HYDROTHERMAL LAUMONTIZATION AND MICROFRACTURE FORMATION IN RESERVOIR ROCKS AT THE YUFUTSU FIELD, HOKKAIDO, NORTHERN JAPAN

Abstract

Reservoir rocks at the Yufutsu oil‐ and gasfield (southern Hokkaido, Northern Japan) comprise Eocene conglomerates and Early Cretaceous granitoids in which networks of fractures < 0.02 mm across are widespread. These basement reservoir rocks were fractured during normal faulting in the late Oligocene — early Miocene. In the early to middle Miocene, the fractured rocks are thought to have been permeated by acidic formation waters derived from Eocene coal measures. Laumontite (a Ca‐rich zeolite) and smectite/illite (S/I) mixed‐layer clay minerals were precipitated in the microfractures and occluded them.The Yufutsu field is located on the western margin of a duplex structure formed during Pliocene or later shortening. As a result of this regional compression, the preexisting “early” microfractures which had been occluded with structurally‐weak laumontite and S/I mixed‐layer clay mineral aggregates reopened as “late” microfractures. Natural gas, which had been generated in deeply‐buried Eocene coal measures in the synclinal area to the east of Yufutsu field, migrated westwards and accumulated in the re‐fractured reservoir rocks.The duplex structure in central Hokkaido probably extends to the unexplored offshore Iburioki structure to the south. Fractured reservoir rocks similar to those at the Yufutsu field may also occur at Iburioki.