The deep-gas field in the Sekihara-Katagai area recently discovered is situated 10 kilometers southwest of the Nagaoka City. The reservoirs of the gas field are rhyolites of the Nanatani Formation of middle Miocene. Lower formations than the Nanatani Formation are not clear as these have not yet been drilled. The Teradomari and Shiiya Formations of upper Miocene and the Nishiyama, Haizume and Uonuma Formations of Pliocene-Pleistocene overlie the Nanatani Formation, and the first three are mainly composed of several kinds of andesites in this field. These andesites are more strongly altered in compared with those occured in the Higashiyama anticline area.
The Nanatani Formation is composed of mudstones and green tuff, which is basalt, andesite and rhyolites. Rhyolites are the thickest member and from petrographical characters are classified into the following four types: rhyolite A, B, C, and D. Rhyolite A contains hyaloclastites and pyroclastics and rhyolite B, C, and D are lava. Rhyolite C is perlitic and rhyolite B and D are characterized by needle-like plagioclase in groundmass. SiO
2 and K
2O contents (calculated as H
2O free) of rhyolite B and C are 66-77 and 0.16-2.9 weight per cent respectively and the latter is much smaller than those of fresh rhyolites.
Good reservoirs are rhyolite B, C and D, whose pores are not primary ones such as cooling cracks and joints of rhyolite bodies, but are secondary pores formed by hydrothermal alteration. Under hydrothermal condition, leaching of K and Fe in glass of rhyolite and recrystallization of quartz and albite from glass have taken place. Secondary pores are druse (vug) and channel coated by quartz, and micropores between crystal grains of quartz and/or albite.
Although many problems remained unsolved, relation between formation of rhyolite reservoirs and primary migration of oil, gas and water are infered as follows: (1) formation of elongated volcanic piles by products of submarine eruption of rhyolites and deposition of mudstones (source rock) around the piles, (2) generation of hydrocarbon with compaction of mudstone and abnormal increasing of temperature in related to volcanism, (3) primary lateral migration of oil, gas and water into secondary pores of rhyolite B and C and enclosure of them beneath impermeable rock (cap rock), which was transformed from rhyolite A by argillization.
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