Differentiation and quantification of pore-fracture structures in marine and transitional shales.

This article focuses on the characterization and comparative analysis of pore and fracture structures in marine and transitional shales in China, which are critical for shale gas exploration and development. It introduces a novel "visual slope" parameter derived from hysteresis loops of low-temperature nitrogen adsorption–desorption curves to quantify the relative proportions of pores and fractures, establishing that slopes below 0.4 indicate fracture dominance, 0.4–0.8 indicate balanced development, and above 0.8 indicate pore dominance. The study finds marine shales generally exhibit higher visual slopes with well-developed pores and fractures, while transitional shales show lower slopes dominated by fractures. Key controlling factors identified include burial depth and thermal maturity (negatively affecting micropore and mesopore development), total organic carbon content (positively influencing pore development depending on organic matter type), and mineralogy—where quartz and pyrite promote pore and fracture formation in marine shales, and clay minerals hinder pores in marine but promote them in clay-rich transitional shales. Predictive models for pore and fracture development based on geochemical and mineralogical parameters are also established to aid in reservoir evaluation and fracturing optimization.