Topological analysis of micropores within the Quaternary Kakkonda granite, northeast Japan, using persistent homology

Quaternary granitic bodies are considered to host high-temperature geothermal reservoirs containing supercritical fluids exceeding 400 °C. However, both the micro- and reservoir-scale geometry and connectivity of pore networks within granite under such extreme conditions remain poorly understood. In this study, we performed micro X-ray computed tomography (µCT) analyses on drill core samples from well WD-1a, which penetrates the Kakkonda granite in northeast Japan. To characterize the geometry and spacing of micropores, we conducted topological data analysis (TDA) using persistent homology. Using 0th-dimensional persistent homology (PH0), we quantified the geometry and spacing of pores as sets of birth and death values {( b i , d i )}. Their statistical characteristics were further analyzed using principal component analysis (PCA). Three types of pores were identified: (1) alteration-type pores formed at feldspar replacement fronts, (2) relatively large facet-type pores bounded by crystal faces, and (3) cracks occurring along grain boundaries. Integrated PH0 and PCA results revealed variations in micropore topology among samples collected across the hydrological boundary and deeper thermal conduction zone. In the weakly metamorphosed granodiorite sample from the hydrothermal convection zone, abundant alteration-type pores exhibited fine-grained textures and close spacing. The sample from near the hydrological boundary showed both alteration and facet-type pores, but had low µCT-derived connected porosity. In the deeper sample, located within the thermal conduction zone, large facet-type pores were interconnected by grain-boundary cracks, forming extensive networks. The observed differences in pore topology are broadly consistent with the permeability variations measured among samples in the laboratory, highlighting the importance of pore generation mechanisms in evaluating geothermal reservoir potential.