Major controlling factors and evolution patterns of shale reservoirs in the Qingshankou Formation, northern Songliao Basin

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.