Dominant lithofacies and hydrocarbon enrichment characteristics of lacustrine shale reservoirs: A case study of the Qingshankou Formation shale in the Southern Songliao Basin

Abstract

With the growing challenges in conventional hydrocarbon exploration, shale oil has emerged as a critical target for future energy supply. The Qingshankou Formation (K₂qn) in the southern Songliao Basin holds significant shale oil potential. However, the complex laminae architecture and poorly constrained lithofacies have hindered a clear understanding of shale oil enrichment mechanisms, thereby limiting efficient exploration and development. This study focuses on the K₂qn shale in the Changling Sag, where a comprehensive characterization of lithofacies and pore structures is carried out using an integrated approach of core description, geochemical analysis, and multi-scale imaging techniques, including thin-section petrography, micro-XRF, MAPS, QEMSCAN, and fluid injection experiments. Three types of argillaceous laminae are identified to reveal significant textural and structural heterogeneity in the shale. Pore systems are dominated by interparticle pores and microfractures, with microfractures playing a crucial role in enhancing pore connectivity and fluid transport. The results also indicate that reservoir quality is jointly controlled by organic matter content, mineral composition, and laminae characteristics. Importantly, two enrichment models are proposed that the layered mixed shale with medium to high organic matter represents a migration-accumulated model influenced by external hydrocarbon input, and the laminated siliceous shale and the mixed shale, both with moderate to high organic content, follow in-situ accumulation patterns dominated by thermal transformation of indigenous organic matter. These findings offer critical geological insights to guide more accurate reservoir prediction and targeted exploration in the lacustrine shale systems.