Characterizing the permeability of drillhole core samples of Toki granite, Central Japan to identify factors influencing rock-matrix permeability

Abstract

Rock matrix permeability is mainly controlled by microcracks and is important for long-term solute transport, but may be highly heterogeneous. This study aims to identify the factors influencing the permeability of granite matrix by analyzing: physical properties, crack distribution, mineral composition, and the concentration of main elements in 48 core samples taken from three deep drillholes in the Mizunami area, central Japan. All drillholes intersect the Tsukiyoshi fault at depth. Permeability of core samples, measured by a gas permeameter, largely increases in the fault and fracture zones, which may be caused by the development of cracks as the total length of cracks extracted from thin-sections is correlated with measured permeability. Although a significant correlation is identified between permeability and P-wave velocity, a representative elastic property of rock, this correlation is enhanced by classifying the samples into two groups by the Mn/Fe concentration ratio. This ratio is identified as a key factor to separate lithofacies differentiated by the source magma. Thus, lithofacies is another control factor for permeability due to the difference in mineral composition. Moreover, permeability shows significant negative and positive correlations with Si and Ca concentrations, respectively. These concentrations are probably affected by dissolution of silicate minerals and calcite generation in the hydrothermal alteration process, which can develop both pore space and crack network and consequently, increases rock permeability in addition to crack generation and development. Therefore, a combination of hydrothermal alteration before the emplacement of granite and strong faulting are the predominant processes for controlling permeability.