We present the depositional environments and geostatistical modeling of the spatial distribution of mud content within a sequence stratigraphic framework for a CO2 reservoir at Nagaoka, Japan. Carbon dioxide was injected into strata interpreted as deltaic or coastal plain deposits characterized by upward-shallowing successions. The spatial distribution of the mud content, as estimated by geostatistical modeling, shows sandy sediments prograding from the lower to the upper part of the reservoir. Geophysical monitoring during and after CO2 injection suggests that CO2 migration within reservoir rocks is inhomogeneous; CO2 breakthroughs were observed at two observation wells, but were not observed at one observation well (OB-3), although the wells are located at similar distances from the injection well. The results of geostatistical modeling also indicate the heterogeneous distribution of muddy sediments. The heterogeneity of the CO2 distribution can be explained by the distribution of muddy sediments around well OB-3, which is located at the most distal (offshore) location. Genetic interpretations of the spatial distribution of mud can be useful for predicting and estimating the distribution of injected CO2 in a reservoir.
We investigated the petrology and geochronology of Pre-Neogene basement core samples obtained at Kukizaki (southern Tsukuba observatory well) in 2006 by the National Research Institute for Earth Science and Disaster Prevention (NIED). The depths of the excavated core samples of the basement rocks were 801-804 m, 944-947 m, 1,111-1,114 m, and 1,114-1,117 m. The basement rocks consisted mainly of mylonitized hornblende biotite tonalite (shallower cores) and hornblende gabbro (deeper cores). In the 944-947-m core, the mylonitic foliation dips northward at low to high angles (average dip, ~40°), as determined from an oriented sample. The thickness of the mylonite zone is at least 200 m. Kinematic indicators within the oriented mylonite sample indicate a top-to-the-southwest thrust/sinistral shear sense. The K-Ar hornblende age obtained on the gabbro (depth, 1,115 m)is 66.1±1.1 Ma, and the SHRIMP U-Pb zircon age of tonalite (depth, 803 m) is 86.3±0.6 Ma. Initial 87Sr/86Sr ratios (SrI), calculated using the U-Pb ages of whole-rock samples of both tonalite (depth, 803 m) and gabbro (depth, 1,115 m), are 0.7067 and 0.7073, respectively. These isotopic ages and SrI values of rocks from the Kukizaki core are concordant with those of the Ryoke granitic rocks, including the Iwatsuki mylonite obtained at a depth of 3506.6 m in the Kanto Plain, whereas the ages are discordant with the Tsukuba granitic rocks, which are younger in age (ca. 60 Ma) and show higher SrIs (>0.711). However, zircon fission-track age dating of the tonalite sample (depth, 803 m) yielded an age of 16.9±0.6 Ma (Miocene), which is discordant with the U-Pb age but is similar to the K-Ar biotite age of 17.65±0.29 Ma of the Iwatsuki quartz porphyry (depth, 3346-2864 m), suggesting that thermal effects of the acidic intrusion at Iwatsuki extend eastward to Kukizaki. Considering the isotopic data and the sinistral/thrust shear sense in the Kukizaki core, we infer that the mylonite zone in the Kukizaki core is the northeastern extension of the mylonite zone observed in the Ryoke belt along the Median Tectonic Line in the Kanto region.
A new geological map has been prepared of an area in Hiroshima where numerous debris flows occurred on account of heavy rainfall on 20 August 2014. Rocks in the area are mainly the Late Cretaceous Hiroshima Granites and its roof pendant, which consists of the Late Cretaceous Takada Rhyolites, a Jurassic accretionary complex correlative of the Kuga Group, and schists correlative of the Triassic-Jurassic Suo metamorphic rocks. All these rocks have been intruded by porphyritic rhyolite dikes. The most disastrous debris flows were due to the collapse of steep slopes composed of the porphyritic rhyolite dikes. Many other disastrous debris flows occurred because of small collapses of steep slopes along the ridge lines composed of schist that had experienced contact metamorphism, and also of porphyritic granite.