Impact of High Temperature on the Evolution of Shale Pore Structure Complexity: A Quantitative Study Based on Shannon Entropy and Lempel-Ziv Complexity

Summary The pore structure in shale oil reservoirs exerts a crucial control over hydrocarbon accumulation and migration. Taking medium-low maturity shale (R0 = 0.71–0.90%) from the Beibuwan Basin as the research object, we study thoroughly investigate the dynamic evolution of multiscale pore structures under high-temperature conditions (0–1,000°C) and their optimization mechanism for fluid percolation and transport efficiency. A comprehensive approach, combining gas adsorption experimental [low-temperature nitrogen (N2) adsorption and carbon dioxide (CO2) adsorption), scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-pressure mercury injection experiments, along with quantitative indicators such as Shannon entropy and Lempel-Ziv complexity (LZC), was utilized to reveal the pore network reconstruction process from the perspectives of full pore size range and spatial heterogeneity. The results indicate that the evolution of shale pore structure exhibits two critical temperature thresholds: The first threshold (400–500°C) is where organic matter pyrolysis leads to a significant increase in pore volume and specific surface area, accompanied by an increase in Shannon entropy. However, the LZC value fluctuates minimally, and pores are mostly isolated and dispersed, resulting in limited improvement in connectivity. The second threshold (500–600°C) is where high-pressure mercury injection data show a sharp increase in permeability, and the LZC value reaches its peak, indicating that the pore-fracture network achieves efficient connectivity, forming a highly complex, interwoven multiscale percolation pathway. At higher temperatures (>600°C), mineral phase transformation further induces stress fractures, which synergistically interact with organic matter pyrolysis fractures (microcracks formed by the pyrolysis of organic matter and mineral phase transformation) to construct a pervasive permeable network within the matrix. This study reveals that the evolution of the shale pore network follows a dynamic process from “pore increment but limited connectivity” to “interconnected pore-fracture and efficient transport,” with the synergistic response between 500°C and 600°C being key to significant optimization of pore connectivity. This structural optimization enhances the pore-throat matching and effective fluid pathway development, providing a crucial micromechanistic basis for evaluating the fluid transport potential and enhanced hydrocarbon recovery in high-temperature shale reservoirs.