Conducting mechanism and saturation model of mature lacustrine shales: A case study of the first member of the Qingshankou Formation in the Changling Sag, Southern Songliao Basin

Shale reservoirs, characterized by intricate fluid occurrence,elevated clay and organic matter (OM) contents, and diverse pore structures, present complexities that obscure the primary controlling factors of lacustrine shale conductivity and render existing saturation models insufficient in accuracy. Taking the medium-high maturity lacustrine shale of the Qing1 Member in the Changling Sag, Southern Songliao Basin as an example,  we use 2D nuclear magnetic resonance and pressure-maintained and sealed coring techniques to obtain the original fluid information of shale reservoirs. By combining geochemical, mineralogical, and geophysical properties, we investigate the impact of mineral components, physical properties, and fluid occurrence on the electrical conductivity properties of shale reservoirs. The clay and carbonate minerals play a primary role in  influencing the conductivity of shale, whereas effective porosity plays a secondary role. Conductivity variations within single lithologies are affected by total organic carbon and fluid types. A novel resistivity-saturation interpretation model, which is a function of saturation, for mature lacustrine shale (MLS) is developed based on lithologic distinctions and the influence of OM. This model identifies two primary conductive pathways: free water conduction in matrix pores and additional conduction from clay-bound water. The bound water cementation indexmwb and conductive bound water fraction Swb are introducedto reflect the impact of OM on clay additional conductivity. Compared with the Archie model developed for clay-free rockand the Indonesian model used for shales, the MLS model offers a more accurate calculation of oil saturation in MLS. Our approach makes a step ahead toward reducing uncertainty in  the evaluation of MLSs as potentially economic oil reservoirs.