南長岡ガス田における火山岩貯留岩の分布と変質作用

Abstract

Volcanic rock units in the Minami-Nagaoka gas field are very heterogeneous because of their facies change and intense alteration. ln this study, a geological model of the Minami-Nagaoka gas field is constructed to explain such heterogeneity for optimizing further exploitation. This model emphasizes two aspects of this volcanic reservoir. The first is distribution of primary volcanic body, and the second is secondary alteration processes. Primary volcanic body is identified on the basis of the difference of the primary plagioclase type in phenocrysts. This volcanic rock is divided into 6 zones according to types of primary feldspar in phenocryst; zone 1, oligoclase; zone 2, oligoclase with labradorite and anorthite; zone3, albite; zone 4, oligoclase and andesine; zone 5, labradorite and anorthite; zone 6, albite. It is clarified that the very productive intervals are within the glassy rhyolite of “zone 3” which extends widely in this field. The glassy rhyolite in zone 3 of the southern area is characterized by perlitictexture, and dissolution pores are dominant in such glassy part. In the northern area, such glassy rhyolite underwent intense silicification and dissolution pores are underdeveloped compared with those in the southern area. It is inferred that such heterogeneity of porosity is due to the difference in recrystallization of glass by the influence of the overlying volcanic activity. Dissolution pores are generated by the process of montmorillonitization in which cations such as potassium and sodium leached out. Montmorillonite changes to sericite (illite) when provided with potassium and to chlorite with iron and/or magnesium. In the lower part of zone 3, chlorite occurs as fill of the pores. This means that the iron and magnesium leached from underlying mafic or intermediate rock units deteriorated reservoir quality with chlorite precipitation in the lowermost interval.