include elevated total organic carbon (TOC), elevated S₁ + S₂ values, and superior hydrogen index (HI). Primary source of organic matter is attributed to saline lacustrine algae, as evidenced by a strong linear correlation between liptinite and TOC content and high proportion of lamalginite. The analyzed organic matter exhibits thermal maturity levels ranging from immature to low-mature level. Organic matter enrichment in these evaporative lacustrine source rocks is controlled by a combination of factors, including paleoclimate, sediment supply, organic matter sources, and depositional conditions. Paleoclimate reconstructions indicate that the Eq³ interval was deposited under cold and arid conditions, with low Ga/Rb, SiO₂ and high K₂O/Al₂O₃, Al₂O₃ + K₂O + Na₂O, accompanied by weak chemical weathering with minimal sediment recycling. Depositional environment demonstrates strongly reducing conditions and exceptional organic matter preservation, as indicated by low Pr/Ph ratios, exceptionally high Gamm/C₃₀ (βα + αβ) hopane indices, rominent extended tricyclic terpanes ratios (ETR) and widespread pyrite occurrence. Elevated gammacerane, β-carotane content, along with high Sr/Ba, B/Ga and DBT/P radios, suggest high paleosalinity. Water column stratification further enhanced paleoproductivity and provided optimal conditions for organic matter preservation. Hydrocarbon-generating organic matter is predominantly derived from aquatic algae and halotolerant bacteria, with negligible contributions from terrestrial higher plants. Laminated algal mats dominate the organic composition, indicating extensive preservation of algae. This study is designed to elucidate the mechanisms governing organic matter enrichment in evaporative lacustrine basins and provides a robust foundation for shale oil exploration in the Qianjiang Depression.