In diagenesis of mud deposits, abnormal high pressures are expected at certain depths and stages of sedimentation. The high pressures are considered to be due to excess free water released from rigid ice layers of clay and zeolitic minerals in alterative temperature and overburden pressures. In addition to the process which schemed by W. D. KELLER as montmorillonite plus potassium to illite and glauconite mica, the alteration of vitric tuff to clinoptilolite and clinoptilolite to analcime were very common in the Funakawa Formation of the Akita oil-field. High pressure mudstones were investigated by core analysis, well-log analysis, X-ray measurements and bottom-hole evidences. A reservoir is built up with fluid migrated into porous media along innumerable of fractures and pore spaces opened by excess high pressures and extensional stresses in the muddy rocks, of which pre- and post-alteration mudstone close and hold the containing pressure. Calculations for several models of high pressure domain show that reservoir maintenance was continued for a period of million years for an ordinary impermeable cap rock.
This is the fourth of a series of paper reporting the results of a study of the porphyrin pigments and polycyclic aromatics in Neogene Tertiary rocks, Northeast Japan. The present paper gives the results of analyses on 92 muddy rocks from the Chuetsu oil field, Niigata Prefecture (Fig. 1). These data, along with earlier analytical results from the previous reports (Taguchi, Sasaki & Ushijima, 1969; Taguchi, Sasaki, Endo & Sato, 1970; Taguchi, Sasaki & Ushijima, 1970) are discussed from the standpoint of sedimentological study. The tendency of the stratigraphic distribution of these compounds in the study area is generally the same as in other areas reported before. However, the average contents of the nickel porphyrin of the Teradomari Formation in Higashiyama and Nishiyama areas show rather different value from correlatives of the Teradomari in the other areas and A-type of vanadyl porphyrin is detected preferentially not only in the early or middle Miocene strata as reported previously but also in the younger formation as the Shiiya of late Miocene. Polycyclic aromatics were found with 100 per cent positive indications in the study area, and showed higher concentration in the samples of the Nanatani Formation in the Higashiyama area. Particularly, it is very interesting that histogram which shows spectral variations in wavelengths of absorption peaks for polycyclic aromatics in the Higashiyarna area has similar pattern to those in some coal fields. The results mentioned above are showed more clearly and are compared with those of other areas by employing the triangular diagrams as seen in Fig. 6. It is clear that both areas of Higashiyama, Niigata District and Kanita, Aomori District have the different patterns from those of Akita and Yamagata Districts. The two unique patterns in Higashiyama and Kanita areas are called “Higashiyama-type” and “Kanita-type” respectively. Other patterns belong to “Yashimatype” which were reported previously. The characteristics of Higashiyama-type may be explained well by the fact that Higashiyama area is located at the marginal part of the Niigata sedimentary basin. Imokawa area also lies in the marginal part, but it does not show similar pattern to the Higashiyama. This may be attributed to the fact that Imokawa area corresponds to the invasion area which have developed roughly in the NW-SE direction in the time of deposition.