天北•羽幌地域の石油地質

深部構造特性と炭化水素ポテンシャル

抄録

The thick Cretaceous-Tertiary sediments which cover the Tenpoku and Haboro area, the northern part of Hokkaido, is believed to possess substantial hydrocarbon potential. More than seventy anticlinal structures have been recognized in this area, but to date only eight wells have penetrated the Paleogene section while four have drilled to Cretaceous sediments.
Four types of anticlinal structures are recognized as interpreted from well data and seismic profiles. The Kitakawaguchi feature represents the first type of anticline. Two wells have been drilled on this structure and seismic profiles are available. A gentle antiform exists as the Paleogene section drapes across thrust-faulted Cretaceous sediments. A more pronounced Neogene anticline is superimposed upon the pre-existing structure. The later structure was formed by flowage of Miocene shales and associated thrust-faulting. The Miocene shales exhibit abnormally high pressure and steep dips. Small rim synclines commonly exist on the flanks of this type of structure.
The Uttsu Anticline illustrates the second type of anticlinal feature. The MITI Enbetsu well was drilled to test this structure. The anticline was formed by deep-seated vertical thrust-faulting which affected Paleogene and Cretaceous strata. The formation of the anticline produced an important effect on Tertiary sedimentation. Similar features are thought to exist along the Horonobe and Magaribuchi-Toyotomi fault systems.
The Rikibiru structure is an example of the third type of anticline and is located in the southern part of the study area. The MITI Rumoi was drilled on the western flank of the feature. Dipmeter logs indicate that this deep Cretaceous structure is quite huge and broad with gently dipping flanks. The thick sequences of coarse-grained Miocene sediments deposited in this area contrast with the predominantly shaly strata seen to the north. The formation of the Rikibiru Anticline is thought to have formed due to loading of the thick Miocene section. Shale flowage did not occur in this area due to the lack of thick shale sequences.
The Maruyama Anticline illustrates the fourth type of anticlinal structure. No wells have been drilled to date on this type of feature. These anticlines are broad and gentle-dipping and are surrounded by rim synclines similar to those exhibited by the first anticline type.
Growing mechanism, timing and distribution of each type of anticline are discussed with the view of petroleum geology.