[S-GC38] Solid Earth Geochemistry, Cosmochemistry

[SGC38-P03] Characteristics of changes in thermal properties in normal consolidation process of sedimentary soft rocks

*Tomoki Yamada¹, Weiren Lin¹, Kazuya Ishitsuka¹, Nana Kamiya², Masahiro Miyazaki¹, Kodai Tomomatsu¹ (1.Graduate School of Engineering, Kyoto University, 2.Faculty of Science and Engineering, Doshisha University)

Keywords:sedimentary soft rocks, thermal properties, consolidation

Since the mechanical characteristics of earthquake faults in subduction zones are strongly affected by temperature, it is important to understand the thermal structure of the sedimentary rocks that compose the subduction zone. For this purpose, it is necessary to know the thermal properties of the sediments during consolidation process occurring in sedimentation process. In previous studies on soft marine sediments, the relationship between consolidation and thermal properties was confirmed under hydrostatic pressure conditions. However, changes in thermal properties during the consolidation process of sedimentary soft rocks under K0 conditions have not been experimentally confirmed. In this study, we conducted a consolidation test using a rigid ring to simulate the natural consolidation condition and measured the thermal properties before and after the test to investigate the change of thermal properties with porosity change caused by consolidation. By applying the porosity-thermal properties relationship obtained from the measurements to the porosity data during the consolidation test, we estimated continuous change in thermal properties in the normal consolidation region. The samples used in the experiments were sedimentary soft rocks of the Umegase Formation of the Kazusa Group, distributed in the eastern part of the Boso Peninsula, central Japan. Block samples were cut from outcrops, cored parallel to the bedding plane, and formed into cylindrical specimens with 20 mm in length and 25 mm in diameter. Five specimens were made from the same core in the same direction. The specimens were saturated with water before the experiment. Consolidation tests were conducted under drainage conditions by applying a uniaxial constant strain rate load with the lateral strain of the specimens restrained by a consolidation ring. In order to investigate as wide a range of porosity changes as possible, different maximum loading pressures were applied to each of the five specimens in the consolidation tests. In addition, after one round of experiment was completed, the same specimen was used for repeated tests, if possible. (In such cases, the loading pressure was changed to be greater than the previous one.) Before and after the tests we measured weight, porosity, thermal conductivity, thermal diffusivity, volumetric heat capacity, and, for reference, resistivity and P-wave velocity. All specimens showed distinct yielding in consolidation tests up to 20~80 MPa, leading to the normal consolidation state. We observed a strong correlation between the porosity and thermal properties measured before and after the consolidation test. As the porosity decreased with consolidation, the thermal conductivity and thermal diffusivity tended to increase, while the volumetric heat capacity decreased. From these data we obtained empirical formulas for thermal properties and porosity. By using them, estimated thermal properties were calculated from five consolidation test data that well represented the normal consolidation state. As a result, the thermal conductivity and thermal diffusivity of sedimentary soft rocks under normal consolidation were estimated to increase logarithmically with increasing pressure, while the volumetric heat capacity decreased logarithmically. These trends can be explained by the fact that the overall thermal properties of the soft rocks become more similar to those of the sediment grains as the porosity of the soft rocks decreases and pore water is expelled due to consolidation. Comparing the estimated values and measured values of porosity and thermal conductivity of the upper and lower formations around the sampling points, it suggests that the trends of measured value are consistent with the changes caused by this consolidation test, and our experiments were able to simulate the conditions of natural consolidation.