The development of petroleum deposits in a basin may generally be controlled by the shape of the basin and by the sedimentary environments. It is very important to clarify the evolutionary changes of the paleoenvironment and paleogeography of the basin for the elucidation of the process of the oil accumulation. Studies on the paleoenvironment and paleogeography of the Niigata Neogene sedimentary basin by the combinations of the isopach, lithofacies and biofacies maps are undertaken parallel with paleoecological analysis of the foraminifera by using computer based on massive recent ecological data. The ecological analysis of foraminifera of more than 200 wells in the Niigata sedimentary basin makes it possible to construct a series of paleoenvironmental maps showing the basin evolution from the Teradomari to the Haizume stages. The biofacies changes and their relationship with thickness of the respective stratigraphic units by the isopach map are analyzed. Consequently, the paleobathymetrical conditions, the bottom confiiguration of sedimentary basin and changes in the structure of the Niigata sedimentary basin can be worked out. The general features of the sedimentary basin during the time of the respective stages which yielded foraminifers as related with development and migration are revealed as shown in the Figs. (1-24). The quantity-distribution (foraminiferal number, arenaceous and shallow foraminifera contents, planktonic/benthonic foraminifera ratios) of foraminifets not only served to indicate the paleogeographical features above mentioned but also are good guides in paleoenvironmental analysis of the respective stages of the Niigata basin. The foraminiferal number in sediments is useful in inferring total organic contents and in determining the organic type, both of which reflect the productivity of marine organism_??_.
The types of organic matter in 296 Neogene Tertiary rocks from 11 wells of the Niigata Plain were determined by the visual kerogen method, and the stratigraphical and areal distributions of kerogen types were studied in relation to the paleoenvironment reported in Part 1 by Maiya et al. (1980). The sapropelic (amorphous) kerogen and arenaceous foraminifera contents have similar stratigraphic trend pattern. The type of organic matter reflects the precursor of kerogen and its depositional environment, and the arenaceous foraminifera contents represent the paleodepth in this area according to Maiya et al. (1980) These suggest that the type of organic matter closely relates to the paleoenvironment. The relationship between the paleogeography and the kerogen type revealed that the Niigata Plain at the Teradomari to the middle Nishiyama stage has the general tendency such as the deep sea is abundant in the sapropelic kerogen and as the shallow sea is rich in the woody, coaly, and herbaceous kerogen. However, the case that the sapropelic kerogen was not necessarily related to the paleodepth was partialy occurred. This may be considered that amounts of the sapropelic kerogen decreased as following results. (1) the autochthonous sapropelic kerogen was diluted by the allochthonous woody, coaly, and herbaceous kerogen; and (2) the sapropelic kerogen was oxidized easilier than the others, in the case of the input of the sea water by the turbidity current and mass sliding in the deeper depth.
Parameters as coal rank of Eocene to Oligocene coal samples from Kushiro and Ishikari coal fields in Hokkaido were studied. It is shown that the sporinite fluorescence is superior than vitrinite reflectance as coal rank parameter on the basis of volatile matter content and calorific value. This is based on the fact that chemical and physical properties of vitrinite is liable to be controlled by the difference of plant materials, paleogeography and diagenesis. On the other hand, spores and pollens were scattered without any connection to the location within the sedimentary basin of coal deposition in the limited area (from Kushiro to Ishikari coal fields) and in short geologic time (from Eocene to Oligocene). In the course of coalification, the variation of distribution on n-alkane occurres at the reflectivity of 0.64-0.71%, the variation of n-alkanes content occurres at reflectivity of 0.63-0.64% and CPI value of different maceral changes over the reflectivity of 0.6%. It is concluded that the diagenesis stage shifts the catagenesis stage over the reflectance from approximately 0.6-0.65%, this is different from the result of Tissot and Wclte (1978).
Many physical and chemical parameters to indicate the maturation levels of organic matters in sediments have been proposed up to the present time. Although it is desirable to measure directly the maturation levels of source rocks from synclinal area where petroleum is believed to be generated, rock samples from such synclinal areas are seldom obtained because exploratory wells are generally drilled on and around anticlinal crests. To estimate the maturation levels of sediments of which any parameters not measured, several investigaters have proposed some time-to-temperature relationships in organic metamorphism. However, these are obtained mainly from the Paleozoic and Mesozoic sediments and therefore, not applicable directly to the oil bearing Neogene systems in Japan. The authors, first, examined the interrelationships between time and temperature of carbonization on coals. The pyrolysis data showed that same value of vitrinite reflectance was provided by a doubling of the reaction rate for each 10°C rise in temperature. We, therefore, studied the time-to-temperature relation in reflectance values measured on some MITI well samples in the Nishi-Kambara area on the basis of a doubling of the reaction rate. The relation can be expressed by log R0=0.0026*Te+0.088*logt-0.52 where Te is the maximum paleo-temperature (°C) and t is the effective burial time (Ma) during which a specific rock has been within 14.4°C of its maximum temperature (Tc). The present maturation levels of the Nishiyama and Shiiya formations might have been decided during the time from approximately 0.2-0.4 MaBP to the present. The maturation level of 0.5% reflectance in the area may be distributed in the temperature range between 95 and 110°C.
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.
The current state of isotopic dating is reviewed with special emphasis on K-Ar dating. Characteristics of K-Ar, Rb-Sr, U-Th-Pb, Sm-Nd, fission-track, 14C and 210Pb methods are very briefly described. The principle of constructing a geological time scale is explained and the newest scale is presented. K-Ar dating is most useful and is now routinely used in geology. Problems of error estimate, materials and argon loss are discussed in some detail. Careful selection of material and interpretation of results in terms of geological setting are very important in isotopic dating.
The author has studied on the timing of primary hydrocarbon migration in the main oil and gas fields of the Northeast Japan, mainly from the geohistorical aspect, and also speculated its relation-ship with hydrocarbon generation through geochemical and coal petrographical study. As a conclusion, the principal timing of primary migration is at the early stage of hydrocarbonization of kerogen, that is to say, with in the mature range of 0.5 to 0.8% in vitrinite reflectance (R0), and also, hydrocarbons generated at the late stage should have been unable to accumulate in significantly large quantities. In the Japanese oil bearing Neogene systems, biginning depths of oil formation are generally from 1500 to 3000m, and their corresponding subsurface temperatures are from 348 to 388K (from 75 to 115°C), respectively. This variation of temperatures necessary for hydrocarbon generation is related to that of effective heating time of Miocene and Pliocene sediments. Most of oil and gas fields in Japan are formed during very young geologic ages. Therefore, as controlling factors for the formation of important oil and gas accumulations, deep burial with thick sediments and presence of deep and large synclines adjacent to anticlines or higher geothermal gradient more than 3.5°C/100m in case of thin sediments are required. These necessary conditions are fully satisfied in the main oil and gas fields in the Northeast Japan.
Geochemical data from 13 wells in the Sea of Japan, Northern Honshu Area, were studied for their relationship to subsurface temperature. The basic geochemical data consisted of elemental analysis in kerogen, isotopic and organic analyses of mudstone, optial kerogen analysis and X-Ray mineral percentages of mudstone cuttings. These data were crossplotted against presumed present-day subsurface temperature. A data analysis of the resulting crossplots implied that migration of oil occurs at a temperature range of 90°C to 105°C (Zone of flushing action) throughout the geologic section without respect to geologic age. The thermally matured zone (termed the zone of peak oil generation) probably corresponds to a temperature of 130°C.
Conditions of the oil zone for wells in Hokkaido were estimated in this study. The results indicate that the primary migration of petroleum is prosperous in the early mature stage as widely accepted by many authors at the present time. Wells used here are classified into following three groups because of the areal differences of conditions; (1) Tempoku well group: burial depth of 2400-3600m, paleotemperature of 100-130°C and vitrinate reflectance of 0.47-0.56% are recognizable as the upper limit of the oil generation zone. Corresponding formations range from the middle part of the Masporo Formation to the upper part of the Cretaceous system. In other words, formations younger than the middle part of the Masporo Formation are immature. (2) Hidaka well group: burial depth of 2400-2900m, paleotemperature of 70-95°C and vitrinate reflectance of 0.50-0.54% are recognizable as the upper limit of the oil generation zone. Corresponding formations range from the middle part to the base of the Fureoi Formation. (3) ‘ Green Tuff Region’ well group: burial depth of 1600-2500m, paleotemperature of 60?-100°C and vitrinate reflectance of 0.50-0.57% are recognizable as the upper limit of the oil generation zone. Corresponding formations range from the Nina Formation to the upper part of the Fureoi Formation (the upper part of the Atsuta Formation).
Carbon isotope ratios (C13/C12) and biological markers are applied to problems of geochemical correlation of crude oil-source rock in Hidaka region, Hokkaido. Possible source rocks based on geological data include Phoronai shales (Paleogene Tertiary) and/or Urakawa shales (Cretaceous). The investigated crude oil produced from Phoronai Formation was separated into saturated, aromatics, heterocomponents and asphaltenes. δ-values of these fractions are -26.4%, -24.5%, -25.1% and -25.7%, respectively. Mean δ-values of Phoronai shales and Urakawa shales are -25.3% and -24.0%. From these results, Urakawa shales are regarded as source rock. Mass chromatography of the predominant fragments of biological markers is successful for the determination of internal ratios of individual components such as 5β/5α steranes and 17α/17β triterpanes. The results obtained on the mass chromatogram patterns and the ratios of geometrical isomers show that Urakawa shales are similar to the crude oil.