Petroleum Geologic Evaluation for the Cretaceous System along the Central Parts of Hokkaido, Japan

Abstract

The purpose of this paper is to discuss the oil possibilities of Cretaceous system along the central parts of Hokkaido, chiefly concerned v, ith detailed discussion and consideration related to the following points of view. First, as the direct evidect of potential Cretaceous system, the occurrences of oil and or gas showings primary in situ or originated from the Cretaceous system were investigated and then their crude oil and gas in character were examined, hereby from the viewpoint of geochemical evolution of the crude the comparison of crude bearing in the Cretaceous system to the one produced from Neogene in Northeast Honshu or Paleogene were taken place. From this results, it became clear that the characters of the crude bearing in the Cretaceous system were light to medium, and extraordinaly abundant in gasoline and kerosene fraction as compared with the crudes belong to the tertiary system. By gas chromatic analysis, they are abnormal type in crude oil which are rich in cyclohexane and cyclomethylhexane, naphthenic compounds, poor in below n-hexane and rich in n-octane, this indicates that they belong to older type of oil and it is as well believed that they have been subjected to the distillation by migration or thermal distillation action for a long time. Furthermore, it is interesting that the crude oil contained in the Poronai formation, Paleogene are similar in character with those belong to the Cretaceous system. Showings are most predominant over the Hakobuchi group and the middle Yesso group are next to them. The sandstones as the valuable reservoir rocks have good developed in the Hakobuchi group qualitatively and qualitatively as well. To the contrary, the sandstones belong to the middle Yesso group have their predominace with a view of their wide distributional area and continuity. The mudstones supposed to he the mother rocks of oil are almost classified to “Q-type mother rock” (correspond to upper Teradomari and Funagawa formations), and it seems to indicate that either the migration of petroleum were relatively fast through them or the petroleum were squeezed out at the early stage from them. As a whole, as to the organic contents extracted from the mudstones of the Cretaceous system, they are inferior to the one found in oil fields of Northeast Honshu but concerning the total amounts of mudstones, those are superior to these, and the total original reserves estimated by Emery's method amount to 731, 000, 000 tons, and it supposed that they show about 5, 4 folds as much as the sum total reserves in Niigata and Akita prefectures. Several uplift areas are infered by means of the preparation of the isopach maps and hereby the distribution of oil and or gas showings are concentricaly abundant along the marginal parts of the sedimentary basins situated around these uplift areas or west of them. Their distributions are independent of the structures or are apart from the anticlinal axis found on surface today. These differ from the occurrences of showings originated from Tertiary system and so it is suggested that the petroleum migration may have taken place prior to or earlier stage of the deformation formed by Cretaceous system today. Moreover the possibilities of cretaceous oil seem largely promising for accumulation by migation beneath the present Tertiary system, along the marginal parts of the sedimentary basins located west of uplift areas or around them.