秋田・新潟のグリーンタフの変質と貯留岩性状

Abstract

Reservoir property of Green Tuff is controlled by the initial volcanic rock facies and alteration. Some hydrothemal alteration increases the secondary porosity and permeability. Altered basalt lava is the main reservoir in the Yurihara oil field. Four depth zones are defined by characteristic secondary minerals: 1) smectite, 2) thomsonite-corrensite, 3) laumontite-albite, 4) laumontite-prehnite-pumpellyite-chlorite. These alteration is estimated to have taken place at sub-seafloor right after the basaltic eruption by hydrothermal fluid originated from seawter. The main oil production zone is from the prehnite zone. High temperature (150-220°C) dehydration alteration have prevented porosity reduction caused by low temperature alteration. Green Tuff in the Katakai gas field is mainly composed of rhyolite. They are mostly recrystallized to form quartz, albite, sericite, calcite, dolomite, siderite, and chlorite. Rhyolite lava is rich in Na and albite, poor in K and sericite. In contrast, hyaloclastite (or pumice tuff) is rich in K and sericite, poor in Na and albite. The alteration in Green Tuff in the Katakai gas field is divided into dolomite-dominant type alteration, which is rich in Fe-Mg carbonate and poor in chlorite, and chlorite-dominant alteration. Dolomite type rhyolite lava is rich in intercystalline pore and dissolution pore, poor in pore plugging clay minerals, and serves as productive reservoir. Andesitic rocks in the MITI Sado-oki are suffered from diagenesis and hydrothermal alteration. The center of hydrothermal alteration zone is reached out by acidic fluid, and shows high permeability.