Evolution Patterns and Anisotropic Connectivity Characteristics of Pores and Fissures in Oil Shale After Steam Heating at Different Temperatures

This paper presents a thorough investigation into the evolutionary patterns of pore–fissure networks and their anisotropic connectivity characteristics within oil shale. We utilized CT digital core analysis after steam heating at varying temperatures. The study revealed that untreated oil shale has a densely compacted internal structure without distinguishable pore–fissure networks. However, steam exposure at temperatures exceeding 314 °C induced penetrating cracks along the bedding plane. This significantly modifies the mass transfer properties in the parallel bedding direction. Beyond 382 °C, continuous thermal cracking occurred, leading to numerous fissures along sedimentary bedding planes. This was accompanied by clustered pores formed through organic matter pyrolysis. These aggregated pores gradually interconnected adjacent parallel fissures, forming distinctive pore–crack clusters. Notably, as the temperature surpassed 500 °C, these pore–crack clusters continued to expand perpendicular to the lamination plane, profoundly influencing the mass transfer performance in this orientation. This phenomenon underscores the fundamental mechanism altering oil shale's mass transfer behavior perpendicular to the layer plane. From the perspective of percolation theory, the percolation threshold parallel to the lamination orientation was approximately 3%, with the transition around 300 °C. Conversely, the percolation threshold vertical to the sedimentary rock layers was approximately 14%, with the transition at temperatures surpassing 500 °C.