Diagenesis is defined as the process involving physical and chemical changes in sediment after deposition that convert it to consolidated rock. In the paper, the physical and chemical factors controlling diagenesis and the stage of diagenesis in sediments were reviewed. Especially, diagenetic changes of pores, pore-fluids and mineral grains in the Neogene argillaceous sediments of Japan were reported in detail. On the other hand, latest theories on the generation and migration of petroleum by many investigators in Japan and elsewhere were also reviewed.

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For the estimation of paleo-temperature, vitrinite-reflectance, kerogen or bitumen indicators have been generally utilized. The writer and his colleagues analyzed the paleo-temperature gradient during Neogene period at many wells in Hokkaido and Niigata areas of Japan by utilizing the transformation temperature of authigenic minerals in the argillaceous rocks.
Analyzed gradients are ranged as follows. In Hokkaido, from 2.2°C/100m to 4.7°C/100m, and in Niigata, from 1.9°C/100m to 3.6°C/100m.

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Neogene Tertiary composed of Atsumi, Kamigo, Mogami and Shonai Groups are developed on the basement of pre-Tertiary granites in the Shonai basin of Yamagata, Japan. Mogami Group which is composed of Aosawa, Kusanagi, Kitamata, Tateyama, Maruyama and Kannonji Formations, has been studied from the petroleum geological viewpoints in order to examine its potentiality of oil and natural gas.
Six routes in mountaineous area of the eastern part of Shonai basin have been surveyed in detail and two hundreds of mudstones and siltstones in Mogami Group were collected. Mineral composition of these samples were analyzed by X-ray diffractometer using zinc oxides as internal standard sample. From these data, the writers settled the following five mineral assemblages. These assemblages include A₁ (montmorillonite-illite-poor chlorite-quartz-analcite and/or heulandite-calcite-dolomite zone), A₂ (poor montmorillonite-illite-poor chlorite-cristobalite-clinoptilolite-calcite-dolomite zone), A₃ (montmorillonite-illite-poor chlorite-cristobalite-clinoptilolite-calcite zone), B₁ (montmorillonite-illite-chlorite-kaolinite-cristobalite-clinoptilolite zone), and B₂ (montmorillonite-illite-poor chlorite-kaolinite-plagioclase-clinoptilolite zone). Stratigraphically, assemblage A₁ may be equivalent to Aosawa Formation; A₂ to Kusanagi and lower part of Kitamata F.; A₃ to upper part of Kitamata, Tateyama and Maruyama F.; B₁ to lower part of Kannonji F.; and B₂ to upper part of Kannonji F. (Jozenji Facies).
Significance of these mineral assemblages were discussed from the sedimentary petrological points of view. As a result, it is clear that these assemblages are quite suitable for the analysis of depositional environments, source materials and diagenesis in argillaceous rocks of Mogami Group.
The mineral assemblages in argillaceous rocks of the Mogami group at five wells drilled in the Shonai plain have been studied. Settled five mineral assemblages are somewhat different from those of mountaineous area. However, these assemblages may clear the stratigraphic problems between Aosawa and Kusanagi Formations which have been identified by lithological and paleontological data only.

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Argillaceous sediment include various minerals such as clays, silicas, silicates, carbonates, etc. Mineral assemblages in argillaceous rocks are a modification of that originally presented in sediments and have been formulated by a combination of various geologic factors such as source materials, depositional environments, diogenesis, metamarphism, and weathering.
Early diagenetic changes of various minerals in sediments have been controlled mainly by chemistry of interstitial or connate water. On the other hand, the most important factor for the late diagenetic transformation of minerals might be the temperature in addition to reaction time and pressure.
Paleotemperature analysis by authigenic minerals can be utilized for the possible prediction of hydrocarbon pools in the exploration area.

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Since 1967, the writers have studied the stratigraphical and geographical distributions of carbonate rocks of the Funakawa, Onnagawa and Nishikurosawa formations of the Neogene in the Akita oil fields, Japan. They considered the origin of these carbonate rocks from the geological and mineralogical points of view. The results are summarized as follows:
1. Stratigraphically, these carbonate rocks are generally distributed in the following formations;
uppermost horizon of Nishikurosawa formation
uppermost horizon of Ormagawa formation
lower horizon of lower Funakawa formation
uppermost horizon of upper Funakawa formation
Geographically, the carbonate rocks are remarkably distributed in the Onnagawa and Nishikurosawa formations along the southern coast of Oga peninsula, in the Nishikurosawa formation of the eastern part of Kameda-machi and in the Onnagawa formation of the Southern part of Yajima-machi.
2. The detrital substances derived from the inorganic and organic matters are the main source materials for these carbonate rocks, while the chemical deposits from the sea water occupy only a part of the carbonate rocks. Depositional environments of these rocks are ranged from the inner-neritic to bathyal conditions.
The supply of both calcium and magnesium is necessary for the formation of carbonate rocks from the source materials because of the poorness of these elements in them. Concerning the carrier waters of calcium and magnesium and the geologic stages of supply of these elements, it is considered that the supply of calcium and magnesium from the salt water originated from the earlier lithification during the period of syn-diagenesis is more effective to the transformation of source materials than the supply of these elements from the water originated from the ana-diagenesis, epi-diagenesis and hydrothermal alteration. Also, the additional supply of these elements from the volcanic detritus in the sea may sometimes be very important.
In the studied area, the formation of chemical deposits is very rare, and genetically, this formation is mainly controlled by the following two factors; one is the abnormal increase in magnesiun and calcium contents of the sea water caused by the supply of large amount of volcanic detritus in the sea, and the other is the temperature rise of the sea water caused by the same phenomenon.

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Transformation of clay mineral facies in mudstones have been controlled mainly by overburden pressure and geothermal temperature. In primary migration of proto-petroleum from source rocks to reservoirs, expulsion of absorbed water in the swelling clay minerals by overburden pressure is considered to be very important. For the solution of these problems, study of compaction on various clays under high pressure and certain temperature is necessary.
The writers, therefore, manufactured the triaxial, hydrostatic compaction apparatus which can increase the pressure to 1500kg/cm² and the temperature to 150°C in maximum.
Compaction on Na-montmorillontie and kaolinite clays have been conducted under the condition at 500kg/cm² and room temperature. Water content in montmorillonite clay decreased gradually in straight, while those of kaolinite clay decreased rapidly by compaction. After 6 day's continuous experiments, there were no apprent changes in the mineralogical form of these clays.

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The importance of carbonate reservoir and source rocks is underscored by the fact that about half of the world's petroleum is produced from carbonates. In as much as stratigraphic and structural habitats of many oil and gas pools in carbonate rocks suggest indigenous origin of their hydrocarbons, many petroleum geologists are studying the origin and occurrence of petroleum in these rocks.
The writers have been studying the origin and distribution of Neogene carbpnate rocks in the Akita Oil Fields of Japan since 1967. In the present paper some data on clay minerals and organic matter present in these rocks are presented. An attempt is made to relate the chemical and mineralogic composition of these rocks to density, porosity and other parameters. The results of the present study can be summarized as follows:
(1) On the basis of clay mineral content, carbonate rocks can be grouped into those containing predominantly (a) kaolin clay or (b) montmorillonite clay. In addition, there is a group poor in clay mineral content.
(2) Carbonate rocks containing kaolin clay formed in inner-neritic depositional environment, whereas carbonte rocks poor in clay mineral content or those containing montmorillonite clay formed in outer-neritic to bathyal environments.
(3) Carbonate rocks containing kaolin clay have high density and relatively high effective porosity Carbonate rocks low in clay mineral content have high density and low effective porosity.
(4) The amounts of organic extracts, hydrocarbons and total organic carbon are approximately equal to 1/2 of those usually found in shales.
In the future the authors will attempt to correlate the Ca/Mg ratio of the carbonate portion of the rock (excluding clays) and the porosity. In addition, insoluble residue contents will be plotted versus degree of dolomitization. The amounts of clays in cerbonate rocks will also be determined quantitatively.

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Neogene strata occur in the riverside of Ara-kawa, northern Hiki hills of Saitama, Japan, and they are grouped into the Yorii, Tachigase, Kozono, Arakawa, Fukuda, Tsuchishio, and Yagii formations.
From the mineral composition of sedimentary rocks, it is cleared that the Neogene in the studied area are grouped into the following five facies. Facies 1: illite-chlorite-heulandite-quartz-plagioclase-calcite-tremolite. Facies 2a: montmorillonite-illite-chlorite-heulandite-cristobalite-siderite. Facies 2b: montmorillonite-illite-chlorite-analcite-plagioclase-calcite. Facies 3: montmorillonite-illite-mordenite-cristobalite-siderite. Facies: 4: montmorilloniteillite-kaolinite-clinoptilolite-quartz-gypsum.
These facies are almost equivalent to stratigraphic zonations. Facies 1: lower and middle Kozono formations. Facies 2a: upper Kozono and lower to middle Arakawa formations. Facies 2b: upper Arakawa and lower Fukuda formations. Facies 3: upper Fukuda formation. Facies 4: Tsuchishio and Yagii formations.
Source materials and depositional environments were important to the formation of these facies. Diagenesis affected strongly to Facies 1 and 2, and weakly to Facies 3, while it was ineffective to Facies 4.

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The Cretaceous in the central Hokkaido are grouped into the upper, middle, and lower Ezo groups.
These groups overlie the Jurassic Sorachi group and are covered by the Paleogene Ishikari group. The dominant carbonate rocks of these groups are limestones such as micrite, sparite and biolithite in the form of layers and nodules. Limy lithic wackes occur sometimes in the middle Ezo group. Carbonate facies are found remarkably in Tomitoi formation of the lower Ezo group, and Saku and Shubu (main part) formations of the middle Ezo group.
Average effective porosity of the layered carbonate rocks in the studied area is approximately three per cent and is considerably lower than that of oil-bearing Neogene Tertiary in the northwestern Honshu. The average ratio of carbonate rocks in thickness to total outcrops is nearly five per cent and is almost equal to that of the northwestern Honshu. These data indicate that the Cretaceous carbonate rocks in the studied area have a lesser character as a petroleum reservoir.

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Four carbonate strata of Miocene are exposed in Akita area of northern Honshu, Japan.
Investigations with the electron-microscope revealed that these carbonate rocks are composed mainly of skeletal debris such as diatoms, forminifera, radiolaria and nannoplankton.
The paleo-environments were bathyal to innerneritic. During syn-diagenesis, calcium and magnesium derived from the sea water as well as from volcanic detritus within the basin altered the originally siliceous sediments to dolostone (in deep, cold waters) and limestone (in shallow, warm waters).
After the consolidation, the primary texture of carbonate rocks was destroyed gradually by dehydration and recrystallization during ana-diagenesis.

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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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The main reservoir rocks in giant oil fields of the world are sandstones and carbo-nate rocks. In Japanese oil fields, however, the reservoir rocks are such sedimentary rocks as sandstones (59per cent), tuffs (37per cent) and carbonate rocks (1per cent), and volcanic rocks 3per cent).
To form good reservoirs in a basin, these rocks must have high porosity and permeability and be developed extensively with adequate thickness. The latter refers to vertical and horizontal dimensions of reservoirs in a basin, and is controlled chiefly by geological and geographical factors
during deposition. The former includes petrophysical properties of reservoirs and are a modifica-tion of primary petrophysical properties resulting from subsequent physical and chemical processes after deposition. These processes are compaction, fracturing, solution, cementation, and transform-ations such as dolomitization and calcitization. Generally, most important and essential mass properties of reservoir rocks, i.e. effective porosity and permeability, are mutually related. Also, effective porosity is positively correlative with absolute porosity. This indicates that absolute po-rosity must be one of the important factors for the evaluation of reservoir rocks.
The writer and his colleagues, therefore, have studied the origin of porosity in reservoirs of various sedimentary basins in Japan and Canada from a petrophysical viewpoints. As a result, he came to a conclusion that the areal and quantitative analysis of each processes relating to origin of absolute porosity must be most important for the evaluation of reservoirs.
Examples of this evaluation, the Cretaceous sandstones of central Hokkaido, Japan, have been discussed in this report. Limy lithic wackes of the Cretaceous Ezo Group of Hokkaido have moderate grain density (2.68g/cm³ in average) and low absolute porosity (4.7per ecnt in average). The most important processes that decreased the porosity were compaction and areal cementation by calcite and chlorite. Porosity increased slightly as a result of solution and fracturing. There-fore, these sandstones may be poor as a petroleum reservoir.

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In this paper, the writer describes the mineralogical study of glauconite from the various sedimentary rocks of Neogene Tertiary system in Niigata oil field area by the methods of optical observation and X-ray diflraction analysis, and lastly he concludes as follows:
1) These glauconite samples are classified into mica like type (a₁, a₂), montmorillonite-illite mixed layer like type (b₁, b₂), montmorillonite like type (C) and mineral mixtured type (D).
2) It is considered they must be derived from such a source materials as volcanic detritus.
3) Glauconitization will be promoted strongly in an anaerobic sedimentary environment having a larger quantity of organic materials and a slower sedimentary velocity with lesser sediments, because the sufficient contact action by sea water is necessary for glauconite formation.
4) As a similar glauconite are formed in a resembling environment, so the correlation by using glauconite is available only such a formations which has deposited in the same basin having a fixed environment.

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Green-tuff Region is defined as existence of abundant submarine volcanic rocks of Early Miocene and overlying oil-bearing pelitic clastic sediments of Late Miocene-Pliocene age. The region is also characterized by predominant distribution of landslide phenomena in the outcropping area of pelitic sediments. The region has been folded and faulted since Late Plocene age, moreover hilly and mountainous area is considered to be uplifting recently.
From paleotemperature analysis by authigenetic minerals and studies of the relation between porosity and burial depth, amounts of uplifting movements are estimated, and it is clarified that overlying Pliocene sediments has been eroded out throughout the Quaternary age, and severe fracturing occurred in the Neogene sediments. This caused the action of groundwater that accerelate landslide phenomena in the Green-tuff Region.

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