Data acquisition of groundwater flow and mass transport properties in sedimentary rocks 2. Considerations of mass transport properties based on pore size distribution, chemical composition and air permeability of the Horonobe specimen

Pore size distribution, chemical composition and air permeability are analyzed on diatomaceous mudstone specimens of Koetoi and Wakkanai formation sampled from boreholes HDB-1, HDB-2 and HDB-5 which drilled by Japan Nuclear Cycle Development Institute in Horonobe area. Permeability of Koetoi and Wakkanai formation is related to diagenetic facies changes of diatom fossils. Diatom fossils included in the Koetoi formation are belonging to Opal-A facies. The fossil remains original feature of diatom. Permeability and porosity of Koetoi formation is decreases by compaction of stratum due to increasing of depth. The diagenetic facies of diatomaceous fossil is changes into Opal-CT facies at Wakkanai formation. Macroscopic crystals of Opal-CT, such as cristobalite, are crystallized in the pore of hardshale in Wakkanai formation. Low permeability of Wakkanai formation should be caused by crystallization of these minerals. Pore throats which have a diameter between 20 to 40 angstrom are much concentrated in the mudstone with high SiO2 content. Permeability of the Wakkanai formation tends to low in the core samples which have high SiO2 content. Pore throat radius concentration ranged from 20 to 40 angstrom is not observed in the core specimens sampled about 600 m depth below the formation boundary between Koetoi and Wakkanai formation. The feature of cumulative curves of pore throat radius distribution changes into monomodal and pore throats which have a radius about 100 angstrom are dominated in the core specimens. Porosity and permeability of these samples are higher than the core samples around 200 to 300 m depth below the formation boundary between Koetoi and Wakkanai formation and the feature of the cumulative curve of pore throat radius distribution of these core specimens is some resemble to Masuhoro formation which under raying Wakkanai formation. The lowest permeability is observed in Wakkanai formation at 200 to 300 m depth the formation boundary of Koetoi and Wakkanai formation. Abnormal high-pressures have been reported at the boreholes HDB-2 and Kita-kawaguchi SK-1. Both abnormal high-pressures are detected more than 200 to 300 m beneath the facies boundary of Opal-A and Opal-CT. Wakkanai formation below 200 to 300 m depth from Opal-A and Opal-CT boundary has the possibility of functioning as a seal rock, which can keep the high pressure, disturb ground water flow and delay radionuclide migration. (author)