Lithofacies classification and reservoir property of lacustrine shale, the Cretaceous Qingshankou formation, Songliao basin, northeast China

The lacustrine shale of the Qingshankou Formation in the southern Songliao Basin is a key target for shale oil exploration and development in China. This study presented a comprehensive investigation of shale lithofacies characteristics and the heterogeneity of shale reservoir property using various analytical techniques, including TOC analysis, X-ray diffraction (XRD), optical microscopy, X-ray fluorescence (XRF), scanning electron microscopy (SEM), modular automated processing system (MAPS), nitrogen adsorption, and mercury intrusion capillary pressure experiments. The findings indicated that (1) Eight shale lithofacies were identified based on the “organic carbon contents - mineral compositions” classification scheme. Laminated shale was predominantly composed of poor-TOC mixed lithofacies (PML), medium-TOC siliceous lithofacies (MSL), medium-TOC mixed lithofacies (MML), rich-TOC siliceous lithofacies (RSL), and rich-TOC mixed lithofacies (RML), with interbedded and mixed laminae structures. (2) The shale reservoir space was primarily influenced by interparticle pores and microfractures. Nanopores played a critical role in pore space, with mesopores significantly contributing to storage capacity. MML shale was identified as the most favorable reservoir due to its superior pore structure. (3) TOC exhibited a significant negative influence on shale pore development. Clay minerals were the main contributors to reservoir space, while authigenic brittle minerals typically occupied some pore space. As organic matter content increased, generated dissolution pores helped alleviate the negative effects of feldspar and quartz on pore structure. Additionally, more bitumen and clay cementation reduced the impact of clay minerals on the shale pore network. (4) Carbonate skeleton laminae exerted a significantly negative effect on shale reservoir properties. Furthermore, a moderate presence of felsic particles in argillaceous and mixed laminae effectively preserved clay mineral-associated pores and enhanced interparticle pore development. These insights are instrumental for advancing shale oil exploration strategies.