The Gulong Sag in the Songliao Basin is an important region of China where significant breakthroughs have been achieved in the exploration of medium- and high-maturity lacustrine shale oil. However, the major controlling factors and evolution patterns of shale reservoirs in this sag remain poorly understood. This study focuses on shales in the 1st member of the Qingshankou Formation (also referred to as the Qing 1 Member) in the Gulong Sag. By integrating multiple analytical techniques, including Rock-Eval pyrolysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption, high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR), we systematically analyze the organic geochemical characteristics of the shales, as well as the reservoir space types and their sizes and distribution patterns. The analytical results show that the shales in the sag contain organic matter dominated by high-quality Type Ⅰ kerogen. These shales were deposited in a semi-deep to deep lacustrine anoxic environment and are currently in oil generation state with moderate to high maturity. Reservoir development in the shales is jointly governed by the sedimentary environment, diagenesis, organic matter type, and thermal evolution. Specifically, intense compaction and cementation in the early stage lead to substantial loss of primary pores. With an increase in thermal maturity, the dissolution of minerals such as feldspars, together with the hydrocarbon generation of organic matter, collectively contribute to secondary pore growth. Consequently, reservoir spaces in the shales progressively evolve into an organic-matter- and clay-hosted composite pore system dominated by nano-scale pores. The reservoir space evolution pattern of shales in the Qing 1 Member established in this study provides an important theoretical basis for shale oil exploration in the Gulong Sag. Furthermore, other relevant patterns and research approaches involved in this study may provide references for the exploration of other shale oil plays.

The practice of oil and gas exploration and development shows that the transformation from “outside source” to “inside source” is an inevitable choice for the sustainable development of petroleum industry. Recently, the breakthrough of unconventional oil and gas in semi-deep lacustrine facies shale in the Qingshankou Formation (K₂qn) in the northern Songliao Basin has proved that it has broad resource prospects. The sedimentary paleoenvironment controls the accumulation of organic matter and the distribution of lithofacies, which is the basis for the prediction of shale oil desserts. In this paper, by means of experimental methods of biomarkers and element geochemistry, parameters such as paleoproductivity, paleoreoxidation, and paleosalinity of the lake basin in the northern Songliao Basin were recovered to clarify the paleoclimate evolution characteristics during the formation of Qingshankou Formation, and to compare the paleoenvironment with that of other major shale oil and gas resource enrichment basins in China. The biomarker compounds in the Qingshankou Formation samples predominantly exhibit a unimodal distribution of n-alkanes, with major peaks at nC₁₈, nC₁₉, nC₂₀, and nC₂₁. The Pr/Ph ratio ranges from 0.44 to 1.31, with an average value of 0.87, indicating a general dominance of phytane. Among the major elements, CaO, Na₂O, and P₂O₅ are relatively enriched, while among trace elements, Sr shows the highest enrichment, with Ba, V, Cr, Ni, Cu, Rb, and Y being relatively depleted. The research results indicate that the Songliao Basin developed under warm and humid paleoclimate conditions. Among the sub-basins, the Gulong Sag was relatively more humid compared to the Sanzhao Sag. The lower section of the Qingshankou Formation exhibited warmer and more humid characteristics compared to the middle and upper sections. Influenced by the transgression of the Paleo-Pacific Ocean from the east, the salinity of the lake basin water was relatively high, with a higher degree of salinization observed in the eastern Sanzhao Sag. During the depositional period of the Qingshankou Formation in the northern Songliao Basin, overall paleoproductivity levels were high. The basin predominantly exhibited a dysoxic to anoxic reducing environment, which provided favorable conditions for the accumulation and preservation of organic matter.