A strong earthquake of M7.2 (MW6.7-6.9) occurred on 14 June 2008 in the north-central part of Northeastern Japan. It was named the Iwate-Miyagi Nairiku Earthquake in 2008. Several seismic models show a 25-30km long reverse fault in the NNE-SSW direction and a westward inclination. Although some geological thrusts with a similar trend are known, as shown in Fig. 1, none of them has been recognized as an active fault. Surface faults associated with the earthquake were found at several points along the geological thrusts. The quake induced many mass-movements in the mountainous and hilly area, which is composed mostly of Neogene to Pleistocene volcaniclastic rocks (Fig. 1), near the seismic fault and particularly on the western side (upper block). Most of the damage caused by the earthquake was closely connected to mass-movements. A large-scale block glide amounting to about 7×107m3 in the volume of dislocated mass (Figs. 2, 3) occurred in the northern half of the pre-existing landslide area composed of poorly-consolidated pyroclastic flow deposits with a welded cap around 5Ma (Np3 in Fig. 1). The almost horizontal slip surface of the glide is considered to have been formed in the underlying lacustrine beds. A large collapse, occurring at a slope on which snow remained on Kurikoma Volcano, composed of Quaternary andesitic lavas and pyroclastics (Qm2 in Fig. 1), formed a debris flow (Fig. 4) which buried a lodge at Komanoyu Spa. Many landslides checked river channels. One of them was observed at Ogawara, where a slide of a spur composed of Pleistocene pyroclastic deposits checked the Ichihasama River (Fig. 5). The collapse of Matsurube Bridge, a three-span 95 m-long bridge over a tributary of the Iwai River, was also entirely caused by a landslide at the north-facing slope of a narrow ridge (Fig. 6). A surface fault appeared in rice fields at Mochikorobashi on the side of a tributary of the Koromo River (Fig. 7). The trace of the water's edge and lines of planted rice indicate a co-seismic uplift and a right-lateral offset with a clockwise turn of the trend (Fig. 8).
A 30,000-yr record of vegetation history has been constructed using a phytolith analysis of a tephra sequence located at the Kawahara No.3 archaeological site, southwestern foot of Aso Volcano, central Kyushu, southwestern Japan. The sequence was divided into three zones: Zone 3 (>ca. 31 cal ka), Zone 2 (31-13.5 cal ka), and Zone 1 (13.5-0 cal ka) in ascending order. Gramineae phytoliths were predominately detected at most horizons, whereas a small amount of arboreal phytolith was observed mainly at Late Holocene horizons. Zone 3 was dominated by a phytolith of Sasa (mainly Sasa sect. Crassinodi), a cool-temperature dwarf bamboo. However, vegetation composed mainly of Sasa dwarf bamboo had declined slightly in Zone 2, which corresponds to the Last Glacial Maximum. During the Holocene (Zone 1) the Sasa grassland dominated consistently, but phytolith of Pleioblastus (a warm temperature dwarf bamboo) and arboreal such as Distylium increased from 7,300 years ago.
The IPCC published a special report on Carbon dioxide Capture and Storage (CCS) in 2005, stating that CCS is one of the promising options for mitigating carbon dioxide emissions into the atmosphere. Among several CO2 storage options, storing CO2 in saline aquifers is the most promising because of the large storage potential, estimated at from about 2,000 Gt CO2 to more than 10,000 Gt CO2. In this article, we first describe current global trends of CCS technology development and national policies. Some CCS technologies are already in practical use in several countries and are economically viable. Close attention has been paid recently to deep saline aquifer storage, which is expected to have a large storage potential of about 2,000 Gt CO2 throughout the world. We then focus on the mechanisms of deep saline aquifer CO2 storage. In deep saline aquifer storage, chemical reactions in the water-rock-CO2 system play important roles for trapping CO2 in the aquifer formation, as well as physical trapping by overburden impermeable cap rocks and residual gas trapping mechanisms. We also stress the importance of the long-term monitoring of the storage aquifer because CO2 would be trapped stably in the formation for a long time. It is thus important to develop effective monitoring techniques for the behavior of CO2 in the aquifer. Physical as well as chemical monitoring techniques should be used for storage aquifer monitoring. We conclude this article with discussions about storage potential in Japan and some important issues related to deep saline aquifers. Deep saline formations are distributed widely in Japan, and have the potential for the geological storage of 146 Gt of CO2. It is therefore economically feasible to use deep saline formations near large emission sources such as coal-fired power plants and integrated steel works. To realize CCS in Japan, it is important to make further advances in studies on the basic physical and chemical trapping mechanisms of water-rock-CO2 system, and in studies on the geological characteristics of aquifer formations.
Several key questions need to be answered when CO2 geological storage is to be undertaken worldwide. How should CO2 be stored underground? Can trapping be assumed in saline formations and can CO2 be retained for long periods safely in the subsurface? The first Japanese pilot-scale CO2 sequestration project in Nagaoka was undertaken to provide answers to these questions. The injection site is located at the Minami-Nagaoka gas field in Nagaoka City, 200km north of Tokyo. Supercritical CO2 was injected into an onshore saline aquifer at a depth of 1,100m. CO2 was injected at a rate of 20 to 40 tonnes per day over an 18-month period, with a cumulative amount of 10,400 tonnes. A series of monitoring activities, which consisted of time-lapse well logging, crosswell seismic tomography, 3D seismic survey and formation fluid sampling, was carried out successfully to monitor CO2 movement in the sandstone reservoir. This paper presents an overview of the results obtained from both field and laboratory studies to examine the spatial-time distribution of CO2 and various trapping mechanisms in the reservoir. CO2 breakthrough at two of the three observation wells was clearly identified by changes in resistivity, sonic P-wave velocity and neutron porosity from time-lapse well logging. Each velocity difference tomogram obtained by crosswell seismic tomography showed a striking anomaly area around the injection well. As the amount of injected CO2 increased, the low-velocity zone expanded preferentially along the formation up-dip direction during the first two monitoring surveys and less change around the CO2-bearing zone could be confirmed from the following surveys. Unfortunately there was no significant change in 3D seismic results due to CO2 injection. The pilot-scale project demonstrated that CO2 can be injected into a deep saline aquifer without adverse health, safety or environmental effects. The Nagaoka project also provides unique data to develop economically viable, environmentally effective options for reducing carbon emissions in Japan.
To evaluate the long-term behavior of a CO2 storage site, it is necessary to understand the geochemical reactions induced by CO2 injection into the water in an aquifer. We observed changes in the chemical composition of the formation water at the Nagaoka CO2 storage site, Japan, where a total of 10,400 tonnes of CO2 was injected into a 12-m-thick sandstone section of the Haizume Formation located at a depth of 1,100 m. The formation water was collected from the CO2 injection well during a pumping-up test carried out before the injection. Before the injection, the total dissolved solid in the sampled water was 8,000 ppm, indicating that 50,000 ppm of CO2 can be dissolved maximally in the formation water due to its solubility. After the CO2 injection, the Cased Hole Dynamics Tester (CHDT) tools sampled formation fluids by penetrating casing, and plugging the test hole in a single trip at the depths determined by the well logging. The fluid sample collected at a depth of 1114.0 m was almost free CO2. The other two fluid samples from depths 1108.6 m and 1118.0 m were mainly the formation water. At the depth of 1118.0 m, we observed an increase in the concentration of HCO3 due to the dissolution of injected CO2. Concentrations of Ca, Fe, Si, Mg and Mn also increased. These elements were potentially provided by the dissolution of hornblende, serpentine, pyroxene, chlorite, and gypsum. In particular, Ca, Mg, and Fe are important to neutralize acidified water and to fix CO2 as a carbonate.
The geochemical characteristics of deep groundwater, or formation water, are essential in all processes of geochemical trapping in an open aquifer CO2 storage. We have been constructing a database of groundwater chemical compositions in deep aquifers in Japan (“Formation-water database”). The database have two major objectives; (1) to be a dataset on groundwater of reservoir depths for evaluating CO2 solubility; and, (2) providing model water compositions for geochemical modeling and experiments in our study of underground CO2 storage. More than 2600 datasets are collected from literature on geochemistry of groundwater reported from 10 selected areas in Japan; the areas of investigation include populated cities in which large point sources of CO2 are located. The accumulated data indicate that groungwater of reservoir depth (>800m) is generally dilute in composition compared to average seawater, suggesting a high potential of CO2 solubility. Systematic geochemical differences are also observed between groundwater hosted in marine and freshwater sediments.
This paper reviews the present status of studies on water-rock interactions associated with the geological sequestration of carbon dioxide (CO2), and stresses the need for a more realistic kinetic approach. Most studies on CO2-water-rock systems based on laboratory experiments, numerical simulations, and natural analogues suggest the fixation of injected CO2 in formation water as stable minerals. However, these results assume the formation of final equilibrium phases by chemical thermodynamics. The slow geochemical reactions and their reaction pathways should also be considered for a detailed prediction of the time scales needed for CO2 fixation. The slow reaction rates, uncertainty about reactive surface area, and difficulty of controlling surface and environmental conditions are major problems associated with this kinetic approach. Phase-shift interferometry (PSI) , which observes the surface topography of minerals at the nanoscale level, has the potential to overcome some of the above difficulties for determining the precise dissolution rates of minerals with knowledge of surface topography.
Environmental assessments and safety control during and after CO2 injection are essential for CO2 geological storage, and we are required to evaluate long-term environmental changes and safety. However, long-term changes are difficult to detect directly because the leakage of CO2 is expected to be small and the evaluation is, sometimes, requested to cover more than 1,000 years. To solve this problem, a natural-analogue study, which inquires into environmental changes at present through a comparison with past geological phenomena, is one possible approach. When the Matsushiro earthquake swarm began in 1965, a large volume of subsurface water accompanied by CO2 gas was discharged along fracture zones. A natural-analogue study on the CO2 discharge during the earthquake swarm should be helpful to create a scenario of leakage and a guideline for the safety of CO2 geological storage. Surveys of the CO2 content in soil gas and CO2 flux emissions from the surface were carried out with carbon isotope ratio measurements to understand the current state at Matsushiro, and to make a conceptual model for environmental assessments and safety control. From geological and geophysical points of view, it is said that deep water gushing out from the surface caused the swarm of earthquakes. As this deep water is still gushing out, we planned to measure CO2 concentrations in soil gas and CO2 flux to examine present CO2 activities at Matsushiro. Because CO2 in the soil is also produced by activities of microbes, however, we decided to measure the isotope ratio of the carbon to distinguish CO2 in deep groundwater origin from that produced by microbes. We selected five survey lines and three survey areas based on previous geochemical measurements and fissure distribution during the earthquake swarm, and measured CO2 concentration in soil, CO2 flux, and isotope ratio. Although there were survey points on the thick fan deposit where CO2 concentration in the soil and CO2 flux were high, the isotope ratio indicated that the carbon is produced by the activity of microbes. On the other hand, the isotope ratio of the samples collected from the thin fan deposit area shows deep subsurface water as the origin. An investigation well was drilled into the basement. Subsurface water samples were collected near the bottom of the well in the igneous rock formation. Geochemical analyses and carbon isotope ratio measurements show higher concentrations of chloride and abiogenic CO2, indicating that groundwater of a deep origin with CO2 is still rising. We are now making a conceptual model of hydrogeological history at the next step. This natural analogue study of CO2 seepage could indicate the importance of understanding shallow hydrogeological characteristics in a CO2 storage field.