Data Acquisition for Hydrology and Transport Properties of Sedimentary Rochs, Pore Size Distribution and Chemical Composition of the Horonobe Area

Hara, Akio

In case of select of the location for repository of high level radioactive waste, it is very important to estimate the facies distribution of heterogeneous sedimentary rocks and its physical property which strongly effect hydrological behavior. Due to its microscopic grain size, physical and chemical methods are usually used to understand the characters of argillaceous rock. In this study, chemical composition and pore size distribution ware analyzed with 21 samples of diatomaceous mudstone sampled at HDB-1 well in the Horonobe area. Well log data were also used for data interpretation to detect relevance between physical and chemical properties with argillaceous rock of the Horonobe area. Following tendency was detected through data interpretation. (1)Porosity of argillaceous rocks is decreasing with depth incretion. Comparing with same depth sample, the level of natural gamma-ray is higher in low porosity sample. Generally, the level of natural gamma-ray is in proportion to AlO composition. Therefore mudstone sample which has high contents of AlO trends to have low porosity. (2)Mudstone samples of the Koetoi formation, diatom origin silica is belonging to Opal-A phase, have more than 40% of porosity. 60 to 80% of pore has radius between 500 to 2000 angstrom. On the other hands, mudstone samples of the Wakkanai formation, diatom origin silica is belonging Opal-CT phase and porosity of mudstone is less than 40%. Bimodal concentration of pore radius, several tens angstrom and several hundreds angstrom, are observed. (3)The mudstone of the Koetoi formation and the Wakkanai formation contain 35 to 55% of diatom origin silica in weight volume. Median pore radius of mudstone of the Koetoi formation is getting larger in proportion to the amounts of diatom contents. Howevere, in the Wakkanai formation, median pore radius trends to be smaller with increasing of diatom fossil contents.