第32回日本情報地質学会講演会 講演要旨集 039-040頁 2021年

Testing of Drill Core Permeability and Porosity in Fault Zones: Implications
for Groundwater Flow Under Basins and Geothermal Resource Mapping
Jacek Scibek*, Taiki Kubo* and Katsuaki Koike**Department of urban Management, Graduate School of Engineering, Kyoto University, Katsura C1–2, Kyoto 615–8540, Japan. E–mail: jacek.scibek2@mail.mcgill.ca (J. Scibek)

Key words: Fault zones, Permeability, Porosity, Permeameter, Drillcore, Geothermal
resources, Groundwater resources

1. Introduction
In Japan, it is recognized that much of the geothermal
water resources are channeled through fracture zones
that occur along tectonic lines or faults. Many wells that
produce thermal water (e.g. onsen or hot spring bathwater, heating water, and high-temperature water andsteam for power production), either source the thermal
water from permeable sedimentary or volcanic strata thatcover the fracture zones in the crystalline basement, or
directly from faulted crystalline rocks. There is always asignificant interest in the hydro-physical properties ofsuch fault zones, because it can help with modelling andcalculations of water flow and heat transfer.

porosity and permeability. The more detailed results
allow us to map the rock permeability continuously along
the drill core, correlate it to detailed rock deformation andmineralogical properties, porosity, and in-situ interval

2. Data Sources and Methods
Permeability to gas or water is recognized as the main
physical property of rocks for controlling the rate of fluid
flow through low-porosity rocks. Porosity of rocks is
directly related to the poro-elastic or storative propertiesof any aquifers in rocks, and also correlated withpermeability. However, the sampling and results of theseproperties depend on the scale of investigation andvarious methods exist (Fig. 1).
One of the major sources of information is the drill corefrom exploration boreholes and wells. At Kyoto University,
we have been testing the drill core from active fault zones
and from granitic rocks, using practical N2 gas probe
permeameter directly on spots on drill core (Scibek, 2019;
Scibek and Annesley, 2021).
One fault is the Tsukiyoshi normal fault at Mizunami,
Gifu (granite), part of a pull-apart zone in a small basin.
That research site has been studied by JAEA and Kyoto
University for two decades, as an analog site forradioactive waste repository in granitic rocks.
The second research site is the drill hole and drill corefrom the Nojima fault zone on Awaji Island, Hyogo. Thatfault cuts granitic rocks on the edge of Osaka basin, islinked to the active fault system in that region, recently
displaced during the 1995 Kobe earthquake. The
exploration hole was drilled after that disaster to study
the fault structure and properties.

3.Example from Nojima faultForm our studies on active faults, such as Nojima fault
at NIED drill hole from Awaji Island, we study how therock deformation and alteration appear to relate to rock

Figure 1. Conceptual diagram of permeability and porosity
testing scales on drill core and in-situ in fault zones under
sedimentary basins that may host geothermal resources.
Modified after Scibek (2019, 2020).

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hydraulic tests. Such data can be used to relate these
physical properties and to model the hydrogeologic system
there. The local structure, downhole probe porosity
estimates, and our recent rock drill core matrix
permeability results in the NIED Hirabayashi drill holecan be briefly illustrated (Fig. 2). In 1999, NIED andUSGS tested only a few small core plug samples forpermeability (Lockner et al., 2009).
The downhole Neutron probe gives an initial estimate
of in-situ porosity. Across the Nojima fault zone, theporosity is surprisingly high, compared to the host granite(<1% porosity). We noted that a much more porous andpermeable 100 to 200m thick "aquifer" has formed along
the steeply-dipping fault zone over a period of tectonic
activity (Fig. 3). The drill core samples shown const of
weakly pulverized and hydrothermally altered granite ofhigh porosity at present time, compared to low-porosity
original host granite rock. The porosity type was
examined under digital microscope at 50 to 200 
magnification directly on drill core at many test spots andaround the test spots.

Figure 3. NIED Hirabayashi drill core at about 1050m depth.
Photo by authors in 2021 at NIED facility in Tsukuba, Ibaraki.

requires GIS analysis, analytical calculations of upflow
velocity and permeability, and we work toward numericalmodelling of sub-regions.
The regional geostatistical modeling efforts ofgeothermal gradients, rock properties, hot spring
occurrence, and structure, will help in the overall
understanding. Regional and world-wide data ofhydraulic properties of fault zones are also available invarious rock types (Scibek, 2020). We use these data tocompare our site-specific results.
Acknowledgement: The authors thank Dr. Kentaro
Omura at National Research Institute for Earth Scienceand Disaster Resilience (NIED), Tsukuba, for help withdrill core access and testing.

References

Figure 2. Example of fault zone properties at NIED drill hole,
Hirabayashi drill site, Awaji Island. (a) Neutron-probe in-situ
porosity, NIED data, and (b) drill core rock matrix permeability
tested by Kyoto University.

4. Discussion
In mature tectonic fault zones, the fault activity
appears to create highly porous and locally permeable"aquifers" in the crystalline host rocks that the fault cuts.
Later hydrothermal alteration may partly seal the porespaces that were created during deformation in parts ofthe fault zones. Such "fault aquifers" may exist in
geothermal areas, in addition to channels in fractured
more fresh rocks. Furthermore, porosity can be sensed byelectrical geophysical methods at regional scale, while
permeability is difficult to test and requires many in-situ
and drill core measurements.
Hot spring flow systems along fault zones, and we areworking on integrating historical data with the newest
databases of thermal springs in Japan (Sakaguchi andMurata, 2020). Thermal spring flow systems in fracturezones in the brittle crust offer one method of estimating
the permeability of these structures in the brittle crust(e.g. Muraoka et al ., 2006). The work on hot springs

Lockner, D. et al. (2009) Geometry of the Nojima Fault at
Nojima-Hirabayashi, Japan – I. A Simple Damage
Structure Inferred from Borehole Core Permeability.
Pure and Applied Geophysics, vol. 166, pp. 1649–1667.
Sakaguchi, K. and Murata, Y. (2020) Hot spring analysis
value data, geothermal information database, June
2020 edition, Geological Survey of Japan, AIST.
Muraoka, H., Sakaguchi, K., Nakao, S. and Kimbara, K.
(2006)
Discharge
temperature–dischargeratecorrelation of Japanese hot springs driven by buoyancy
and its application to permeability mapping.
Geophysical Research Letters, vol. 33, L10405.
Scibek, J. (2019) Global compilation and analysis of fault
zone permeability. PhD Thesis, McGill University,
Montreal, Canada.
https://escholarship.mcgill.ca/concern/theses/2n49t631d
Scibek, J. (2020) Multidisciplinary database ofpermeability of fault zones and surrounding protolith
rocks at world-wide sites. Scientific Data, vol. 7, 95.
Scibek, J. and Annesley, I. R. (2021). Permeability testingof drill core from basement rocks in the fault-hosted
Gryphon U deposit (eastern Athabasca Basin, Canada):
Insights into fluid-rock interactions related to deposit
formation and redistribution. Natural Resources
Research, https://doi.org/10.1007/s11053-021-09811-x