北海道天北地域の石油探鉱に関する地質学的考察

Abstract

Oil exploration has been carried out since 1914 in the Tenpoku Area, Hokkaido. Several oil fields were discovered in the early stage of the exploration. Main producing horizon of those fields was the Masuporo Formation however, all of these fields were minor in their scale bacause of the poor development of sandstone reservoir.During 1970 to 1975, the fourth five-year-exploration plan was carried out by the Government and three deep wells (MITI stratigraphical wells) were drilled after the basic seismic works in the area. These activities were followed by several seismic survey and two deep wildcats which were carried out by JAPEX.Based on the results of a frame work of the area, by reviewing available data obtained from the past exploration activities, the authors had some geological considerations for the petroleum exploration of the area and concluded as follows:(1) The result of the compilling work on the source rock evaluation, which is based on the hydrocarbon, non-carbonate carbon and their ratio, shows that the Wakkanai, Masuporo, Onishibetsu and Chikubetsu Formations have possibilities as oil source. However, data on the maturation are not enough to be discussed on the charactristics of the area in this stage.(2) Sandstone of the Masuporo Formation is the most possible reservoir rock. The sandstone well develops in the eastern half of the area such as the east of the Horonobe fault, core of the Masuporo anticline and lower part of the Masuporo Formation in the Omagari R-1 well however, in the western half, the sandstone is not well develops in such wells as Okumenashi SK-1, Masuporo SK-1D, MITI Wakkanai, Toyotomi R-1 and MITI Hamayuchi.(3) Palaeogene formations are missing in the central part of the Kiyokawa uplift where MITI Enbetsu was drilled. It is presumed that there is a higher possibility of the development of coarse clastic rocks in the Miocene formations, especially in the Masuporo Formation, around the Kiyokawa uplift.(4) Kiyokawa uplift and its surrounding area is located at the margin or near the boundary of the palaeogene sedimentaly basin. It is considered that the area situated on the isoclinal high in the Miocene and the migration of hydrocarbon is expected from both of the Miocene and the Palaeogene sediments.(5) It is considered that the most of the recognized anticlines have grown up since the Early-Middle Miocene on the base of seismic interpretation therefore, possible hydrocarbon migration is expected in the earlier stage of the sedimentation of the Miocene formations.(6) The average scale of anticlines in the area is about 5km2 of the clousure, which introduce about 1.3 million kl of oil reserve from the reasonable parametres.(7) Structures around the Kiyokawa uplift are considered to be prospective, Miocene sequence as main target and Palaeogene to upper Cretaceous sequence as second target. According to the results of the above, the exploration can be extended toward north to the Maruyama anticline via the Horonobe anticline.