1:45 PM - 3:15 PM
[HCG21-P02] Verification study on borehole investigation technologies (3): Geological structures of an accretionary complex
Keywords:geological disposal, accretionary complex, geological structure, Neogene period sedimentary rocks, stratigraphic division
The Yokosuka verification study which aims at systemization and improvement of the borehole drilling for survey and assessment of geological conditions, has been conducted since the 2006 fiscal year as a collaborative study between Nuclear Waste Management Organization of Japan and CRIEPI (Central Research Institute of Electric Power Industry). Geological structure model for 3 km square area including the Yokosuka CRIEPI site has been constructed and updated its model. The Hayama Groups, which is considered to be an accretionary complex of Neogene period, is distributed around this area. Since blocks or discontinuities in an accretionary complex have potential pathways for radionuclide migration, it is important to understand their locations and properties, and to clarify the structural stratigraphy (units) and internal structures of an accretionary complex. In the deep borehole investigation YDP-4, we developed geological structure survey methods for the accretionary complex. This information is applied to be reflected in the development of more accurate hydrogeological model and the evaluation of the validity of the interpretation of groundwater ages.
YDP-4 borehole was drilled to a depth of 720m, with all-core drilling except for the two sections (0-24.5m and 420-550m). The boring core under alluvium consisted of mudstone dominated lithology similar to the Hayama Group with interbedded fine-grained tuff and tuffaceous sandstone. Although some faults with small-scale gauge and faulted breccia were observed in this lithology, no clear stratigraphic boundary structure was recognized. The mudstone in YDP-4 was classified into three groups: mudstone, mudstone (fractured) and mudstone (deformed zone). The observation by X-ray CT showed that mosaic-like fractures without apertures developed uniformly in the mudstone over the entire depth, while distribution of open fractures were varied with depth. Mudstone (fractured) showed that a high density of gravels with various grain sizes. The results of X-ray diffraction analysis of the mudstones indicate that the degree of diagenesis is low throughout the entire depth range. Although the number of microfossils such as calcareous nannofossils, radiolarians, and foraminifera was very small at all depths, the increased frequency of analysis at depths with relatively large numbers and a wide range of depositional ages suggests that the composition is roughly consistent with the biostratigraphic sequence of the middle to lower part of the Hayama Group shown in a previous study. The magnetic susceptibility of mudstone and mudstone (fractured) showed almost constant values, while that of tuffaceous lithology showed generally higher values, possibly due to the influence of magnetic minerals. The magnetic susceptibility of the mudstones at depths of 300 m to 400 m was relatively low, possibly due to the influence of fracturing or mineral alteration.
In conclusion, all lithologies in borehole YDP-4 are considered to be of the Hayama Group, which biostratigraphic age is roughly consistent with the middle to lower part of the Hayama Group. X-ray CT observations show that the density tends to change at certain intervals regardless of the lithology. This trend is linked to the change in magnetic susceptibility, which may reflect increased pore size (decreased density) and mineral alteration due to fracturing. Thus, we found the possibility of stratigraphic unit classification by interpreting the trends and characteristics found by multiple measurement methods. Appropriate sampling locations and frequency of data acquisition improved the accuracy of dating by microfossil stratigraphy. In the future, from the viewpoint of reflection in the hydrogeological model, we would like to clarify the factors affecting permeability and its distribution in the geological structures of accretionary complexes.
YDP-4 borehole was drilled to a depth of 720m, with all-core drilling except for the two sections (0-24.5m and 420-550m). The boring core under alluvium consisted of mudstone dominated lithology similar to the Hayama Group with interbedded fine-grained tuff and tuffaceous sandstone. Although some faults with small-scale gauge and faulted breccia were observed in this lithology, no clear stratigraphic boundary structure was recognized. The mudstone in YDP-4 was classified into three groups: mudstone, mudstone (fractured) and mudstone (deformed zone). The observation by X-ray CT showed that mosaic-like fractures without apertures developed uniformly in the mudstone over the entire depth, while distribution of open fractures were varied with depth. Mudstone (fractured) showed that a high density of gravels with various grain sizes. The results of X-ray diffraction analysis of the mudstones indicate that the degree of diagenesis is low throughout the entire depth range. Although the number of microfossils such as calcareous nannofossils, radiolarians, and foraminifera was very small at all depths, the increased frequency of analysis at depths with relatively large numbers and a wide range of depositional ages suggests that the composition is roughly consistent with the biostratigraphic sequence of the middle to lower part of the Hayama Group shown in a previous study. The magnetic susceptibility of mudstone and mudstone (fractured) showed almost constant values, while that of tuffaceous lithology showed generally higher values, possibly due to the influence of magnetic minerals. The magnetic susceptibility of the mudstones at depths of 300 m to 400 m was relatively low, possibly due to the influence of fracturing or mineral alteration.
In conclusion, all lithologies in borehole YDP-4 are considered to be of the Hayama Group, which biostratigraphic age is roughly consistent with the middle to lower part of the Hayama Group. X-ray CT observations show that the density tends to change at certain intervals regardless of the lithology. This trend is linked to the change in magnetic susceptibility, which may reflect increased pore size (decreased density) and mineral alteration due to fracturing. Thus, we found the possibility of stratigraphic unit classification by interpreting the trends and characteristics found by multiple measurement methods. Appropriate sampling locations and frequency of data acquisition improved the accuracy of dating by microfossil stratigraphy. In the future, from the viewpoint of reflection in the hydrogeological model, we would like to clarify the factors affecting permeability and its distribution in the geological structures of accretionary complexes.