The Ishikari oil and gas producing depression of the surrounding region covers an area lying between 42°36' and 43°50' N. lat., and 141°30' and 142°15' E. long., which occupies the western part of central Hokkaido.
In the rolling hills of this region, 4 oil fields have only been developed and produced about 300, 000 kl in cumulative crude, up to 1955. After 1956, however, 2 oil and 3 gas new fields have been discovered by the result of the recent exploration on this depression, and stratigraphically most of the producing horizons are restricted to the marine Fureoi and Morai groups of middle to upper Miocene in age.
This report is showing 90 anticlines, 59 synclines, 6 tectonic zones and 8 uplifts on this area. From the isopath maps on the depression, the writer explains the subsurface structure at each formations of Miocene to Pliocene and so-called Paleozoic in age.
The structural development history are summarized as follows:
I. All the new found oil and gas-bearing structures were generated at later Atsuta stage, these were widespread generally during pre-Morai-post-Atsuta in time and developed to one of the peak at Morai stage, accompanied several flucturation such as clear differentiation of the sedimentary basin and remakable westwards transformation of the sinking belt. And on some parts of the dipression, the folding have been succeeded at Takikawa stage, and is completed at the Quternary age.
II. Some groups of the folding, it is distinguished there were the relation with several uplift zone at Takinoue-Fureoi stages after post-later Oligocene Poronai-pre-early Miocene Momijiyama in age. Few of these uplifts have been reflected the relief of the paleostructures in pre-Tertiary age, such as well-known Shintotsugawa and Umaoi anticlines. In the depositional position of the sedimentary basins, the former belongs on the shelf of post-so-called Paleozoic and the latter on the hinge line of Cretaceous to Paleogene Tertiary deposits. There are not a few unknown part on this area, but judging from the oil geologic condition as to deposition of sediments and the geohistorical factors of the established facts, the writer thinks that it should be more concentrated future prospecting and exploration for petroleum and natural gas at the folding zone of eastern part of this area.