Altered rhyolites as oil and gas reservoirs in the Minaminagaoka gas field (Niigata Prefecture, Japan)

Abstract

Subsurface geology and petrography of volcanic rocks of the recently discovered Minaminagaoka gas field show that good reservoirs in the field are altered rhyolites of the mid-Miocene Nanatani Formation. The Nanatani Formation is composed of mudstone, lava and pyroclastic rock with a composition of rhyolite, andesite and basalt. Rhyolites are divided into rhyolites A, B, B', C and D according to petrographical features. Rhyolite A is a hyaloclastite and pumiceous tuff of glassy rhyolite. Rhyolites B, B' and D are aphyric rhyolite lavas characterized by needle-like plagioclase in groundmass. Rhyolite C is perlite lava. Rhyolites B, D and C are suitable for reservoirs, whose pores are not primary, but are secondary macroscopic druses and micropores formed by hydrothermal alteration. Micropores are openings among crystal grains of quartz and albite and are several tens of micrometers in size. Alteration of rhyolitic rocks is of hydrothermal type and is characterized by a smectite (or smectite/chlorite mixed-layer mineral)-sericite-albite-quartz-carbonate mineral assemblage in the upper zone, but clay minerals in the lower zone are chlorite and sericite. Alteration of andesite and basalt is similar to that of rhyolite in general, but in the Kitafukazawa bore hole, andesite and basalt below 4640-m depth have chlorite-laumonite-prehnite-(pumpellyite-epidote) assemblage which is correlated to the assemblage of the higher part of the laumonite subfacies of zeolite facies. Formation temperature of secondary quartz, estimated by the measurement of the filling temperature of fluid inclusions, is ∼ 150°C and is nearly the same as the present temperature at the bottom of the hole. From the geological environment, characteristics of alteration and paleogeotherm, formation of rhyolite reservoirs and migration of oil and gas can be summarized as follows: (1) formation of volcanic piles by submarine volcanism of rhyolite, andesite and basalt, and deposition of mudstone around the piles; (2) generation of hydrocarbon in mudstone along with increasing temperature related to volcanism; and (3) formation of rhyolite reservoirs by hydrothermal alteration, followed by lateral migration of oil and gas into secondary pores.