Insights into the Transformation Patterns of Overmature, Organic-Rich Shale Reservoirs under High-Temperature Conditions: Evidence from Physical Experiments

The efficient development of shale oil and gas is crucial for China’s energy security. Shale exhibits characteristics such as well-developed bedding planes, strong heterogeneity, low porosity, and low permeability. Previous studies have demonstrated that high-temperature thermal treatment can damage the internal structure of shale, serving as an important method for horizontal well stimulation and effectively enhancing shale oil and gas production. However, the high-temperature damage mechanisms in over-mature shale remain unclear, and the optimal reservoir stimulation temperature has yet to be determined. This study focuses on over-mature organic-rich shale from the Niutitang Formation in Hunan Province, employing scanning electron microscopy, low-temperature gas adsorption, and high-pressure mercury intrusion porosimetry as research methods to investigate the pore evolution patterns of over-mature shale under high-temperature conditions and reveal its high-temperature damage mechanisms. The findings indicate that the pore structure distribution of over-mature shale exhibits multi-stage changes with temperature. Influenced by rock deformation and mineral composition, the micropore volume of over-mature shale reaches its maximum after high-temperature treatment at 600℃ but decreases as the temperature continues to rise. After treatment at 400–500℃, the mesopore volume and specific surface area undergo significant increases, followed by a notable decline, while the specific surface area reaches a minimum at 600℃, suggesting the existence of an optimal temperature range (500–600℃) that maximizes mesopore development. With increasing temperature, the fractal dimension of macropores approaches 2, displaying regular morphology and favorable connectivity between pores. Therefore, the optimal stimulation temperature for over-mature shale lies within the range of 500–600℃, maximizing pore volume and specific surface area development, thereby providing essential migration pathways for shale oil and gas production. This research offers theoretical guidance for high-temperature stimulation of over-mature shale.