This paper presents new findings from oil and gas exploration engineering.

Objective Geological investigations of petroleum potential, supported by the China Geological Survey (CGS), suggest that the Upper Triassic black shales represent the most significant source rock interval in the Qiangtang Basin. However, the organic geochemical characteristics and hydrocarbon generation potential of these source rocks in the deep basin are still under ongoing research due to the lack of deep drilling activities. Research methods The QZ−16 well, located in the eastern part of the North Qiangtang Depression, has encountered the deepest Upper Triassic strata in the basin to date, revealing significant gas anomalies and a substantial amount of bitumen. This provides a new opportunity to enhance the understanding of deep basin source rocks. Here, organic geochemical analyses of the Upper Triassic black shales were conducted to investigate the organic matter quality, quantity, and levels of thermal maturity as well as the oil−source relationship between the source rock and bituminous oil.

Results Results show that the TOC content of the Upper Triassic black shales in the QZ−16 well is generally poor to fair organic matter richness, ranging from 0.12−1.09 %, with an average of 0.47 %. These values are higher than the average TOC limit (0.3%) of over−mature source rocks with type II kerogen. The degree of thermal maturity is suggested to be in the over−mature stage based on significantly high Tmax (536−602 ℃) and vitrinite reflectance values (Ro = 2.44%−2.77%). The chloroform asphalt A and hydrocarbon generation potential (S₁+S₂) may not reliably reflect the organic matter abundance due to the source rocks being in the over−mature stage. Kerogen type index (8.75−18.5), Ph/nC₁₈ (0.65−1.06), Pr/nC₁₇ (0.34−0.61) and C₂₇−C₂₈−C₂₉ sterane characteristics indicate a mixed organic matter source comprising of lower plankton and higher terrestrial plants of Type II₂ kerogen. Most source rock intervals were deposited under strongly reducing conditions, exhibiting a brown−black kerogen color, which align with biomarker parameters, indicating that the thermal evolution of the black shale is over mature. The oil source correlation parameters reveal a strong affinity between the Triassic bituminous oil seedling and black shale in the QZ−16 well, ng that the oil is self−generated and self−stored.

Conclusion The results of this study provide a new reference for evaluating source rocks and analyzing hydrocarbon generation potential in the Qiangtang Basin.