Abstract: The mudstone of the Nadu Formation in the Baise Basin is a critical layer for shale oil exploration in Guangxi Province. This formation is predominantly characterized by massive matrix mudstone. In contrast, shale oil enrichment layers in other petroliferous basins are primarily dominated by laminated shales, which differs significantly from the matrix mudstone in the Nadu Formation. Recent exploration findings suggest that the petroleum generation conditions within the Nadu Formation of the Baise Basin are favorable and the potential of shale oil resources are considerable. However, quantitative characterization and evaluation of shale oil resources in this basin remain insufficient, leaving the characteristics of oil content, mobility, and their controlling factors unclear. To address this, this study focuses on the mudstone of Nadu Formation in the Baise Basin. By integrating conventional methods such as core observation, thin-section analysis, porosity and permeability testing with advanced techniques including low-pressure nitrogen adsorption, high-pressure mercury intrusion, nano-CT scanning, two-dimensional nuclear magnetic resonance, and multi-stage pyrolysis, we have conducted multi-scale characterization of shale oil occurrence and storage space within the Nadu Formation. Additionally, the oil-bearing properties and mobility of the matrix mudstone were analyzed. This research not only provide a scientific foundation for the assessment of shale oil resources in this region but also offer valuable insights into the exploration of similar dense matrix shale oil in China. The results indicate that the matrix mudstone of the Nadu Formation in the Baise Basin exhibits a tightly compacted structure, with micropores and mesopores being the dominant pore types. Pore morphology is predominantly spherical or tubular, distributed in isolated patterns within the rock matrix. Comparisons between pre- and post-extracted samples reveal that shale oil primarily resides in layered fractures less than 10 nm. These pores exhibit small throat radii and poor connectivity. The organic matter content in the first member of the Nadu Formation is relatively high, correlating with elevated oil content. However, due to its low maturity, shale oil predominantly exists in an adsorbed state, with mobility estimated at approximately 20%. Conversely, the second and third members of the Nadu Formation display lower organic carbon content and reduced oil content compared to the first member. Nevertheless, these members exhibit slightly higher maturity levels, with mobile oil content in interbedded sandstone layers reaching over 60%. A comprehensive analysis concludes that the oil-bearing capacity and mobile oil ratio of the matrix mudstone in the Nadu Formation of the Baise Basin are relatively low. Therefore, interbedded sandstone layers are more promising target for exploration.

Key words: shale oil, matrix shale, oil mobility, Nadu Formation, Baise Basin