Distribution Characteristics and Fractal Dimension of Continental Shale Reservoir Spaces Based on Lithofacies Control: A Case Study of the Lucaogou Formation in Jimsar Sag, Junggar Basin, Northwest China

The significant heterogeneity of continental shale reservoirs within the Permian Lucaogou Formation of the Jimsar Sag presents a major challenge for shale oil exploration. This study aims to quantitatively characterize the pore structure complexity of different lithofacies to identify favorable “sweet spots.” By integrating geochemical, petrological, and high-resolution pore characterization data with fractal theory, we introduce a comprehensive fractal dimension (Dc) for evaluation. Five distinct lithofacies are identified: massive felsic siltstone (MFS), bedded dolostone (BD), bedded felsic dolostone (BFD), laminated dolomitic felsic shale (LDFS), and laminated mud felsic shale (LMFS). Pore structures vary significantly: MFS is dominated by mesopores (100–2000 nm), BD and BFD exhibit a bimodal distribution (<30 nm and >10 μm), while LDFS and LMFS are characterized by nanopores (<50 nm). Dc analysis reveals a descending order of pore structure complexity: BFD > LMFS > LDFS > MFS > BD. Furthermore, Dc shows positive correlations with clay mineral and feldspar contents but a negative correlation with carbonate minerals. A significant negative correlation between Dc and measured permeability confirms its effectiveness in characterizing reservoir heterogeneity. We propose that MFS and LDFS, with higher pore volumes and relatively lower Dc values, represent the most favorable targets due to their superior storage and seepage capacities. This study provides a theoretical foundation for the efficient development of continental shale oil reservoirs.