Geological characteristics and in-situ retention mechanisms of clay-rich lacustrine shale oil in the Songliao Basin

The Cretaceous Qingshankou Formation in the Gulong Sag of the Songliao Basin, located in the eastern of the Eurasian Plate, exhibits unique shale oil properties, including high clay mineral content (>40 %), high organic matter maturity (>1.0 %), and low shale oil density (∼0.78 cm³/g). This represents the first globally documented shale oil produced from clay-rich shales. This study systematically investigates the characteristics and controlling factors of reservoir space, oil-bearing properties and the formation and enrichment processes of shale oil. Findings reveal that the Qing-1 Member predominantly comprises laminated argillaceous shale, bedded mixed shale, and massive calcareous shale. The kerogen is mainly Type I, characterized by well-developed lamalginite and high organic matter (OM) abundance, with an average Total Organic Carbon (TOC) of 2.27 %. Thermal maturity (R_o) in oil-producing intervals typically exceeds 1.0 %. The main pore types are intergranular pores, intragranular pores, OM-hosted pores, exposed pores after OM degradation, and microfractures, with pore development influenced by TOC, clay mineral content, and R_o. The Gulong shale is notably oil-rich, with oil films present observed in different types and sizes of pores, and oil content significantly affected by TOC and R_o. The coupling of hydrocarbon evolution, fault activity periods, and regional stress changes governs the in-situ retention of light oil within the sag and dictates the reservoir formation sequence and spatial distribution of tight and conventional oil outside the source rock. This study offers critical insights for the exploration and development of continental clay-rich shale oil reservoirs.