Abstract
The Cretaceous in the central Hokkaido are grouped into the upper, middle, and lower Ezo groups.
These groups overlie the Jurassic Sorachi group and are covered by the Paleogene Ishikari group. The dominant carbonate rocks of these groups are limestones such as micrite, sparite and biolithite in the form of layers and nodules. Limy lithic wackes occur sometimes in the middle Ezo group. Carbonate facies are found remarkably in Tomitoi formation of the lower Ezo group, and Saku and Shubu (main part) formations of the middle Ezo group.
Average effective porosity of the layered carbonate rocks in the studied area is approximately three per cent and is considerably lower than that of oil-bearing Neogene Tertiary in the northwestern Honshu. The average ratio of carbonate rocks in thickness to total outcrops is nearly five per cent and is almost equal to that of the northwestern Honshu. These data indicate that the Cretaceous carbonate rocks in the studied area have a lesser character as a petroleum reservoir.
