Physical diagenesis and rock properties of the On'nagawa siliceous mudstone in the central and southern parts of the Neogene Akita sedimentary basin
Hiromi HondaNaoya YoshitakeShuji YamamotoTakafumi AshidaTomohisa KawamotoTakashi TsujiToshio Todoroki
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Abstract:
The On'nagawa Formation in Akita Sedimentary Basin is composed of siliceous mudstone and volcaniclastics. The volocaniclastic facies have been productive reservoirs of oil and gas fields in the Neogene Akita Basin, North Japan. The mudstone facies have been recognized to be potential source rocks for hydrocarbons pools in the reservoirs. After a long period of oil and gas production in the basin, a potential exploration concept of a new play in the siliceous rocks is required to keep the productivity of the basin.This study reports a basic description of physical properties of the siliceous mudstone facies in order to provide a basis for the idea of shale play in the On'nagawa Formation. Wireline logs (GR, Δt, ρb) are basic data for our description of the physical rock properties as well as descriptions of cutting lithology.Keywords:
Neogene
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ABSTRACT Diagenetic changes in textures and mass properties of Neogene marine noncalcareous siliceous rocks corresponding to silica-phase transformations (opal-A opal-CT quartz) were studied in subsurface and surface sections of northern Japan. In opal-A diatomite, siliceous tests are well preserved, no cementation is observable, and pores exist chiefly as inter- and intragranular micropores (2 10µ). During the opal-A to opal-CT transformation, siliceous tests are extensively dissolved to form fine aggregates of opal-CT in matrix and cement of opaline porcelanite. An abrupt decrease of as much as 15% in porosity occurs between opal-A and opal-CT zones in subsurface sections, a decrease which may be due to the destruction of intragranular pores in siliceous tests by dissolution. Ultramicropores (10 µ) as molds and chambers of siliceous tests in quartzose porcelanite. Porosity of Neogene noncalcareous siliceous rocks in subsurface sections of northern Japan gradually decreases with the increase of burial depth, and silica-phase transformations have a relatively small effect on it. This gradual decrease in porosity, which corresponds well to that of Neogene mudstone and clean, fine-grained sandstone, indicates that little additional silica cementation occurs during burial (Iijima and Tada, 1981). At a depth of 4.5 km and a temperature of 130° C, quartzose porcelanite holds porosity of 10-15%. Burial depth of around 10 km might be required to form dense quartzose chert without porosity by burial diagenesis of siliceous rocks if extrapolating from the burial depth-porosity relation. This depth is too large. Some additional mechanisms are required ther than mechanical compaction, silica-phase transformations and clay alteration. Pressure solution-reprecipitation of quartz is the most probable one.
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The On'nagawa Formation in Akita Sedimentary Basin is composed of siliceous mudstone and volcaniclastics. The volocaniclastic facies have been productive reservoirs of oil and gas fields in the Neogene Akita Basin, North Japan. The mudstone facies have been recognized to be potential source rocks for hydrocarbons pools in the reservoirs. After a long period of oil and gas production in the basin, a potential exploration concept of a new play in the siliceous rocks is required to keep the productivity of the basin.This study reports a basic description of physical properties of the siliceous mudstone facies in order to provide a basis for the idea of shale play in the On'nagawa Formation. Wireline logs (GR, Δt, ρb) are basic data for our description of the physical rock properties as well as descriptions of cutting lithology.
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