Organic geochemistry, petrology, and conventional and unconventional hydrocarbon resource potential of Paleogene saline source rocks in eastern China: The Biyang Sag of the Nanxiang Basin

Basins containing Paleogene organic-rich shales formed in a saline lacustrine environment are widespread in eastern China, and they provide sources of hydrocarbons in numerous petroleum fields. However, research on these basins has been patchy, which limits our understanding of the hydrocarbon source rocks. To address this problem, we selected the relatively understudied Nanxiang Basin as the study area and, more specifically, the third member of the Paleogene Hetaoyuan Formation (Eh₃), which is the main hydrocarbon-generating layer in the Biyang Sag of this basin. In this paper, we report on the organic petrology and geochemistry of Eh₃, and further analyze both its conventional and unconventional hydrocarbon potential. Our results show that the organic matter (OM) within the shales of Eh₃ had bioprecursors that were mainly phytoplankton, bacteria, and benthic algae, with relatively small contributions from animal and higher plant materials. The shales have an average OM abundance (total organic carbon, or TOC) of 2.96 wt%, thus meeting the criterion of a good-quality source rock. Their organic-matter is mainly type I to type II, and in terms of thermal maturity they range from low maturity to mature, thus displaying the characteristics required to generate oil. The results of our molecular geochemical analyses show that the sediments of Eh₃ in the Biyang Sag belong predominantly to deep or semi-deep lake facies, and they were deposited in a reducing environment and may have been affected by a marine incursion. The evidence for this includes Pr/Ph values of <1.0, samples falling in the marine and salt lacustrine regions on the Pr/nC₁₇-Ph/nC₁₈ diagram and values of C₂₆/C₂₅ for the tricyclic terpanes that are <1.3, thereby indicating the influence of sea water. Eh₃ in the Biyang Sag has good potential to be a conventional and unconventional petroleum resource, particularly at the depocenter of the sag. Vertically, the third submember of Eh₃ (Eh₃³) is proven to be the most favorable conventional petroleum resource, whereas the second submember (Eh₃²) is the most favorable unconventional petroleum resource. Therefore, the saline lacustrine sequences are good for both conventional and unconventional petroleum resources.