Biomarkers in source rocks are crucial in uncovering biological source and depositional environment. However, the detection of critical biomarkers such as steranes and terpanes in high-maturity source rocks is often challenging. On one hand, biomarkers in high-maturity source rocks are inherently present at low concentrations; on the other hand, the detection of these low-concentration biomarkers can be hampered by the high concentrations of interfering compounds, such as n-alkanes and other branched/cyclic alkanes. In this study, we developed a sample pretreatment protocol of stepwise extraction, using the combination of solvent in varying polarities (i.e., n-hexane and dichloromethane) and samples of different sizes (i.e., blocks >3 cm, 2.5–5 mesh, 10–20 mesh). The specific procedures are as follows: (1) removing the high amounts of interfering compounds during the first-step extraction to increase the relative proportion of biomarkers in saturated fraction by extracting coarse samples with relatively weak-polar solvents; (2) analyzing the soluble organic matter extracted in the second step by Gas chromatography-mass spectrometry (GC-MS) for biomarker (i.e., steranes and terpanes) detection by extracting samples of 100–200 mesh with solvents of stronger polarity. This method was demonstrated using organic-rich shales with high maturity (R_o = 1.42%) from the Qingshankou Formation (Fm.) in the Gulong Sag, and it was validated using the medium maturity (R_o = 1.0%) shales from the Qingshankou Fm., Sanzhao Sag, Songliao Basin. The results show that under routine Soxhlet extraction combining GC-MS analysis, sterane and terpane biomarkers in the Qingshankou shales are unidentifiable, regardless of whether urea adduction is applied or not. In contrast, these biomarkers can be successfully identified utilizing stepwise extraction coupled with GC-MS analysis. The effectiveness of biomarker identification is influenced by the interplay of both the shale particle size and organic solvent polarity. The integration of biomarker indexes was then employed to interpret the biological source and the depositional environment of the organic matter, providing detailed insights of the hydrocarbon generation potential of shales. This information can further provide additional guidance for selecting favorable resource zones for shale oil exploration in the Gulong Sag, Songliao Basin.

Occurrence and mobility of shale oil are prerequisites for evaluating shale oil reserves and prioritizing exploration targets, particularly for heterogeneous lacustrine shales. The Qingshankou Formation in the Gulong Sag, Songliao Basin is a classic lacustrine pure shale reservoir that contains abundant shale oil resources. The predicted geological reserves of the shale are 1.268 × 10⁹ t. In this study, field emission scanning electron microscope (FE-SEM), the modular automated processing system (MAPS), pyrolysis-gas chromatography (Py-GC), low-pressure nitrogen gas adsorption (LPNA), Soxhlet extraction, pyrolysis, and 2-D nuclear magnetic resonance (NMR) were integrated to describe the shale oil components, microscopic occurrence, mobility, and the effective pore size distribution. Meanwhile, the related controlling factors are discussed.

The shale oil in the Qingshankou Fm exists dominantly in the matrix pores of the clay minerals, with small amounts distributed in the intergranular pores of terrigenous clastic grains, intercrystalline pores of pyrite, intragranular pores of ostracod shells, and micro-fractures. Shale oil is distributed in the pore spaces of variable sizes in different lithofacies. The clay mineral-laminated shales are characterized by the broadest range of pore size and largest volume of pore spaces with shale oil distribution, while the ostracod-laminated shales have limited pore spaces retaining oil. Furthermore, the proposed integrated analysis evaluates the shale oil molecules existing in two states: movable, and adsorbed oil, respectively. The result illustrates that movable oil takes up 30.6%–79.4% of the total residual oil. TOC, mineral composition, and pore structures of the shale joint together to control the states and mobility of the shale oil. TOC values are positively correlated with the quantities of shale oil regardless of the state of oil. The mineral components significantly impact the state of shale oil. Noticeable differences in the states of oil were observed following the changing types of minerals, possibly due to their difference in adsorption capacity and wettability. Clay minerals attract more adsorbed oil than movable oil. Felsic minerals generally decrease the occurrence of total and adsorbed oil. Carbonate plays a positive role in hydrocarbon retention of all the shale oil states. As for the pore structure, the average pore size exerts a critical impact on the total, movable, and adsorbed oil content. The total pore volume and specific surface area of shales play a principal role in controlling the total yields and amounts of adsorbed oil. This research improves the understanding of the occurrence characteristics and enrichment mechanisms of shale oil in terrestrial pure shales and provides a reference for locating favorable shale oil exploration areas.