Amane Waseda

Geochemical characteristics of terrigenous- and marine-sourced oils in Hokkaido, Japan

Crude oils in Hokkaido, Japan, are divided into two genetic groups based on bulk properties, carbon-isotope compositions and biomarker distributions. One group is characterized by high carbon-isotope canonical variables (Sofer, 1984), high wax and low sulfur content, high Pr/Ph ratios, low C27/(C27+C29) sterane ratios, relatively low Ts/(Ts+Tm) ratios, and the absence of tricyclic terpanes and C35 homohopanes. These characteristics suggest that the oils were generated mainly from terrigenous organic matter in source rocks deposited under highly oxic conditions. The other group is characterized by low canonical variables, low wax and high sulfur content, low Pr/Ph ratios, high C27/(C27+C29) sterane ratios, relatively high Ts/(Ts+Tm) ratios, and the presence of tricyclic terpanes and C35 homohopanes, properties that suggest that they were generated from marine shales. Based on the biomarker distributions, the terrigenous oils correlate to Paleogene coals and coaly shales, whereas the marine oils correlate to Miocene marine shales. The atomic H/C ratios of the coals indicate that they have oil-generating potential. The geographic distribution of the terrigenous and marine source rocks, and their thermal maturity control the distribution of the oils in Hokkaido. The geographic distribution of the Paleogene coal-bearing formations corresponds to the distribution of the terrigenous oils in central Hokkaido. The marine oils are restricted either to areas of high heat flow related to volcanic activity, or to the area with thick Neogene and Quaternary sediments, where the Miocene source rocks are buried deep enough for oil generation.

Sequence stratigraphic architecture of a differentially subsiding bay to fluvial basin: the Eocene Ishikari Group, Ishikari Coal Field, Hokkaido, Japan

Abstract The Eocene Ishikari Group, deposited in a bay to fluvial basin in central Hokkaido, Japan, provides important information on fluvial sequence stratigraphy in a differentially subsiding ocean-connected setting. The Ishikari Group consists of four million-year-order depositional sequences (Isk-1, Isk-2, Isk-3 and Isk-4), composed mainly of meandering/braided fluvial systems. Each depositional sequence contains marine or lacustrine incursion beds, which show lake or bay to bay-margin tidal facies, at maximum flooding surface horizons. This indicates that marine or lacustrine incursion into fluvial environments took place when the increase rate of accommodation space due to relative sea-level rise exceeded the rate of accumulation. Fluvial channel stacking density varies in response to stratigraphic position within a depositional sequence, namely, high channel density in the lower part of the transgressive interval (Trs: LST to TST) and the upper part of the regressive interval (Rgr: HST), and low channel density in upper Trs and lower Rgr. Multistory channels tend to be developed in lower Trs, whereas single-story channels predominate in upper Trs and Rgr. These patterns in the distribution of fluvial channels resulted from changes in accommodation space in response to relative sea-level changes. Sequence boundaries are recognized at fluvial incision surfaces at the base of Trs, but the development of incised valleys is minor because of the relatively high subsidence rate, which obscured the effect of relative sea-level fall. Syndepositional differential subsidence within the paleo-Ishikari basin resulted in selective development of relatively downstream depositional systems in the more rapidly subsiding area. High-frequency depositional cycles are also prominently developed in the subsiding area where sedimentation rate was sufficiently high to record high-frequency relative sea-level cycles or autocycles of deposition.

Methane Accumulation and High Concentration of Gas Hydrate in Marine and Terrestrial Sandy Sediments

Gas-hydrate-bearing sand-core samples have been obtained from the Canadian Mallik wells, and the Nankai Trough wells of offshore Japan. The chloride-content anomalies in extracted pore waters, core-temperature depression, core observations, and visible gas hydrates, as well as continuous downhole well-log data, confirm the presence of pore-space hydrate as an intergranular pore filling within sandy layers, clarifying the characteristics of subsurface natural gas hydrate in marine and permafrost zones. Gas-hydrate saturations (percent of pore volume) as high as 80% have been measured, which requires enough original pore space in the host sediment to allow the gas to accumulate. Carbon and hydrogen isotopic compositions of methane and hydrocarbon compositions in gas hydrate and gas-hydrate-bearing shallow sediments in the Nankai Trough show that methane is generated by microbial reduction of CO2 and suggest progressive decreases in microbial (biogenic) activity with depth and upward gas migration through the sediment column. In the Mackenzie delta, methane in gas hydrate is generated by thermogenic decomposition of kerogen. Based on the geochemical and geological data, gas migration is estimated to be an active flow to permeable sandy layers in the Nankai Trough, and a long migration of thermogenic gas, generated in deep mature sediments, through faults in the Mackenzie delta. Note that many similarities in appearance and occurrence between the terrestrial (Mallik) and the marine (Nankai Trough) areas exist, and this knowledge and information is crucial to the identification of other hydrate deposits and to assess their eventual energy resource potential.

Geochemical characteristics of Tertiary oils derived from siliceous sources in Japan, Russia and U.S.A

Abstract Siliceous sourced Tertiary oils from the Circum-Pacific area of Japan, Russia and the U.S.A. have a heavy carbon isotope composition, monomodal n -alkane distributions, and nearly identical regular sterane compositions with a predominance of C 27 homologues. These are consistent with open marine depositional environments dominated by diatomaceous organic matter. However, a number of alkane and biomarker parameters such as Pr/Ph, CPI, relative concentration of 28,30-bisnorhopane, and the C 35 /C 34 homohopane ratio indicate more oxic depositional environments for the source rocks of Japan and Russia. In contrast to the California Monterey Formation sourced oils, petroleums with low maturity levels from the North Sakhalin basin, Russia and the Akita basin, Japan have lower concentrations of asphaltenes and sulphur and are characterized by higher API gravities. A correlation of extractable organic matter from source rocks vs the least matured petroleums demonstrates that oil expulsion in siliceous shales of the Akita basin occurs at a maturity level corresponding to R o ≥0.65%, which is in the range of the conventional oil window (R o =0.6–1.1%).

Origin and migration of methane in gas hydrate-bearing sediments in the Nankai Trough

Abstract Carbon and hydrogen isotope compositions of methane and hydrocarbon compositions in gas hydrate-bearing shallow sediments in the Nankai Trough show that the methane is generated by microbial reduction of CO 2 . The δ 13 C values of CH 4 range from - 96 to -63% in the upper 300 m sediments. Both δ 13 C values of CH 4 and CO 2 become more positive with increasing depth. The preferential depletion of 12 CO 2 , progressive decrease in microbial activity with depth and upward gas migration through the sediments column explain the δ 13 C depth profiles. In deeper horizons, the origins of gases change from microbial to thermogenic at around 1500 mbsf (meters below seafloor). Gases shallower than 1500 mbsf have lower δ 13 C values of CH 4 (lower than -59%), while gases deeper than 1500 mbsf have higher δ 13 C values of CH 4 (-48 to -35%), typical for gases generated by thermal decomposition of organic matter. The measured total organic carbon (TOC) in the Nankai Trough is around 0.5%, which is considered too low for in situ formation of gas hydrate. Consequently, some gas migration and accumulation processes are required for the concentrated formation of the gas hydrates (up to 80% in pore space) in the Nankai Trough. This process may be related to the geological setting of the Nankai Trough, where fluid flow containing methane is active through thrust systems within Nankai accretionary prism sediments. There is, however, no indication of thermogenic gases in shallow sediment including the hydrate-bearing intervals, suggesting that the fluid migration is rather local and restricted to the shallow sediments.

Oil-source rock correlation in the Tempoku basin of northern Hokkaido, Japan

Abstract Geochemical correlation studies of seven oils and source rocks from the onshore Tempoku basin of northern Hokkaido were carried out. Low sulfur, high wax and high pristane/phytane ratios of the oils suggest that they were generated mainly from nonmarine source rocks. Source rock evaluation and hydrous pyrolysis data show that coals and coaly shales in the Paleogene Haboro Formation and Cretaceous Hakobuchi Group have oil-generating potential. These nonmarine source rocks are correlated as the major source of the Tempoku oils, using sterane and triterpane distributions and carbon isotope compositions. Correlation studies also suggest a minor contribution from marine source rocks of the Neogene Onishibetsu or Paleogene Magaribuchi Formations to the oils of the Yuchi and Wakkanai fields located in the western part of the basin

Source and Reservoir Rock Distributions in Coal-bearing Non-marine Sediments within a Sequence/Tectono-Stratigraphic Framework: Implications for Non-marine Rock Exploration

Coal-bearing non-marine sediments are one of major targets of hydrocarbon exploration, as it contains high sourceand reservoir-rock potential. Since non-marine sediments are more heterogeneous in general than marine sediments, this paper attempted to construct a realistic and practical model for source and reservoir rock distributions in non-marine sediments within a sequence/tectono-stratigraphic framework, which focuses on controlling factors on the distribution patterns. This study selected 3,000 m-thick Eocene coal-bearing non-marine sediments in Hokkaido, Japan as a case study field to collect basic data for constructing and testing the sourceand reservoir-rock distribution model. In addition to sedimentological field analysis, geochemical source-rock potential analysis was conducted, and relevant published data were also considered to obtain a comprehensive model. The obtained non-marine sequence stratigraphic model indicates that a sequence boundary (SB) of a non-marine depositional sequence can be recognized at the bottom of an incised valley. A depositional sequence can be divided into lower half (Lowstand Systems Tract, Transgressive Systems Tract) and upper half (Highstand Systems Tract). The boundary between two is a maximum flooding surface (MFS), which is recognized in a marine incursion interval within fluvial sediments. Fluvial channel sandstones, which are major reservoir rocks in non-marine sediments, occur frequency in LST, lower TST and upper HST, whereas occur rarely in upper TST and lower HST. Coals and coaly mudstones, which show the highest organic geochemical potential and are considered as the major source rocks of non-marine sediments, occur dominantly in upper TST, if there is no large-scale marine incursion bed. With regard to spatial variation, high potential coal seams dominantly to occur at an area where sediment accumulation rate and accommodation rate related to basin subsidence are balanced within a basin. On the other hand, coal potential tends to be low in a highly subsiding area, where flooding and clastic dilution events are common, and in a subtle subsidence area, where coal preservation potential is low. Introduction Coal-bearing non-marine sediments are on the spotlight in recent hydrocarbon explorations, as it contains high sourceand reservoir-rock potential. In Southeast Asia, coal-bearing non-marine sediments dominantly occur as syn-rift to early post rift sediments, and they are regarded as one of the major targets of hydrocarbon explorations. Since non-marine sediments are more heterogeneous in general than marine sediments, non-marine rock explorations commonly require a special model for sourceand reservoir-rock distributions. In 1990’s, some researchers tried to apply sequence stratigraphic concepts to nonmarine sediments as a new genetic stratigraphic method, and proposed fundamental non-marine sequence models. Although these studies presented general trend of sourceand reservoir-rock distributions, especially in a vertical succession, it seems that we still need more case studies to construct a precise and practical model for the three-dimensional distributions of nonmarine source and reservoir rocks, as actual distribution patterns tend to be more complex and variable in various tectonic settings. Since non-marine sedimentation is strongly controlled by various factors such as tectonics, climate and sea-level change, reexamination of the precise distribution model requires consideration of sedimentation controlling factors during sourceand reservoir-rock deposition.