Abstract

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 albitization.

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.

Introduction

The Minami-Nagaoka gas field, 3 miles (north-south) by 1 mile (east-west), is located in the northern central region of Japan (Fig. 1). There are 17 wells completed in the main reservoir, occurring at a depth of 12,500 ft with the initial pressure of 8,100 psi and the temperature of 350°F. Deep-seated volcanic rocks (rhyolite) of the Middle Miocene age form this gas reservoir and are referred to as "green tuff" be cause of the characteristic greenish color. As a consequence of repeated volcanisms, rhyolite eruptions were deposited one after another forming a thick formation (over 2,600 ft) with a drastic change of lithofacies, divided into lava, breccia, and hyaloclastite.1

Core analyses show that the permeability of breccia facies ranges from 0.1 to 10 md, while most of the hyaloclastite facies yields low values of less than 0.1 md. The average porosity is around 15% in both facies. Lava facies is characterized by the average porosity value less than that of hyaloclastite but the permeability value between the other two facies. Since the volcanic rocks are naturally fractured, in-situ values are supposed to be lower than the core measurements.

As for the productivity, most of the productive wells are located in the southern region of the field and the northern region does not yield economic gas production. Indeed, the breccia facies, which is most favorable as a reservoir rock, is found more frequently in the south than in the north. This biased distribution of productive zones, however, has not been well understood or explained.

In addition to the lithofacies change, intense alteration made the reservoir much more heterogeneous. Due to the severe heterogeneity, the discontinuous layers,2,3 and the large formation thickness, well-to-well correlation is hard to establish by using conventional means, such as log responses. To keep objectivity in zoning the formation, additional petrological tools must be utilized.

This paper attempts to construct a geological model in an objective manner. The objectives are

  1. to gain a clearer view of the formation and

  2. to give a reasonable explanation to the regional difference in productivity.

The key strategy is to focus on the primary volcanic body and the secondary alteration.

Petrological Methods

Petrological tools employed in this study are the refractive index of plagioclase and the microscopic texture of the subject volcanic rock. The primary volcanic body distribution is inferred through the analyses of the refractive index of plagioclase, which indicates the compositional character of magma, with an aid of the microscopic texture. Various types of alterations are inspected by examining the microscopic texture of the rock.

This paper attempts to construct a geological model in an objective manner. The objectives are

  1. to gain a clearer view of the formation and

  2. to give a reasonable explanation to the regional difference in productivity.

The key strategy is to focus on the primary volcanic body and the secondary alteration.

Petrological Methods

Petrological tools employed in this study are the refractive index of plagioclase and the microscopic texture of the subject volcanic rock. The primary volcanic body distribution is inferred through the analyses of the refractive index of plagioclase, which indicates the compositional character of magma, with an aid of the microscopic texture. Various types of alterations are inspected by examining the microscopic texture of the rock.

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