In dealing with heterogeneous thick formations, objectivity of well-to-well correlation is often hard to maintain and this is the case for a volcanic formation, the Minami-Nagaoka gas field. To overcome this difficulty, the refractive index is utilized to identify the different types of plagioclase, with which the well-to-well correlation is established in an objective manner and the formation is divided into six volcanic bodies.

One of the identified bodies corresponds to the productive zone encountered in the southern region of the field. This volcanic body is glassy rhyolite and extensively distributed throughout the field, even toward the northern region, where the productivity is poor. To explain the regional difference in productivity, various types of alterations are examined, including devitrification, montmorillonitization, chloritization, illitization, and albit ization.

Identifying the primary volcanic bodies and examining the secondary alterations enables us to construct a geological model of the subject volcanic formation. Integrated geological interpretation yields a clear view of the formation and a reasonable explanation to the productivity aspects.

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