A study on diffusion and migration processes of ions in sedimentary rock matrix: Measurements and evaluations of physical and chemical properties of Horonobe Mudstone

The physical and chemical properties of Horonobe mudstone (HDB-1 borehole) were measured and evaluated as a function of depth from ground surface (8 points). Micropore structural observations by SEM, constituent element analyses by EDS, mode analyses and the measurements of swelling properties and exchangeable cations (EC) in addition to the measurements of porosity, dry density, solid density, pore-size distribution and the specific surface area of pores were measured as the physical and chemical properties. It is considered that clearly physical gap exists between mudstone samples 4 (depth: 288.33 m) and 5 (depth: 370.95 m), and that geological formations with different properties neighbor between those depths, judging synthetically the results of measurements by water saturation method and Hg porosimetry, the results of the measurements of swelling properties and the results of micropore structural observations by SEM. In the results of the measurements of EC, the sum total of EC was in the range of about 10-35 meq/100 g in Koetoi formation, and showed a tendency to increase with an increase of depth. On the other hand, it was in the range of about 30-40 meq/100 g in Wakkanai formation, and were approximately constant. This indicates that cation exchangeability is potentially high in Wakkanai formation. The results of mode analyses indicated that rock is diatomaceous earth in whole Koetoi formation and in the upper part of Wakkanai formation (mudstone samples 1-5: ∼370.95 m), and is tuffaceous mudstone in area deeper than there. It was found that diatomaceous earth is composed of mainly some kinds of diatom fossils. Moreover, from the results of SEM observations and EDS analyses, many diatom fossils were found in Koetoi formation and in the upper part of Wakkanai formation (mudstone samples 1-5: ∼370.95 m), but the number of diatom fossil decreased in mudstone sample 6 (depth: 453.28 m), and diatom fossils were few found in mudstone sample 7 (depth:528.77 m) and in area deeper than there. Considering for the kind of diatom, euryhaline diatom fossils were found in all formations, but oceanic diatom fossils came to be found in mudstone sample 2 (depth: 193.92 m) and in area deeper than there. Synthesizing these matters, diatoms in mudstone sample 1 (depth: 47.28 m), where is equivalent to the upper part of Koetoi formation and in area narrower than there, is considered to have grown and have sedimented in narrow sea area relatively near the coastal area, and diatoms in mudston sample 2 (depth: 193.92 m), where is equivalent to the main part of Koetoi formation and in area deeper than there, is considered to have grown and have sedimented in far sea area from the coastal area. Finally, considering from that sediment structure came to be found and that the number of diatom fossil, which can see keeping the shape, extremely decreased in mudstone sample 5 (depth: 370.95 m) and in area deeper than there, the border between both formations is considered to exist between mudstone samples 4 (depth: 288.33 m) and 5 (depth: 370.95 m). (author)