The Potential of the Oldest Shale Oil in China: Mesoproterozoic Hongshuizhuang and Xiamaling Shales in the Yanshan Basin

Shale oil exploration and development have made significant breakthroughs in Paleozoic marine shales and Mesozoic–Cenozoic lacustrine shales. However, the shale oil potential of older Precambrian sediments remains poorly constrained. Our two newly drilled boreholes reveal shale oil shows in the Mesoproterozoic Hongshuizhuang and Xiamaling shales in the Yanshan Basin, China. In order to further evaluate the shale oil potential, multiple experimental methods were employed, including TOC and thermal maturity analyses, routine and step-by-step Rock-Eval pyrolyses, X-ray diffraction, field emission scanning electron microscopy, low-temperature nitrogen adsorption, and hydrocarbon vapor adsorption. The results show that Hongshuizhuang shale, which is characterized by type II kerogen and low-medium maturity of R₀ = 0.65–0.92%, is a set of good source rocks, while the source rock quality of Xiamaling shale is highly variable. Moreover, the hydrocarbon generation potential of prokaryote-dominated Mesoproterozoic source rocks is comparable to that of eukaryote-dominated Phanerozoic source rocks, implying a favorable factor for shale oil accumulation in Precambrian formations. The main storage space for shale oil in both formations is provided by inorganic pores, in which the adsorbed oil is predominantly retained in micropores to fine mesopores (<15 nm) and free oil mainly stored in mesopores to macropores. Adsorbed oil exists as a multilayer oil film, with its average thickness increasing as a Langmuir adsorption curve with pore size. The evaporative hydrocarbon content of the Hongshuizhuang Formation ranges from 2.53 to 12.53 mg/g and is dominated by adsorbed hydrocarbons (mean 6.01 ± 2.10 mg/g), while the evaporative hydrocarbon content of Xiamaling Formation is much lower, with an average value of 2.4 ± 2.4 mg/g. TOC content, mineral composition, thermal maturity, pore volume and surface area are important factors affecting the degree of shale oil enrichment. The compilation and comparison show that the Hongshuizhuang Formation is a promising target for the oldest shale oil exploration. Furthermore, the results suggest that the combination of step-by-step pyrolysis and hydrocarbon vapor adsorption is a practical method for estimating both free and adsorbed hydrocarbon contents and revealing their storage space within shale nanopores.