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Stable carbon isotope behaviour of natural seepage of deep underground ¹³C-rich methane detected along a fault zone and adsorbed in mudstone: Tokyo Bay area, Japan
- Nakata, E., Tanaka, S., Suenaga, H., Mahara, Y., Nakamura, T., Nakagawa, K., Ohta, T.
- Applied geochemistry 2012 v.27 no.9 pp. 1710-1723
- biodegradation, carbon dioxide, drilling, gas production (biological), groundwater, methane, natural gas, oils, sandstone, seepage, shale, stable isotopes, tectonics, wells, Japan
- Gas was sampled regionally, including by drilling into faults, in the South Kanto gas-field around Tokyo Bay, Japan. Gas samples were collected from cores in a gas sampling container immediately after drilling. A value of δ¹³C₁=−44.3‰ was obtained for gas in the container and δ¹³C₁=−36.3‰ for seeping gas in a fault zone. However, typical CH₄ in this dissolved-in-water gas-field is mainly depleted in ¹³C, and δ¹³C₁ values range from −66‰ to −68‰ owing to microbial degradation of organic material. ¹³C-rich CH₄ is so far uncommon in the South Kanto gas-field. Seepages were observed from the surface along the north–south fault zone. The natural gas is stored below the sandstone layer by impermeable mudstone underlying the boundary at a depth of 30m. Gas seepages were not observed below a depth of 40m. Gas rises along the fault zone dissolved-in-groundwater up to the shallow region and then separates from the groundwater. ¹³C-rich CH₄ (adsorbed CH₄) was found to have desorbed from drilled mudstone core samples taken at depths of 1400–1900m in the main gas-production strata. Similarly, ¹³C-rich CH₄ was found in black shale overlying the oceanic crust forming part of a sedimentary accretionary prism underling the Tokyo region. It also appears in the spring-water of spa wells, originating at a depth of 1200–1500m along a tectonic line. Methane generated by microbial degradation of organic material through CO₂ reduction in the South Kanto gas-field mainly originates as biogenic gas mixed with a small amount of ¹³C-rich CH₄, derived from thermogenic gas without oil components in strata. It is assumed that ¹²C-rich CH₄ is easily detached from core or pore water through gas production, whereas ¹³C-rich CH₄ is strongly adsorbed on the surfaces of particles. The ¹³C-rich CH₄ rises along the major tectonic line or up the 50m wide normal fault zone from relatively deep sources in the Kanto region.