Neogene argillaceous rocks of Nishikurosawa, Onnagawa, Funakawa, Tentokuji, and Sasaoka Formations are well developed in Akita Oil Field, Japan. The composition of minerals in about two hundreds samples obtained from some well were analyzed by X-ray diffractometer. The mode of occurrence of the constitutions were also observed under the microscope. From these data, the writer classified these samples into the following two rock groups. These rock groups include Group I (clay mineral-silica mineral rock group) and Group II (zeolite-bearing clay mineral rock group). The rocks of Group I are composed of montmorillonite, illite/montmorillonite mixed layer clay minerals, silica minerals (α-cristobalite and/or quartz) and plagioclase with concomitant illite, chlorite and others. The rocks of Group II are composed of not only the same constitutions in the rocks of Group I but also zeolites (clinoptilolite and/or analcime), and of more or less volcanic glass and diatoms which are not so common in the rocks of Group I. Progressive transformation of (1) α-cristobalite to quartz, (2) montmorillonite to illite/montmorillonite mixed layer clay minerals in both rock group and (3) clnoptilolite to analcime in Group II are evident from their zonal distribution in each well, graditional changes in amount with increase of depth in well and so on. With the progress of burial, the changes of the mineral assemblages also advance. Consideration and discussion on the distribution of authigenic minerals in each well lead to the conclusion that the sequential transformation of sillica minerals and zeolites may be attributed to the substantial burial depth and time. However, the question is still remained in the transformation of clay minerals. It would be true that the changes of the mineral assemblages took the different course with the conformity of the burial history of rocks, as is shown in CA type and AC type sequences in Group II.
It has been generally accepted that kerogen is the source material of petroleum. The authors have performed experiments of isothermal pyrolysis as one of fundamental studies for the kinetic evaluation of petroleum souce rocks. Samples are the kerogen isolated from mudstone of the Onnagawa Formation in Oga Peninsula, Akita Prefecture, and Recent sea tangles. These samples and N2 gas are sealed in Pyrex tube, and pyrolised at 220°-300°C. The thermal decomposition of these samples are accompanied with the generation of bitumen, and oil is considered to be formed through intermediate formation of bitumen. The authors made a rection model based on the assumption that the generation of bitumen can be represented by the combination of several 1st-order reactions which occur independently and in parallel. This reaction model is compared with the experimental data, and then the results are as follows: 1) Only a part of kerogen takes part in the formation of bitumen (Fig. 1, Table 2, 3). 2) The generation process of bitumen is not a simple 1st-order reaction but is approximated by the above-mentioned reaction model (Fig. 2, 3, 4, 7, 8). 3) Characteristics of the decomposition of kerogen and/or generation of bitumen are represented in the forms of frequency distribution of kerogen that consists of several types of chemical bonds (Table 2, 3). 4) The proportion of chemical bonds represented by low activation energy in the building-block of Recent sea tangle is larger than that of Tertiary kerogen (Table 2, 3). 5) The main part of kerogen seems to show high energy level as the maturation of kerogen.