Features of fractures forming flow paths in granitic rock at an LPG storage site in the orogenic field of Japan

The biggest underground storage facility for Liquefied Petroleum Gas (LPG) in the world has been constructed in sub-seabed Cretaceous granitic rock located in the Seto inner-sea area of western Japan. This LPG facility is being operated using a ‘water curtain system’ in order to control the flow of groundwater and pore pressure and thereby ensure long-term safe LPG storage. To design this ‘water curtain system’ it was essential to understand the hydrological and structural features of flow-path (FP) fractures in the granitic host rock. Therefore, detailed FP mapping, structural characterization and grouting after hydrological examination were carried out. As a result, two types of FP were distinguished in the host granitic rock. One is a ‘planar type’ fracture, which constitutes a major FP at the site. Hydrologically, the fractures of this type are well-connected and readily controlled by grouting. The other type is a ‘micro-sheeting type’ fracture, which is characterized by an assemblage of micro-fractures. Fractures of this second type were formed under an unloading stress field while the granitic body underwent uplift. However, these fractures have quite low groutability owing to the poor connectivity of their micro-pores. The physical characteristics of the fracture fillings (e.g. calcite) within both kinds of FPs reflect the processes of FP formation and evolution of the host rock. The structural and mineralogical features of the FPs can be explained by the development of fractures and fluid circulation due to the relatively rapid uplift of the granite during orogenesis. The two kinds of FP can be considered typical of groundwater conductive features in an orogenic stress field. Inevitably such features will be encountered in other plutons formed at continental margins.