Abstract
1) In the Niigata Sedimentary Basin and the surrounding areas, the basement rocks of the Green Tuff formations belong to the Ashio and Joetsu Belts in the northern part, and to the Maizuru Belt in the southern part. Recent paleontological researches indicate that the formations of the Ashio Belt are mostly of Triassic to middle Jurassic ages. The Joetsu Belt is divisible into two zones: Tanigawadake and Katashina, the latter being correlatable to the Maizuru Zone.
2) The middle Miocene Jonai Group and its correlatives distributing in the three areas (Jonai, Yuzawa and Tanigawadake-Minakami) overlie the rocks of the Joetsu Tectonic Belt. They were formed in the same sedimentary basin. Recently plant and foraminiferal fossils were discovered from them. The common characteristic volcanism was eruption of basaltic magma in a marine environment. The successive unique sedimentation of conglomerate was also a common event throughout the three areas during successive ages.
3) The reservoirs of the Katakai gas-field are rhyolites of the Nanatani Green Tuff, which is composed of lavas and pyroclastics of basalt, andesite and rhyolite. Rhyolites are thickest and are petrographically classified into five types (A, B, C, D and E). Rhyolitic hyaloclastites are intercalated between rhyolitic lavas, and seem to develop especially on the rather steep slopes underlain by lavas. They represent possibly the submarine pyroclastic flow deposits.
Alteration (silisification, chloritization, albitization and sericitization) is mainly due to hydrothermal action, and partly to diagenesis. Carbonatization is conspicuous, and the paragenesis of the carbonate minerals (calcite, dolomite, siderite and ankerite) is peculiar in different horizons, and might have concerned genetically to the high content of CO2 in gas phase and of HCO3- in formation water.
