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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.