If we are well informed of the time of formation of certain petroleum deposits, it would be possible to save some of the exploration drillings in the adjoining areas, because we should be then able to condemn in advance the structures, which had not been formed in time, to be barren. Bisides, the knowledge about the time of accumulation of petroleum in general, is expected to throw much light upon the fundamental probeems of petroleum geology, i. e., the problems of the origin and migration of petroleum. However, it is usually very difficult to make clear the exact time of formation of petroleum deposits. That the time of accumulation postdates the time when the trap came out to being is an axiom, but it does only tells the earliest possible time of the formation of the deposits. The saturation pressure of the oil of a pool, in connection with its reservoir pressure and its volume, may, in some instances and under certain assumptions, tell more exactly the time of the accumulation. If the oil in a pool shows different property at the different portions of the pool, there may be some possibility that the difference tells the course of migration of the oil and eventually the time of accumulation.
1) Sedimentary basins in Akita region are divided into several structural basins, and there are many oil fields. The existence of these oil fields is closely related to the genetic process of these structural basins appeared on shelf zone in the Miocene. 2) The extractable organic matter of mudstones increases toward the geologically lower horizon and shows two prominent peaks; one is in the Onnagawa formation and the other in the lower part of the Tentokuji formation. The main productive reservoirs have been discovered in both formations. 3) The resemblance of each reservoir conditions of various fields suggests similarity in genetic process of most oil fields. 4) Growth of each oil field structure occurred concurrently with volcanic activity. 5) Therefore, it is concludcd that most structural traps were formed at about the time of the organic movement which occurred soom after the deposition of the formation containing main reservoirs.
Main oil reservoirs of the Sarukawa field are of the lower Tentokuji formation. In this paper, the writer discusses the time of their formation, considering (a) porosity of mudstones calculated from sonic logging data, (b) isopach and depositional conditions, and (c) nature of crude oil. The writer concludes that oil migration had been initiated before the end of the early Tentokuji age, that the formation of oil pools had finished before the late Tentokuji age, and that part of oil remigrated after the Shibikawa orogenesis.
In this paper the writer describes a structural history and accumulation of oil in Akita plain which is one of the most important area in Akita sedimentary basin, northeast Japan. The first movement in this plain occured at Yabase structural trend in age of last Funakawa stage, and then a large amount of oil could migrate into the reservoir rocks of Onnagawa formation in the Yabase oil field. In last Tentokuji stage, there were two main anticlines in this plain, one of them at Yabase district and the other at Asahikawa, but the shallow pool at Asahikawa was eroded out, while the pool of Tentokuji formation in Yabase was preserved.
In this paper, the writer discusses the time of oil accumulation in the Nigabo oil field. At Kotaki and Kamihama pools of this field, oil accumulated at about the time of the folding of pre-Sasaoka stage. Even though oil had once accumulated to form pools, a greater part of it was driven out of them by erosion, while oil accumulated almost concurrently with the folding of the last tectonic phase of the Shibikawa stage at Innai and Katsurazaka of the same field. Investigation into oil and gas accumulation in the Nigabo oil field arrives at the conclusion that the time of structural movement is another factor which controlled the oil accumulation. The reservoirs in the Funakawa formation at Innai pool is one of such examples. Even though these reservoirs were folded with black shale, oil could not accumulate in them, because the folding was too late to make an oil pool.
In this paper, the writer explains (a) the relationship between the Akita sedimentary basin and the Shonai basin, (b) the types of the oil and gas traps in the Shonai basin, and their geological positions in this basin, (c) the characteristics of each field, and (d) the time of migration of oil and gas. The writer concludes that the oil and gas pools are found near the hinges or ridges in the sedimentary basin, and that the migration of oil and gas occurred concurrently with the sedimentation of the source rocks.
It is believed that the distribution of chemical components in crude oil gives expression greatly to the geological environment, and that the chracteristic composition is related to the geological age. Oil produced in Akita area is considered to be related to each horizon from the viewpoint of oil evolution, and some of them are found to have been biochemically corroded by bacteria brought by meteoric water, showing evident increase in iso-paraffins.