One of the most important problems in petroleum geology is time and mechanism of petroleum migration. Since many years ago many geologists have seemed to believe that mechanical compaction (except that in the earliest stage of diagenesis) is the main mechanism of petroleum migration ("theory" of primary migration in early diagenetic stage.) Recently such geochemists and geologists as Powers, Burst, Kartsev et al. insist that interlayer water expulsion from montmorillonite clay is the main mechanism ("theory" of primary migration in late diagenetic stage). This new "theory" is reasonable to some extent, but it cannot explain how the petroleum in carbonate source rocks has migrated into reservoirs,
In this introductory paper I cannot deny these two "theories", because plural "theories" may be consistent in case we study innumerable oil and gas fields in the world that are geologically various. At the end of this paper I propose a few methods of determining time and mechanism of petroleum migration and accumulation.

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Petroleum such as oil and gas generated from kerogen under low temperature will migrate primarily from source rocks to reservoir rocks. Much water will be necessary as a carrier of these hydrocarbons especially in the case of oil. Many investigators have considered that the expulsion of interlayered water from montmorillonite and mixedlayered mineral by transformation during diagenesis would be most important as a carrier of oil.
The writers, however, studied in detail the expulsion mechanisms of interlayed and interstitial water and the change of pores and grains (especially minerals) in argillaceous rocks during each stage of diagenesis, and examined the depth and temperature on generation of hydrocarbons in the various sedimentary basins in Japan and elsewhere. As a result, we reached another conclusion different from their theory on primary migration of petroleum as follows.
During the first stage of diagenesis (early compaction stage), argillaceous rocks have porosity beyond 30 per cent and generation of oil is not vigorous in general. Biochemical methane generated earlier will escape considerably by rapid dewatering of interlayered and interstitial water. During the early third stage of diagenesis (recrystallization stage), argillaceous rocks have 10 to 5 per cent porosity and generation of hydrocarbons is active in usual. Interlayered water by the transformation from montmorillonite to mixedlayered mineral will expel. However, the expelled water is small in content and also is difficult to migrate outside because of the lack of proper paths in the rock. Therefore, almost of oil generated during this stage could hardly migrate, although gas could move. During the late third stage of diagenesis, argillaceous rocks have porosity below 5 per cent and generation of oil become weak while formation of thermochemical methane is still active. Migration of oil from source rock will be quite difficult as the content of carrier water is very small.
On the contrary, during the second stage of diagenesis (late compaction stage), porosity of argillaceous rocks is between 30 and 10 per cent and content of carrier water is enough for migration of oil. Because dewatering of interstitial water is still active and expulsion of crystalline water from zeolite lattice and those of interlayed water from montmorillonite lattice are considerably vigorous with the progress of diagenesis. Almost the same content of interstitial and interlayered water will expel. Oil could migrate outside through the pores between grains though their genera tion was not so active in argillaceous rocks of Japan generally. Moreover, from the petrophysical viewpoint of reservoir rocks, it is reasonable to consider the migration of oil could occur during this stage.
From this conclusion, it is expected that the big oil will move during the period when the active generation of oil corresponds with the primary migration of water. The stage of primary migration of carrier water in Japanese oil fields is considered to be 1, 500 to 2, 800m at burial depth. Therefore, the possibility of formation of big oil pools will be high at the area where the geothermal gradient is between 3.0 and 3.5°C/100m. Previous evaluation of oil production in the exploration area is possible by the examination of geothermal gradient and vertical change of porosity in argillaceous rocks.

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Porosity dependence of mechanical properties of the clastic sedimentary rocks under high pressure has been shown in the experimental work by the authors (Hoshino et al., 1972; Hoshino, 1974; Hoshino and Inami, 1974). In the present report, such mechanical properties as deformational behavior, strength, compressibility, cohesive strength, and viscosity were studied in terms of porosity based on the results of the above work and also were compared with data from several deep exploratory wells in the central Niigata oil-bearing area, which reconfirmed the results reported by Inami and Hoshino, 1974.
In summary, progress of compaction is divided into three stages based upon differences of mechanical properties as follows, (Figs. 6 and 7). The first stage or viscous compaction-stage: The mineral grains within the clastic rocks are not touched each other yet. The rocks behave like soil and physically rather fluid-like material. The second stage or plastic compaction-stage: The mineral grains come into contact. The rocks consist of the mechanically stable framework constituting of these grains. Physically, they are plastic solid or Bingham substance. Third stage or elastic compaction-stage: Authigenic minerals appear among the mineral grains, and cementation is common in every part within the rocks.
It is considered probable that the most of migration of oil (explusion) are done in the second compaction-stage, which would also provide a favorable condition for accumulation of oil.

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Pore water moves during compaction and causes so called Compaction current and abnormal subsurface pressure in mud. Such behaviors of the pore water should be closely related to the migration of petroleum hydrocarbons. Recently, compaction equation becomes an important target among the petroleum engineers, because the equation would be very useful for numerical evaluation and simulation of these behaviors.
Compaction process has attracted more serious attention to a large number of scientists in soil mechanics for these three decades. The considerable amount of overburden pressure and strain in geologically long time are characteristic nature of the compaction in sedimentary basins. However, Terzaghi's compaction equation which has been used by many workers is not always applicable for such conditions. It may be also difficult to simulate this process by recent compaction equations based on creep mechanism, because the equations are rather complicated and because we have few useful data of these conditions.
In this paper, a case study of a sedimentary basin in the vicinity of Boso peninsula, will be reported for the first, with geological treatment in relation to some physical properties as porosity, density, and the coefficient of permeability. Secondary, an attempt to do a generalization of the compaction equation with new idea of the non-linear variation of the coefficients of compressibility and permeability is described. Then our equation is presented, which would be more suitable for analysis of petroleum migration with large-scaled numerical model.

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In order to initiate primary migration of the petroleum, the drainage areas relating to the traps are to be buried within the critical range in which the petroleum is generated.
Petroleum expulsion from the source beds to the carrier beds can occur regardless of the formation of the traps. However, primary migration is only effective in forming commercial oil pools in cases where the reservoirs form traps and their caprocks have enough sealing capacities. In Japan, the carrying medium for petroleum in primary migration is pore water which is expelled from the source beds.
Where the porosity anomaly of undercompacted argillaceous sediments is buried to the depth of the petroleum generating zone, the porosity anomaly zone plays two parts in forming petroleum deposits: In the first place, it's the potential source of the primary petroleum migration, and in the second place, it's the barrier to the upward migration of the liquid which is expelled from source beds.
Where a permeable bed is sandwiched between two porosity anomalies, the permeable bed recieves the liquid expelled from the anomaly zones, and the liquid as forced to move laterally in the layer. This liquid movement in the permeable layer could form commercial petroleum deposits in the traps.
We have been engaged in petroleum exploration mainly based on the structural control philosophy. However, the writer found that many commercial hydrocarbon deposits were confined within specific stratigraphic intervals relating to the porosity anomalies.
For example, the porosity anomaly which lies immediately above the stratigraphic trap in the Minami-Aga field is important evidence in discovering the target zone of potential hydrocarbon deposits. Combining this porosity anomaly and other geological data, we will be able to point out the target stratigraphic zone for potential hydrocarbon deposits more precisely.

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In this paper, the author discusses the factors controlling petroleum generation in source rocks, migration through carrier beds, accumulation into reservoir rocks, and their timing. This study is based on some of the recognized case histories and the structural developments of the oil fields in Japan.
Petroleum of the Yabase Oil Field migrated into reservoir of the Onnagawa formation during the early Tentokuji stage, whereas such migration into the Katsurane formation occured since the Sasaoka stage. The same phenomena are recognized at the Fukubezawa and Sarukawa Oil Fields, that is to say, petroleum of the Onnagawa formation migrated at the early Tentokuji stage, and petroleum of the lower Tentokuji formation migrated at the late Sasaoka stage. At the Nishiyama Oil Field, the structure was formed during the time from the Haizume to the late Uonuma stages, and the petroleum accumulated at the same time as the formation of the structure, and migrated into the Shiiya formation. Almost all of the petroleum of the Nanatani and Teradomari formations which had already generated, migrated toward the up dip into the "Chuoh Yutai" (Chuoh Oil Belt) before the formation of the Nishiyama structure. Thus it is recognized that there is an extremely close relationship between petroleum generation and migration.
It is further concluded that petroleum is not generated within 1, 200 meter burial depth (subsurface temperature being less than 60°C)in Japan. On the other hand, the generation of petroleum is recognized below 1, 500 meter burial depth (subsurface temperature being higher than 70°C) in many areas. For instance in the Akita Oil Province, argillaceous rocks, that show porosity from 25% to 35% at 1, 500 meter burial depth, generated petroleum and compaction currents forced the petroleum into the carrier zone.

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Isopach map of the stratigraphic interval from the Globorotalia inflata No. 2 zone (top of the lower Nishiyama formation) to the top of the hydrocarbon producing sandstone shows that the definite structural traps were formed after the deposition of the Globorotalia inflata No. 2 zone, and the fault traps were also formed at that time.
Geothermal process in the upper Shiiya formation (main producing zones) yielded hydrocarbon generation at the bottom of the syncline after the time of the deposition of the lower Nishiyama formation.
The hydrocarbon formed thus at the synclinal bottom migrated through the permeable sandstones in various path ways.
The hydrocarbons seemed to migrate along the structurally concave portion of the permeable beds, and finally trapped (Fig. 10).
The sizable hydrocarbon deposits (Id, IIa, IIb in Fig. 7) locate in widespread sandstone layers, whereas rather small deposits are in lenticular sandstones. The sizable deposits are characterized by uniform constituents of the hydrocarbon, and smaller deposits are characterized by the variable contents of the liquid hydrocarbon in gas.
The former hydroarbocns were derived from the synclinal bottoms, and the later hydrocarbons were formed near the present reservoirs.

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During the last decade, many arguments have arisen about the generation, migration and accumulation of hydrocarbons. Improvement of geochemical field and the study of physical properties of hydrocarbons contribute to solve the above subject.
The writers studied Kitakanbara area, northern part of Niigata prefecture, where active hydrocarbon exploration and development have been made. As a result of the exploration and development of the hydrocarbon in this area, many kinds of the data on the subsurface petroleum geology were collected, and published. In this paper the subject is discussed as follows: 1. distribution and deposional mechanism of reservoir facies, 2. a change of chemical composition of hydrocarbon at each producing oil and gas field and thermal effects.
Paleodeposional system of reservoir facies is different at each geological stage. The sands of the Teradomari and Shiiya formation are transported lateraly from local source area. On the contrary, the sands of Nishiyama formation are widely derived from Kushigata mountain range area. Their distribution indicate a close relationship between the migration mechanism and the forming of accumulation of hydrocarbon. A significant change of chemical composition of hydrocarbon reflects the migration distance (mobility) and a grade of water washing. These data show that the Shiiya formation generates hydrocarbon in surrounding Shintainai area, which then migrated to Hirakida gas field with oil and gas segregation facies, and then medium gravity crude oil is transfered to Echigo-Kurokawa under the controls of differential entrapment. These migrations occured between the Nishiyama stage and Haizume stage that is simultaneous developing and forming this basin. This oil varied to the heavy oil after accumulation by water washing effect.

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A significant quantity of hydrocarboneous fluid is produced at a critical depth in sedimentaly basin by the dehydration of caly minerals under the diagenetic conditions of overburden pressure and heat. The dissociated liquid accompanied with the formation water primarily causes an abnormally high pressure zone at the stratum. If any exceptional force does not occasionally act on for basin, the fluid may be stationally pressed out into a porous medium and driven to final reservoir for a considerably long period of several million years. The driving force of migration is however not only the hydro- and lithostatic pressure, but also a tectonic stress. Regarding to a younger oil basin in the tertiary orogenetic system, the emulsive hydrocarbons are not ready to be held in the primary formations, but run off to more stable and porous reservoirs through a large number of cracks and fractures developed temporarily within the basin by means of destructive stresses of earthquakes or volcanism. Numerous seismic and volcanic activities which magnitudes are generally small and occasionally leage, are relevantly connected to the oil tertiary evolution in this country. The diffusive periods and ranges are estimated to be comparable to the order of the build-up times and the dimensions of the events. The migration of hydrocarbon is thus so frequently synchronized to the dynamic activities of the basin that accomplished more rapidly on a farther scale than estimated with static permeation of the fluid.

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The Kawasaki odservation well of Geological Survey of Japan was drilled in the Fujimi Park, adout 1km east of the Kawasaki Station, Kawasaki City, Kanagawa Prefecture. Lithostratigraphical subdivisions of the well are as follows, in descending order. A formation (Alluvium)

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