The Petrophysical Characteristics and Controlling Factors of the Wufeng Formation–Longmaxi Formation Shale Reservoirs in the Fuyan Syncline, Northern Guizhou

ABSTRACTThe physical properties of shale reservoirs are critical to the enrichment mechanisms of shale oil and gas. To clarify the differences in reservoir physical properties and their primary controlling factors of the Wufeng‐Longmaxi Formation shales in northern Guizhou, this study systematically employed multi‐scale characterisation techniques, including mineral composition analysis, scanning electron microscopy (SEM) observation, organic geochemical testing, low‐temperature nitrogen adsorption, and high‐pressure mercury intrusion (HPMI). By combining HPMI and nitrogen adsorption experiments to achieve cross‐scale characterisation of pore structures, we investigated reservoir heterogeneity among different lithofacies and identified key controlling factors. Results indicate that the shales in the study area can be classified into four typical lithofacies: organic‐rich layered siliceous shale (F1), organic‐bearing laminated clayey shale (F2), organic‐poor laminated clayey shale (F3), and organic‐poor laminated mixed shale (F4). Among them, F1 exhibits the highest pore volume (PV) and specific surface area (SSA), characterised by well‐developed organic pores and microfractures; F2 has moderate PV and SSA, with microfractures as the dominant pore type; F3 and F4 show lower PV and SSA, dominated by dissolution pores. The fractal dimensions of the shales range from 2.6114 to 2.6528 (D1) and 2.8040 to 2.9024 (D2), showing weak negative correlations with total organic carbon (TOC) content, BJH pore volume, and BET specific surface area, while weakly positively correlated with clay mineral content. Further analysis reveals that TOC content and microfracture density are significantly positively correlated with PV and SSA across all four lithofacies, serving as the dominant factors regulating reservoir physical property differences. In contrast, mineral composition shows weaker correlations with PV and SSA, acting as secondary controlling factors. Notably, significant differences in preservation conditions exist between the Wufeng‐Longmaxi Formation shales in northern Guizhou and the Sichuan Basin, leading to regional variations in the primary controlling factors of reservoir physical properties.