Abstract
The vitrinite reflectance (Ro) and statistical Thermal Alteration Index (stTAI) of the 12 samples from MITT Sanriku-Oki borehole drilled in the Pacific Ocean off the east coast of Honshu Island were measured to investigate organic maturation and the effect of unconformity on organic maturation in this borehole. This borehole is composed of sediments from the late Cretaceous to Quaternary and has two unconformities at 1, 683 and 3, 535m in depth from sea level. The vitrinite reflectance shows a general tendency to increase with increasing depth having some dispersion, and is not influenced by the unconformities. Although the statistical Thermal Alteration Index regularly decreases with increasing depth, it made a large difference between in the upper and in the lower zones than the unconformity at 3, 535m. This unconformity means that sedimentation was interrupted for about 9 million years between early Maastrichtian and early Late Paleocene. It is suggested that Ro is a maturation parameter more sensitive to temperature than to heating time, but oppositely stTAI is a parameter more sensitive to heating time than to temperature. It is shown with both data of Ro and stTAI that organic maturation reaches the stage of petroleum generation, 0.5% in Ro and 110-145 in stTAI, at 3, 400m in depth.
