Abstract
The authigenic minerals in sedimentary rocks commonly consist of zeolites, silica minerals and clay minerals as shown in Fig. 2. These minerals are mainly formed by burial diagenesis. The reaction temperatures to form these minerals are particularly affected by physical factors; heating time and geothermal gradient. The transformation of the authigenic minerals is similar to the progress of maturation of organic matters in sediments.
From this study, the variations of the authigenic mineral zones in the Cretaceous and Tertiary sediments of Hokkaido, Akita and Niigata oil fields are correlative with the various parameters for determining maturation levels in organic matter. The parameters include vitrinite reflectance, elemental analysis and colouration of kerogen, the ratio of extractable hydrocarbon to organic carbon and carbon preference index. For example, the first occurrences of clinoptilolite, analcime, albite and montmorillonite-illite mixed layer mineral are 0.35, 0.53, 0.68 and 0.50% in vitrinite reflectance, respectively.
The "oil generation zone", which corresponds to vitrinite reflectance of 0.5 to 1.3% and is defined by a large ratio of extractable hydrocarbon to organic carbon, approximately belongs to the analcime zone to the albite zone or the mixed layer mineral zone to the illite zone. The maximum of the "oil generation zone" is distributed in the albite zone and/or the illite zone.
The authigenic minerals are considered to be an important parameter to indicate the maturation levels of organic matter in sediments.
