Recent progress in oceanography, marine sedimentology, and organic geochemistry enables us to evaluate, quantitatively, factors which control the process of marine organic matter production and its degradation during settling through the water column as well as within the surface sediment. Since many of the factors could be evaluated through detailed geological observation and paleoceanographic reconstruction of the depositional setting, it seems possible to incorporate mathematical formulation of this process into the pre-existing models of source rock evaluation. In this paper, I will propose the mathematical formulation of the marine organic matter production-settling-degradation process based on recently accumulating oceanographic data, and try to apply this formulation to evaluate the source rock potential of the Miocene Onnnagawa Formation, northeast Japan.
In order to assess the hydrocarbon potential of the Middle Miocene (10-12Ma) Onnagawa diatomaceous sediments, outcrop samples from seven sites in the Aomori and Akita Prefectures were examined for the distribution of their major and minor elements. Three factors optimal for the good source rock generation, i. e., supply of biogenous materials, small dilution effect by terrigenous detrius, and anoxic bottom water condition, were extracted by multivariate statistical analyses. Spatial distributions of these factors indicate source facies developed in the central deep part of the basin, which was kept stagnant by restricted circulation of sea water. During 10 to 9Ma, paleoceanic condition changed drastically due to the initiation of localized upwelling.
Subsurface geology of the Onnagawa Formation, in the Akita-Yamagata Sedimentary Basin, Northeastern Honshu, Japan, is studied using well logs and geochemical data. This study includes the distribution of lithology and total organic carbon content. Gamma ray (GR) log is a useful indicator of detritus content. Rocks with lower GR value contain less detritus whereas those with higher GR value contain more detritus. In the Ayukawa, Yurihara and Fukubezawa fields, oil and gas reservoirs are composed of porcelanite, chert and dolomite, which contain little detritus. They are characterized by low GR and high resistivity on the log (they will be called ‘LG/HR rocks’ hereafter). It is also observed from the ordinal source rock analyses using cutting samples that rocks with lower GR value tend to have higher organic carbon content. Reservoir-forming LG/HR rocks, which also have high potential as source rocks, mainly occur in the middle part of the Onnagawa Formation. It is observed that they develop in the areas where the formation is moderately thick and contains less amount of tuff. In the areas where basaltic rocks occur in the uppermost part of the underlying Nishikurosawa Formation, we observe the Onnagawa Formation is thin if we exclude volcanic and intrusive rocks. These areas are considered to have formed a palaeogeographic high. It indicates that deposition should be restricted in the latest Nishikurosawa and the early Onnagawa Stages. In the Onnagawa Formation, the higher average total organic carbon content is observed in the areas where LG/HR rocks content is high. High average total organic carbon content is also observed in their surrounding areas where the Onnagawa Formation is relatively thick.
Geochemical studies of the Miocene Onnagawa Formation of northern Japan and the Monterey Formation of California are reported. Geochemical parameters indicative of organic source are similar between the Onnagawa and Monterey Formations, suggesting marine plankton is the major source of organic matter. The degree of preservation at depositional site is a major factor influencing quality and quantity of organic matter. The total organic carbon (TOC) considerably varies among samples collected at millimeter- to centimeter-scale stratigraphic intervals, and TOC shows a distinct positive correlation with detritus content. This close relationship between TOC and detritus content suggests that the organic matter could be largely adsorbed to the surface of detritus particles which might have formed aggregates with a greater resistance to bacterial degradation. The detritus contents varied in centimeter- to decimeter-scale and control organic richness. Biomarker data indicate the Monterey Formation was deposited under more reducing condition than the Onnagawa Formation. This intensive anoxic condition could contribute to the higher contents of organic carbon and sulfur in the Monterey Formation compared to the Onnagawa Formation.
The Neogenesedimentary basins in Akita and Niigata area are typical of marine oil producing sedimentary basins in Japan. Middle Miocene source rocks of the Onnagawa Formation in the Akita oil field are characterized by a high abundance of biogenic silica. Contemporary source rocks of the Lower Teradomari and Nanatani Formations in the Niigata oil field are argillaceous with significant clay-rich clastic rocks. In order to clarify organic geochemical differences between the source rocks from these two basins, source rocks and crude oils were analyzed for biomarkers, aromatic sulfur compounds (ASC), kerogen sulfur concentrations, and oil generation kinetic parameters. Regular sterane compositions of crude oils from both basins are similar, showing major contributions from zoo- and phytoplankton. However, Akita crudes are characterized by low pristane/phytane (Pr/Ph), low diasteranes/regular steranes (Dia/Reg), low oleanane/hopane (Ole/Hop), and high benzothiophenes/dibenzothiophenes (BTs/DBTs) compared to Niigata crudes. Although the potential source rocks showed a significant variation in their organic compositions, kerogen in the Akita basin tends to have the higher atomic S/C and O/C, and lower activation energy for oil generation. Some kerogens from the central part of the Akita basin are classified as Type II-S kerogen, which are generally rich in 24-norcholestane derived possibly from diatom. Kerogens from the southern part of the Niigata basin are characterized by the lowest sulfur concentrations and the highest generation activation energies. Notably, the Onnagawa source rocks have significantly lower Dia/Reg and higher BTs/DBTs than the Teradomari and Nanatani source rocks with the same maturity levels. These organic geochemical differences between Akita and Niigata basins can be interpreted in terms of differences in oxic-anoxic condition and primary productivity at the time of deposition, the consumption of H2S by terrigenous iron at the sediment/water interface, and the abundance of clay minerals which can act as catalysts for the isomerization of regular steranes to form diasteranes during diagenesis.
We describe in detail the paleoenvironment of the Niigata sedimentary basin related to the distribution of the petroleum source rocks based on the analysis of the foraminifera) assemblages in the latest Nanatani, Teradomari, Shiiya and early Nishiyama ages. The Teradomari stage is divided into five zones named LT 1 to LT 4 and UT based on the characteristics of the formaniferal assemblages. These are: LT 1: mixed assemblage of calcareous benthic foraminifera (Globobulimina spp. and Uvigerina spp.) and arenaceous foraminifera (Haplophragmoides spp.) LT 2: arenaceous foraminifera assemblage characterized by the abundant occurrences of Dorothia spp. and Haplophragmoides spp. LT 3: Cassidulina-Gyroidinoides assemblage mixed with arenaceous foraminifera (Haplophragmoides spp.) LT 4: Dorothia spp.-Haplophragmoides spp. assemblage UT: Spirosigmoilinella compressa, Haplophragmoides spp., and Martinottiella communis assemblage. Both LT 1 and LT 3 zones are also characterized by the relatively abundant planktonic foraminifera. The assemblages of LT 2 and LT 4 zones, in which arenaceous foraminifera occur abundantly, indicate that the paleosea of the Early Teradomari age was anoxic environment. Furthermore, on the basis of distribution pattern of Nanatani stage, LT 1 to LT 4 zones, and Upper Teradomari stage, the paleogeographies of the sedimentary basin of the Niigata area in the middle to late Miocene are reconstructed as follows. The paleo-ocean in the Nanatani age was widely present in the Niigata area. The sedimentary basin of the earliest Teradomari age (LT 1-2) was shifted to the limited area in the western part of the Niigata area, such as Kubiki, Nishiyama-Chuo oil field, Minami-Nagaoka, and Minami-Aga areas, in contrast with both the Naka-Kanbara and eastern part of the Niigata areas where no sediment was deposited. On the basis of the paleogeography in the earliest Teradomari age, it is inferred that the organic rich sediments were deposited in the Nishiyama-Chuo oil field to Minami-Nagaoka areas near the edge of sedimentary basin trending northwest to southeast where productivity of phytoplankton was high due to upwelling.
The paleoenvironment and the distribution of source rocks in the Niigata oil and gas fields was detailed (Sato et al., 1995, this volume). We describe the organic geochemical characteristics of oils produced and probable source rocks, and oil-to-oil and oil-to-source rock correlation using aliphatic biomarkers. Petroleum potential evaluation based on the S1+S2 values on Rock Eval pyrolysis revealed that the argillaceous rocks in the Lower Teradomari LT2 and LT4 zones correspond to possible or potential source rocks (Hirai and Miyamoto, 1992). Triterpane and sterane compositions of the oils and bitumens extracted from argillaceous rocks were compared with each other. The results of the triterpane (m/z 191) and sterane (m/z 217) fragmentograms showed that these oils are matured, and two types of oils can be distinguished (Hirai et al., 1992), and their corresponding source rocks are suggested. (1) The Niitsu type oils produced mainly in the eastern oil and/or gas fields with low oleanane/norhopane ratios (lower than 1.0) and relatively low C29/C27 sterane ratios, suggesting that they were generated from the effective source rocks in the Lower Teradomari LT4 zone with larger contribution of marine organic matter. The Shintainai, Shiunji, Aga-Oki, Higashi-Niigata, Matsuzaki, Minami-Aga, Minami-Niitsu, Kuwayama, Yoshida and Mitsuke fields are classified in this type. (2) The Kubiki type oils are produced mainly in the western oil and/or gas fields with high oleanane/norhopane ratios (higher than 1.0) and relatively high C29/C27 sterane ratios, suggesting that they were generated from the effective source rocks in the Lower Teradomari LT2 zone with larger contribution of terrestrial organic matter. The Fujikawa, Minami-Nagaoka, Tamugiyama, Amaze, Nishiyama, Katamachi, Kuroi, Meiji and Gohtsu fields are classified in this type.