Geochemical characteristics and hydrocarbon generation potential evaluation of Cretaceous source rocks in Junggar Basin

To provide a basis for investigating the resource potential and hydrocarbon accumulation patterns of the Cretaceous source rocks in the Junggar Basin, organic petrology, detailed geochemical characterization, and closed-system thermal simulation technology were employed. The study analyzed the geochemical characteristics and hydrocarbon generation potential of Cretaceous source rocks in different blocks of the basin. These source rocks showed strong heterogeneity and generally of low quality. However, high-quality source rocks were also present. Type Ⅰ and Type Ⅱ₁ source rocks accounted for 39.6% of the total, and source rocks with total organic carbon (TOC) content > 1.0% accounted for 17.5%. In the Shawan, Manas and Hutubi area, located in the middle section of the southern margin of the basin, the Cretaceous source rocks had the largest sedimentary thickness, with dark mudstone thickness reaching up to 574 m and burial depth generally exceeding 6 000 m. The organic matter was mainly Type Ⅱ (89.1%), with 8.7% of the source rocks having TOC > 1.0%. Vitrinite reflectance (R_o) values ranged 0.82% to 1.01%, indicating that the source rocks were currently at the peak stage of oil generation. The source rocks had Pr/Ph values of 0.31 to 1.20 and gammacerane/C₃₁ hopane ratios of 0.46 to 8.12, with dominant abundances of C₂₇ and C₂₉ regular steranes. C₂₇, C₂₈, and C₂₉ regular steranes showed V-shaped distributions, indicating deposition in a strongly reducing, saline lacustrine environment. Aquatic algae were well-developed, and organic macerals developed lamalginite that emitted strong yellow fluorescence, indicating strong oil generation capacity. Cretaceous Type Ⅰ and Ⅱ₁ source rocks had maximum oil generation up to 660.0 mg/g and 284.0 mg/g, respectively, with peak oil generation at R_o = 1.0%. Comprehensive study indicates that in the Shawan, Manas and Hutubi area, located in the middle section of the southern margin of the Junggar Basin, the Cretaceous source rocks are thick, rich in oil-generating lamalginite, currently at the peak stage of oil generation, and possess high oil generation capacity, making the area the most favorable target for Cretaceous-sourced oil generation.