Lithological analysis of volcanic rock by using the differential gamma ray log

At the giant oil and gas field in northern part of Japan, most of the reservoir rocks consist mainly of a volcanic complex, and the hydro-carbon productivity of formation is strongly affected by a degree of alteration of rock matrix and existing values of clay minerals in the formation. The acidic and basic rock can be discriminated by the use of a differential gamma ray analysis. The degree of alteration of basic rock can also be discriminated by means of 40K gamma ray value and the formation resistivity crossplot. The lava flow direction of underground formation may be estimated by this crossplot method with a correlation of several number of welds at the specific oil and gas field. (author)