Characteristics of Reservoir Rocks in the Green Tuff

Studies on Hydrocarbon Deposits in the Green Tuff in the Niigata Basin, Japan Part 1

Abstract

Reservoir rocks in the Green Tuff, especially in the Mitsuke oil field, Yoshii-Higashi Kashiwazaki and Minami Nagaoka-Katakai gas fields, are examined using evaluation techniques such as petrological method, core analysis, log analysis and pressure analysis.
These reservoir rocks are mainly composed of subaqueous lavas of acidic volcanic rocks (rhyolite-dacite), and are classified into two types based on their characteristics of pore spaces. The first type, Mitsuke-Yoshii type, is characterized by common occurrence of mega-sized, primary vugs and fractures which play an important role in hydrocarbon production. On the contrary, the second type, Minami Nagaoka-Katakai type, mainly consists of micro-sized, secondary vugs, and its production behavior is similar to intergranular pores.
The areal extent of volcanic bodies having high productivity varies from several hundred meters to about 3 kilometers in diameter. Acidic volcanic rocks occur in restricted areas, and their distribution trend is consistent with the stress field in the Nanatani stage.