北海道石狩北部地域の油・ガス田 その地球化学と地質的背景

Abstract

Northern Ishikari area is one of the markedly distinguished areas for oil and gas fields in Hokkaido. Ishikari, Atsuta and Barato oil fields had been developed and the active exploration since 1983 resulted in the discovery of two gas fields such as Minamikanazawa and Minenobu gas fields.This area is divided into 3 tectonic provinces; The Western, Central and Eastern on the basis of its structural characteristics. The While the Western and the Eastern have clear NNE-SSW and N-S trending folds, the Central has undirectional dome structure and pre-neogene basement at shallower depth. The oil fields are situated in the Western and the gas fields in the Central. Oil and gas occur in the Neogene sandstone reservoirs of which the distribution is limited locally.The change of palaeobathymetry such as shallow in Ponsubetsu age, deep in Atsuta age and shallow in Bannosawa age is assumed on the basis of well and surface geological data and seismic record sections. In addition to this assumption, it is inferred that the area of oil fields was deeper than that of gas fields at least during the deposition of the formations of the above three ages.Source rock characteristics represent a good correlation to the palaeobathymetry. Carbon isotope ratio in kerogen, which reflects the type of organic matter, is particularly concerned with the palaeobathymetry.The source rocks in Barato SK-1 are characterized by more oil prone type than that in Minamikanazawa SK-1 on the basis of the data of S2/S3, carbon isotope ratio in kerogen and sterane composition. In Barato oil field, most part of kerogens in sediments are considered to be the source of oil accumulated, on the assumption that isotope ratio of kerogen is heavier than that of oil when oils and kerogens are genetically related.On the south of Minamikanazawa gas field, deep depression exist and the type of source rock deposited there is estimated to be oil prone. Nevertheless the hydrocarbon produced in Minamikanazawa gas field is only dry gas. This suggests that the hydrocarbons accumulated in Barato oil field and Minamikanazawa gas field are generated near the sturucture. One of reasons for these phenomena is thought to be due to the local distribution of sandstone reservoirs as pathways for generated hydrocarbon.