Study on conducting mechanism and saturation model of mature lacustrine shales: A case study of the first member of the Qingshankou Formation in 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 Changling Sag, Southern Songliao Basin as an example, we use two-dimensional nuclear magnetic resonance (2D-NMR) and pressure-maintained and sealed coring techniques to obtain the original fluid information of shale reservoirs. By combining geochemical, mineralogical, and geophysical properties, we investigated 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 the shale, while effective porosity plays a secondary role. The conductivity variations within single lithologies are affected by TOC and fluid types. A novel resistivity - saturation interpretation model, which is a function of saturation, for mature lacustrine shale (MLS) is proposed based on lithological 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 index mwb and conductive bound water fraction Swb are introduced to reflect the impact of OM on clay additional conductivity. Compared to the Archie model proposed for clay-free rock and the Indonesian model used for shales, the MLS model offers a more accurate calculation of oil saturation in mature lacustrine shale. The proposed approach makes one step ahead towards reduces uncertainty in the evaluation of mature lacustrine shales as potentially economic oil reservoirs.