1997 Volume 62 Issue 1 Pages 69-79
The following four conclusions were deduced particularly from the examination of vitrinite reflectance (Ro) data from MITI exploratory test wells conducted by the Ministry of International Trade and Industry together with some other data from Japanese oil companies.
(1) The regression equation between Ro and maximum burial temperature formulated by disregarding the effective heating time (Barker and Pawlewicz, 1994) is not applicable to the estimation of the temperature gradient of the Tertiary and Quaternary Systems in Japan, because there is significance change of Ro with time within 20-30m.y.
(2) Correlation between Ro and maximum paleotemperature was estimated by isomerization of sterane and hopane biomarkers in the MITI Hamayuchi well core (Sakata and Akiyama, 1989). Maximum paleotemperature gradients of 23 wells in Japan were calculated based on the above correlation data. All of the calculated gradient values are much higher than the measured present temperature gradient values (Table 1). This result suggests that the poor correlation is due to too few Ro measurements in the MITI Hamayuchi core, while the estimation method of maximum paleotemperature using isomerization of sterane and hopane biomarkers is reliable.
(3) The maximum paleotemperatures estimated from the nomogram of the MITI Shimoigarashi well of Niigata Prefecture (Sekiguchi and Hirai, 1980) show good agreement with the measured present temperature gradient in many wells drilled in the Niigata plain. The area around Mt. Yoneyama in the central part of Niigata Prefecture, which shows marked igneous activity in the Pliocene has a high paleotemperature gradient suggesting much higher heat flow than at the present.
(4) The kink in a two-segment vitrinite reflectance profile in MITI Takadaheiya, MITI Kubiki and many other wells in Niigata Oil Field is restricted to the Middle Miocene Nambayama Formation, suggesting perturbation in the thermal gradient caused by heat transfer processes associated with abnormal high-pressures as was discussed by Law et al. (1989) and Hunt (1995).
