Stable carbon isotope behaviour of natural seepage of deep underground C-13-rich methane detected along a fault zone and adsorbed in mudstone: Tokyo Bay area, Japan

作者：Nakata E^*; Tanaka S; Suenaga H; Mahara Y; Nakamura T; Nakagawa K; Ohta T

来源：Applied Geochemistry, 2012, 27(9): 1710-1723.

DOI：10.1016/j.apgeochem.2012.02.007

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 delta C-13(1) = -44.3 parts per thousand was obtained for gas in the container and delta C-13(1) = -36.3 parts per thousand for seeping gas in a fault zone. However, typical CH4 in this dissolved-in-water gas-field is mainly depleted in C-13, and delta C-13(1) values range from -66 parts per thousand to -68 parts per thousand owing to microbial degradation of organic material. C-13-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 30 m. Gas seepages were not observed below a depth of 40 m. Gas rises along the fault zone dissolved-in-groundwater up to the shallow region and then separates from the groundwater. C-13-rich CH4 (adsorbed CH4) was found to have desorbed from drilled mudstone core samples taken at depths of 1400-1900 m in the main gas-production strata. Similarly, C-13-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 1200-1500 m 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 C-13-rich CH4, derived from thermogenic gas without oil components in strata. It is assumed that C-12-rich CH4 is easily detached from core or pore water through gas production, whereas C-13-rich CH4 is strongly adsorbed on the surfaces of particles. The C-13-rich CH4 rises along the major tectonic line or up the 50 m wide normal fault zone from relatively deep sources in the Kanto region.

• 出版日期2012-9