Molecular and Carbon Isotope Compositions of Hydrocarbon Gas in Neogene Sedimentary Rocks in Horonobe Area, Northern Hokkaido, Japan

Abstract

We conducted headspace gas analyses using cores from two 520 m boreholes to evaluate the gas permeability of Neogene sedimentary rocks in Horonobe, Hokkaido, Japan. Analytical data suggest that most hydrocarbon gases are microbial methane. Further analyses of these data indicated that the low concentration and heavy carbon isotope composition of methane could be explained by carbon isotopic fractionation during migration near fractures. The high residual volume of microbial methane observed in the study area suggests that the sedimentary layers have low gas permeability, and are affected less by secondary post-generation processes. The headspace gas analysis is an effective approach for detecting sedimentary layers having a high confinement capability.

The origin of muddy sand sediments associated with mud volcanism in the Horonobe area of northern Hokkaido, Japan

“…Funaki et al . [] reported that the δ 13 C CH4 and C 1 /(C 2 + C 3 ) ratios of a biogenic gas from Neogene sedimentary rocks in the Horonobe area are less than −50‰ and several thousand, respectively.…”

“…The d 13 C CH4 and C 1 /(C 2 1 C 3 ) ratios of the gas in the Haboro and Hakobuchi formations, as determined by head-space analysis, are approximately 230& and <10, respectively [JNOC, 1995]; these values indicate that the gases are of thermogenic origin [JNOC, 1995]. Funaki et al [2012] reported that the d 13 C CH4 and C 1 /(C 2 1 C 3 ) ratios of a biogenic gas from Neogene sedimentary rocks in the Horonobe area are less than 250& and several thousand, respectively.…”

Improvements in Drill-Core Headspace Gas Analysis for Samples from Microbially Active Depths

“…The δ 13 C CH4 values of gases from depths of <1000 m, in the biogenic region, are usually in the range of -70‰ to −60‰, with isotopic compositions becoming heavier as depth increases towards the thermogenic region (e.g., [12,13]). Large variations in carbon isotopic ratios in CH 4 and CO 2 are often reported for depths of <1000 m, with δ 13 C CH4 values sometimes reaching −20‰ (e.g., [14][15][16][17]). These variations are associated with the effects of microbial activity on methane production or oxidation in underground environments [18].…”

“…In a previous study, gas samples from two boreholes (PB-V01 and SAB-1, both~500 m deep) in the Horonobe area, Hokkaido, were processed using IsoJar™ containers [14]. In that study, cores were stored in IsoJar™ containers with water and a few drops of BKC solution [14] for up to three months before headspace analysis.…”

“…In a previous study, gas samples from two boreholes (PB-V01 and SAB-1, both~500 m deep) in the Horonobe area, Hokkaido, were processed using IsoJar™ containers [14]. In that study, cores were stored in IsoJar™ containers with water and a few drops of BKC solution [14] for up to three months before headspace analysis. Because sampling date of cores and analysis date of gases, which are necessary for the calculation of the storage period, have not been presented in Funaki et al [14], these unpublished information are summarized in Tables S1a and S1b.…”