Stable carbon isotope ratio of 13C to 12C is applied to problems of geochemical correlation of crude oil to source rock on the assumption that isotope ratio of kerogen is heavier by ranging from 0.5 to 3‰ in δ13C values than that of oil when oils and kerogens are genetically related. The accumulated oils have δ13C values ranging from -23 to -22‰ and, consequently, those of source kerogens are expected to be in range from -23 to -19‰ in the Niigata basin. The source kerogens appear at two stratigraphic horizons. One of them is the Nanatani Formation to the lower part of the Teradomari Formation (source “D”) and the other is the top of the Shiiya Formation to the base of the Nishiyama Formation (source “S”). The source “S”, which is confirmed for the first time using stable carbon isotopes, is distributed in the area of the Niitsu, Minami Aga and Aga Oki oil fields, but it is absent in the area of Higashi Niigata and Matsuzaki gas fields. From this result, it may be inferred that the source “S” contribute to generation of the oils accumulated in the Shiiya and the Nishiyama Formations in these oil fields.
It is the purpose of the present paper to correct mistake in the equation and some discussion which were studied by author in this Journal, 49 (5) 1984. When the Horner build-up analysis of temperature is not applicable, the approximate formation temperature (TFeq) can be calculated from bottom-hole temperature (BHT) by the revised equations. TFeq=A•(BHT-25)+30 (BHT_??_25°C) A=0.21•GGBHT+0.73 Where, GGBHT is geothermal gradient determined from BHT. The usefulness of the above equations for determining formation temperature is not alterative as that on his previous paper.
The Kisoshisui Tokachi-Oki Well at a total depth of 4, 457.06m which is located offshore Tokachi in eastern Hokkaido, Japan was penetrated into the alternation of conglomerate, sandstone and mudstone, exceeding 2, 000m in thickness. This paper is a study of the stratigraphy, the depositional environment and the distribution of the above sequence in this well. The sequence is correlative with the Okoppezawa Formation of middle Miocene age. Numerous coarse grained sand beds are abundant in older sedimentary rock fragments and contain primary sedimentary structures such as graded bedding, lamination and rip-up clast. The formation in this well is characterized by a rapid sedimentation rate of about 1, 330m/Ma, a lack of microplanktonic fossils and a trace of arenaceous benthonic foraminifers. The geological evidences indicate the Okoppezawa Formation in this well is typical turbidite facies in stagnant deep sea environment of deposition during middle Miocene age. The turbidite facies are distributed in a limited area along the western part of the Doto magnetic belt offshore Tokachi.