火山岩貯留岩の成立条件 孔隙の形成と保存

Abstract

In terms of generation and preservation of porosity, volcanic-rock reservoirs in Japan are studied. A model of generation and preservation of dissolution pores during interaction between volcanic rocks and sea- or formation-water is discussed.The volcanic rock reservoirs can be divided into the primary-porosity type, the dissolution-porosity type and the fracture-porosity type, based on the origin and preservation mechanism of pores. The first type was observed in Miocene pyroclastic rocks in the Yufutsu Oil and Gas field, Hokkaido. They are mainly composed of scoria and fine volcanic glasses which erupted on a volcanic island. Their pores are neither filled with clay minerals nor collapsed by compaction, but supported by unaltered brittle volcanic fragments.The second type was recognized in the Ayukawa Oil and Gas field and in the Akita area, northeastern Honshu. A reservoir in the Ayukawa Oil and Gas field occurs in dolerites which intruded into unconsolidated argillaceous sediments. Its pores were formed by dissolution of feldspar and clay minerals which once replaced mafic minerals. The framework that preserves pores is an initial texture which was strengthened by secondary K-feldspar precipitated around the original feldspar.Reservoirs in the Niigata area occur in Miocene acidic rocks which erupted on a seafloor. Their pores were formed by dissolution of groundmass and feldspar. The framework that preserves the pores is formed by aggregated authigenic quartz such as spherulite.The third type was observed in the Yurihara Oil and Gas field. One of its reservoirs was formed in highly altered basalt which erupted on a seafloor in the Miocene. Its pores were formed by fractures in porous lava flows which were strongly altered to become characteristically brittle.Microscopic observation of the dissolution pores and the experiments by the previous studies in water-rock interaction suggest that the dissolution pores and the framework could be formed with the formation of authigenic minerals during the water-rock interaction when igneous rocks erupted in the seawater or intruded into unconsolidated sediments. If this model is followed, the more water the igneous rock reacts with, the more dissolution pores and authigenic mineralization may occur. Accordingly, constant fluid flows may cause the generation of dense distribution of dissolution pores.