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

1. Introduction

Investigation of the origin of deep hydrocarbons is an important aspect of resource exploration and may lead to an improved understanding of geological environments. In this investigation, gases adsorbed on rock fragments or bore cores were studied by headspace gas analysis (e.g., [1-5]), which provides information on the generation and migration of light hydrocarbons and gases. The IsoJar[TM] (Isotech Laboratories and Humble Instruments, USA) container, which is widely used in such analyses (e.g., [6, 7]) comprises a plastic container of ~600 ml volume with an aluminum screw cap on which there is a rubber septum through which headspace gas can be taken by syringe. The analysis procedure (e.g., [8]) normally involves storage of wet cuttings or cores in the jar with water and an air headspace for several days or weeks, the addition of a microbicide such as benzalkonium chloride (BKC) to minimize bacterial activity, the partitioning of gas into the headspace during storage, and analysis of these gases (e.g., [9-11]). The use of distilled or tap water avoids contamination from dissolved gases. The [delta][sup.13][C.sub.CH4] values of gases from depths of < 1000 m, in the biogenic region, are usually in the range of -70 [per thousand] to -60 [per thousand], with isotopic compositions becoming heavier as depth increases towards the thermogenic region (e.g., [12, 13]). Large variations in carbon isotopic ratios in C[H.sub.4] and C[O.sub.2] are often reported for depths of < 1000 m, with [delta][sup.13][C.sub.CH4] values sometimes reaching -20 [per thousand] (e.g., [14-17]). These variations are associated with the effects of microbial activity on methane production or oxidation in underground environments [18]. There are a number of factors that control the rate of methanogenesis [19], including temperature [20], groundwater salinity [21], pH [22], and pore space [23]. Peak microbial activity occurs at 35-45[degrees]C, which corresponds to depths of <1000m [19, 24]. At greater depths, microbial action decreases as thermogenic production increases with the onset of catagenesis (subsequent to diagenesis at shallower depths [8]). More importantly, pore diameters of at least 1 [micro]m are necessary for in situ methanogenesis, as microbes are in the 1-10 [micro]m size range [25], which suggests that active methane production occurs at depths of < 1500 m [24]. At shallow depths (less than several meters) below the ocean floor, where the concentration of dissolved gas is relatively low, considerable care was taken to avoid contamination and microbial activity (e.g., [26]). Hachikubo et al. [27] adjusted the concentration of BKC in samples to ~2.5% using 25 ml vials to obtain precise depth profiles of gases relative to hydrates. While the concentration and/or amount of microbicide normally added to IsoJar[TM] vessels are often omitted in reports, it is considered that the final concentration in IsoJar[TM] containers should be of the order of 0.01%, which is two orders of magnitude less than that reported by Hachikubo et al. [27]. It is speculated that another possible cause of variations in carbon isotopic composition may be microbial activity in the headspace after sampling, as the amount of microbicide commonly used with samples from microbially active depths might be insufficient to suppress microbial activity.

In a previous study, gas samples from two boreholes (PB-V01 and SAB-1, both ~500m deep) in the Horonobe area, Hokkaido, were processed using IsoJar[TM] containers [14]. In that study, cores were stored in IsoJar[TM] 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. Concentration of the BKC solution and amounts of cores and water also have not been reported in Funaki et al. [14]. In the conventional way of using IsoJar[TM] headspace gas analysis, it is considered that the concentration of BKC solution is lower than 10%, in which case concentrations of BKC in jars are in order of 0.01%. In the construction of the Horonobe Underground Research Laboratory (URL), including two boreholes (PBV01 and SAB-1), water was collected from groundwater at ~50 m depth, and this was used by Funaki et al. [14] as filling water for the IsoJar[TM] containers. Gases dissolved in deep groundwater from the URL were also analyzed, using an evacuated-vial (EV) method [28]. Measured [delta][sup.13]C values for C[H.sub.4] and C[O.sub.2] from both sets of analyses are plotted against each other in Figure 1. Large variations in [delta][sup.13][C.sub.CH4] values from the IsoJar[TM] measurements (Figure 1) were attributed to methane-oxidizing bacterial activity using sulfate ions in the deep underground environment or to isotopic fractionation during gas migration through fractures [14]. However, these possible causes are considered unlikely because (a) geochemical studies in the Horonobe area indicate that reducing conditions are maintained deep underground and sulfate ions are either absent or present at very low concentrations [29-32]; (b) studies of iodine enrichment [33] indicates that any traces of methane oxidation by sulfate in pore waters of sediments would have been erased during upward fluid flow due to compaction during burial; and (c) there is no evidence in the study area of isotopic fractionation in gases during migration [34, 35]. The [delta][sup.13][C.sub.CH4] and [delta][sup.13][C.sub.CO2] values obtained by the EV method (Figure 1) show little scatter plotting in the carbonate reduction field.

Differences between these data sets could be attributed to factors such as aerobic microbial oxidation of methane in the containers after sampling and/or using groundwater from the different depths of core samples. Possible causes were investigated in the present study to improve the methodology of headspace gas analysis using IsoJar[TM]. Gases from the Wakkanai Formation in the Horonobe area were sampled using the methods of Funaki et al. [14] and Miyakawa et al. [28]. The effects of the sampling method (storage period, water type, and additives) on carbon isotopic ratios in C[H.sub.4] and C[O.sub.2] were investigated, and improvements in headspace gas analysis techniques are suggested.

2. Geological Setting

The Horonobe area is located in northwestern Hokkaido, in a Neogene-Quaternary sedimentary basin (Figure 2). Since August 2006, the Japan Atomic Energy Agency (JAEA) has been excavating the URL for a research associated with the development of technologies related to the geological disposal of high-level radioactive waste. Geologically, the URL area comprises marine sediments of the Wakkanai Formation (Neogene siliceous mudstone containing opal-CT) and Koetoi Formation (Neogene-Quaternary diatomaceous mudstone containing opal-A). Burial and subsidence of these formations occurred throughout the Neogene and Quaternary, when they underwent early diagenetic thermal alteration at temperatures of <60[degrees]C [36]. The URL and surrounding geology are depicted in Figure 3. The high-pressure boreholes have steel casings, with valves allowing the sampling of groundwater from multiple depths or zones [37]. The [delta]D vs. [delta][sup.18]O plots for groundwater from the Wakkanai and Koetoi Formations indicate that it is a mixture of local meteoric water and altered seawater [38, 39]. Methanogenic and methane-oxidizing microbial communities have played an important role in these formations [40-42], where secondary microbial gas with [sup.13]C-enriched isotopic values ([delta][sup.13][C.sub.CH4], -74 [per thousand] to -28 [per thousand]; [delta][sup.13][C.sub.CO2], -7 [per thousand] to +31 [per thousand]) formed through C[O.sub.2] reduction af.sub..sub.ter uplift of the area [28].

3. Sampling and Analytical Methods

3.1. IsoJar[TM] Samples

3.1.1. Effect of Storage Period. Core samples including in situ pore water (300-400 g), crushed to pieces roughly 30-50 mm in diameter, were placed in IsoJar[TM] containers with 250-300 g of "filling" water. Hg[Cl.sub.2]-saturated or 10% BKC aqueous solution (10 drops (~0.5ml) of either) was added to suppress microbial activity, and the jars were sealed with an air headspace. Final concentrations of both microbicides in the jars were about 0.01%-0.02%. The jars were kept in the dark at room temperature for 5-92 d before analysis. Details of each experiment are summarized in Table 1 and Table S2.

The cores were obtained immediately after drilling of borehole 350-Fz-01 from the bottom of the east shaft (Figure 3). Although distilled or tap water is usually used in the IsoJar[TM] method, groundwater from depths of 53.5-64.5 m and 350 m in borehole 13-350-C01 (drilled in the 350 m gallery; Figure 3) was used to match the 50 m groundwater used by Funaki et al. [14]. The priority in this study was to evaluate the effects of sampling method on the carbon isotopic ratios of C[H.sub.4] and C[O.sub.2], rather than to obtain accurate in situ values. A large portion of C[H.sub.4] dissolved in groundwater around the URL had already escaped due to the pressure decrease associated with excavation, so it was expected that only small amounts of gas would remain in cores. Groundwater, rather than tap water, was used to compensate for this (with water from borehole 13-350-C01 being used because water was not available from borehole 350-Fz-01). Core samples IJ1-IJ25 were from depths of 384-416 m (Table S2). Core samples IJ26-IJ28 were from a depth of 470 m (Table S2) and kept in vacuum storage for one month after drilling. At these depths, the C isotopic ratios for C[H.sub.4] and C[O.sub.2] had similar values to those from 350 m depth [28].

3.1.2. Effect of Additives. The effective amount of additives was investigated as follows. IsoJar[TM] samples were prepared as described in Section 3.1.1, with up to 20 ml of BKC and Hg[Cl.sub.2] solutions per jar (Table 2 and Table S3). All core samples were from a depth of 480 m in 350-Fz-01. Fresh cores taken immediately after drilling were not available for analysis, and the samples used in this study had been kept in vacuum storage for about six months. Groundwater from borehole 13-350-C01 was used as filling water. Headspace gas compositions were determined after storage in IsoJar[TM] for one month.

3.2. EV Samples. The EV sampling procedure involved the preliminary evacuation of septum-topped 50 ml glass vials containing ~1 ml phosphoric acid (85 wt%). Acidification removed any inorganic carbon as C[O.sub.2], with the C[O.sub.2] concentration being measured as total inorganic carbon (TIC). Groundwater (15-30 ml, from borehole 09-V250-M02#1 drilled from the 250 m gallery; Figure 3) was introduced by a syringe through a 0.22 [micro]m membrane filter to remove microsized carbonate grains and microbes. Samples were stored in the dark at room temperature for 5-98 d, after which ultrapure He was added by a syringe to equalize headspace gas and atmospheric pressures. The sample was left to stand overnight for gas exchange equilibrium to be established between the headspace and dissolved gases. The composition of the headspace gas was determined by GC, with the concentration of dissolved gas being calculated using Henry's law and the ideal gas equation.

3.3. Analytical Procedure. Gases adsorbed on rock fragments in the IsoJar[TM] containers were desorbed into the headspace by ultrasonic shaking. Concentrations of [O.sub.2], [N.sub.2], C[O.sub.2], C[H.sub.4], [C.sub.2][H.sub.6], and [C.sub.3][H.sub.8] in headspace gas were determined by gas chromatography (GC) using a GC7890A Valve System (Agilent Technologies, USA). Carbon isotopic values ([delta][sup.13][C.sub.CH4] and [delta][sup.13][C.sub.CO2]) were determined by GC combustion isotope-ratio mass spectrometry (GC-C-IRMS), using an IsoPrime GC-MS system (GV Instruments, UK), and are expressed in the usual VPDB [delta] notation. The lower limit of determination of carbon isotope ratios ([delta][sup.13][C.sub.CH4] and [delta][sup.13][C.sub.CO2]) requires concentrations of 0.01%. Details of Gc and GC-C-IRMS procedures can be found in Waseda and Iwano [43].

4. Results

Headspace concentrations of the gases analyzed and [delta][sup.13][C.sub.CH4] and [delta][sup.13][C.sub.CO2] values for the IsoJar[TM] samples prepared as described in Section 3.1.1 are listed in Table 1 and Table S2, and the results for the EV samples are listed in Table 3 and Table S4.

Sample contamination by air does not affect the present discussion of isotopic ratios because concentrations of C[H.sub.4] and C[O.sub.2] in air are much lower than those in the free gas from groundwater (where C[H.sub.4] = 74%-100%; C[O.sub.2] = 1%-20%; [28, 34]).

C[O.sub.2] in the free gas undergoes isotopic fractionation during exchange with dissolved inorganic carbon (e.g., C[O.sub.2(aq)], HC[O.sub.3.sup.-], and C[O.sub.3.sup.2-]), with HC[O.sub.3.sup.-] being the dominant aqueous species at around neutral pH. To compare [delta][sup.13][C.sub.CO2] values of the IsoJar[TM] samples with those of the EV samples, a fractionation correction (intrinsic isotopic fractionation factor) of +7.9 [per thousand] between C[O.sub.2] gas and HC[O.sub.3.sup.-] at 25[degrees]C [44] was added to [delta][sup.13][C.sub.CO2] values measured for the IsoJar[TM] samples. The range of room temperatures was 20[degrees]C-25[degrees]C, giving an error of up to -0.6 [per thousand].

Variations in [delta][sup.13][C.sub.CO2] and [delta][sup.13][C.sub.CH4] values with storage time are shown in Figures 4(a) and 4(b), respectively. IsoJar[TM] samples with groundwater (350 m) stored for less than one week give values similar to the EV samples. Storage periods of [less than or equal to] 7 d and [greater than or equal to] 20 d were considered, as no data are available for the interval of 7-20 d.

The [delta][sup.13][C.sub.CO2] and [delta][sup.13][C.sub.CH4] ratios for the EV samples are relatively constant, although [delta][sup.13][C.sub.CH4] values show a small variation (<4%) after 98 d (Figure 4(b)), which is consistent with a previous study [28]. It is possible that some microbes are not removed by 0.22 [micro]m groundwater filtration (e.g., [45]), and this might have caused the slight variation in [delta][sup.13][C.sub.CH4] values. Differences between carbon isotopic values of C[H.sub.4] and C[O.sub.2] ([delta][sup.3][C.sub.CO2] - [delta][sup.13][C.sub.CH4] = isotopic separation factor; [46]) in the Horonobe area decrease slightly (<1 [per thousand]) with increasing depth and temperature according to isotopic equilibrium values, with values at depths of 250 m and 350 m being similar to each other within the natural variation of ~2 [per thousand] (1[sigma]) [28]. Therefore, [delta][sup.13][C.sub.CO2] and [delta][sup.13][C.sub.CH4] values of 250 m groundwater obtained by the EV method (Table 3) were used as reference values for the IsoJar[TM] samples with 350 m groundwater.

The [delta][sup.13][C.sub.CO2] values for IsoJar[TM] samples show two separate trends for 350 m and 50 m groundwater, with both showing a slight decrease over time, from +15 [per thousand] at 5 d to +13 [per thousand] at 92 d and from +5 [per thousand] at 17 d to -1 [per thousand] to +3 [per thousand] at 79 d, respectively (Figure 4(a)). The patterns are the same for both BKC and Hg[Cl.sub.2] additives. The two trends suggest that the [delta][sup.13][C.sub.CO2] ratios represent dissolved gases from different depths. The concentration of C[O.sub.2] in the groundwater, as HC[O.sub.3.sup.-], is relatively high even when degassed at atmospheric pressure (30-50 mmol [kg.sup.-1]; [47]), compared with that of adsorbed C[O.sub.2], so carbon isotopic values should be largely dependent on dissolved C[O.sub.2]. The [delta][sup.13][C.sub.CO2] values of samples with 50 m groundwater (dashed line in Figure 4(a)) are distinct from those of samples with 350 m groundwater, indicating that the former samples are strongly contaminated.

The [delta][sup.13][C.sub.CH4] values for IsoJar[TM] samples (Figure 4(b)) increase markedly with time regardless of additives and fluctuate by more than 30 [per thousand] after 80 d; separate trends for different depths are not evident. The concentration of C[H.sub.4] remaining in the groundwater at atmospheric pressure after sampling is relatively low (~3 mmol [kg.sup.-1]; [47]), and [delta][sup.13][C.sub.CH4] values for the IsoJar[TM] samples mainly represent gases adsorbed on cores. Therefore, while the relatively large amounts of dissolved C[O.sub.2] reduced the effects of isotopic fractionation on [delta][sup.13][C.sub.CO2] values, [delta][sup.13][C.sub.CH4] values were strongly affected.

The results of the effect of additives as described in Section 3.1.2 are shown and discussed in Section 5.2.

5. Discussion

5.1. Methane Oxidation. During methane oxidation, decreasing C[H.sub.4] and increasing C[O.sub.2] concentrations are associated with carbon isotopic fractionation, resulting in enrichment of [sup.13]C in unreacted C[H.sub.4] and depletion in C[O.sub.2] produced. However, there is no clear relationship between headspace gas concentrations and storage period, possibly because of the variability of adsorbed gas levels in natural samples. An apparent carbon isotopic fractionation factor, a, defined as [alpha] = ([delta][sup.13][C.sub.CO2] + 1000)/([delta][sup.13][C.sub.CH4] + 1000), was calculated using the [delta][sup.13][C.sub.CO2] and [delta][sup.13][C.sub.CH4] values in Table 1 (Figure 5). Initial values of a in the Horonobe samples, calculated using the results of the EV method, were around 1.06-1.08, in good agreement with those determined by Miyakawa et al. [28]. The values of a for the IsoJar[TM] samples decreased from ~1.07 to 1.04 with increasing storage time (Figure 5). An earlier study reported a similar trend, where microbial methane oxidation in marine sediments gave an a value of ~1.08 in the methanogenesis zone, decreasing to ~1.02 in the methane oxidation zone [48].

A bivariate plot of [delta][sup.13][C.sub.CH4] vs. [delta][sup.13][C.sub.CO2] (Figure 6) indicates two trends for the IsoJar[TM] samples (Figure 6(b)), as in Figure 4(a). Thus, data plotted in Figures 4-6 indicate that [delta][sup.13][C.sub.CH4] values in samples stored for more than one week were affected by C[H.sub.4] oxidation to C[O.sub.2]. [delta][sup.13][C.sub.CH4] and [delta][sup.13][C.sub.CO2] values reported by Funaki et al. [14] are plotted between the two trends (Figure 6(b)). It seems, therefore, that the variations can be explained by the mixing of the two trends, indicating effects of both methane oxidation and contamination of 50 m groundwater in the IsoJar[TM] container after sampling.

5.2. Effect of Additives. Significant isotopic fractionation occurred in the IsoJar[TM] samples, strongly affecting [delta][sup.13][C.sub.CH4] values despite the addition of BKC or Hg[Cl.sub.2] (Figure 4), suggesting that the amounts of additives used were insufficient to suppress microbial activity. Results of effect of additives as described in Section 3.1.2 are listed in Table 2 and shown in Figure 7. With <10 ml BKC solution or <0.3% BKC concentration, [delta][sup.13][C.sub.CH4] values fluctuated significantly, and isotopic compositions became lighter. This is opposite to the effect of microbial carbonate reduction and may be due to low C[H.sub.4] concentrations (Table 2) resulting from storage of the cores for about six months in a vacuum container (to avoid oxidation by the air and drying), possibly with significant removal of adsorbed gases. An instrumental mass bias of carbon isotope ratio was reported in a very low concentration of hydrocarbons with respect to mass spectrometry [49]. In this study, all the data of C[H.sub.4] concentrations were above the lower limit of determination of 0.01% (Table 2) indicating that large fluctuations of [delta][sup.13][C.sub.CH4] values (Figure 7) are not due to instrumental mass bias. The mechanism of any reaction opposing fractionation is not clear. In a low C[H.sub.4] concentration, the carbon isotope ratio may be easily disturbed by complex microbial metabolism (e.g., methane oxidation and carbonate reduction). With >10 ml BKC solution or >0.3% BKC concentration, the [delta][sup.13][C.sub.CH4] values were relatively constant at about -56 [per thousand] (Figure 7), which is in good agreement with the values for EV samples.

Although the [delta][sup.13][C.sub.CH4] values for IsoJar[TM] samples with Hg[Cl.sub.2] solution are relatively constant (Figure 7), they are consistently 5 [per thousand]-6 [per thousand] lower than those with BKC solution. Concentrations of C[O.sub.2] in the headspace of samples IJ47 and IJ48, to which 20 ml Hg[Cl.sub.2] solution was added, are considerably higher than those in the other samples (Table 2). A possible cause of the decrease in [delta][sup.13][C.sub.CH4] values may be isotopic fractionation associated with mercuric reactions with methane, which generate HCl and lead to C[O.sub.2] outgassing. Although this mechanism is not clear, considerable care should be required using Hg[Cl.sub.2] as a microbicide with respect to carbon isotope fractionation.

6. Conclusions

This study investigated the possible causes of carbon isotopic variations (in [delta][sup.3][C.sub.CH4] and [delta][sup.13][C.sub.CO2] values) in borehole drillings to 500 m depth, as reported by Funaki et al. [14]. The results have led to improvements in the IsoJar[TM] method for the determination of carbon isotopic ratios in C[H.sub.4] and C[O.sub.2] adsorbed on bore cores. It was found that with air in the IsoJar[TM] headspace, microbes oxidize C[H.sub.4] to C[O.sub.2] during storage, accompanied by isotopic fractionation, especially for samples from depths of < 1000 m where microbes are more active. Isotopic fractionation resulted in [delta][sup.13][C.sub.CH4] and [delta][sup.13][C.sub.CO2] values reaching >30 [per thousand] and >2 [per thousand] after 80 d storage, respectively, while samples analyzed within a week of sampling showed no such effect. The significant isotopic fractionation in C[H.sub.4] was due to its low concentration in the sampling container, while the weaker fractionation in C[O.sub.2] was due to its relatively high concentration. The conventional amount of BKC additive (~0.5 ml of 10% solution) was insufficient to suppress microbial activity at least when using in situ groundwater as filling water. The large variations in isotopic compositions reported by Funaki et al. [14] thus appear to have been caused by microbial methane oxidation in the IsoJar[TM] containers after sampling and contamination with groundwater from different depths. Important technique improvements are summarized as follows: (1) if long-term sample storage is necessary, >10 ml of 10% BKC solution should be used or >0.3% BKC concentration is required; (2) analysis within a week of sampling is strongly recommended; and (3) for C[O.sub.2] analysis, groundwater from different depths should not be used.

Data Availability

The all data used to support the findings of this study are included within the article and Supplementary information file.

https://doi.org/10.1155/2018/2436814

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

The authors are grateful to Hironori Funaki (JAEA) and Taiki Ishikawa (Mitsubishi Materials Techno (MMT)) for providing helpful information regarding sampling methods. Amane Waseda (JAPEX) provided valuable comments that greatly improved the manuscript. The authors would also like to thank Yusuke Kitagawa (MMT) and Eiichi Ishii (JAEA) for the help with sampling. Hiroshi Sasamoto (JAEA) and Aaron Stallard (Stallard Scientific Editing) are gratefully acknowledged for improving the manuscript.

Supplementary Materials

Sampling date, analysis date, sampling depth of cores, and carbon isotope ratios of C[H.sub.4] and C[O.sub.2] of headspace gases in IsoJar[TM] samples of Funaki et al. [14] (Tables S1a and S1b). Sampling date, analysis date, sampling depth of cores, and chemical compositions of headspace gases in IsoJar[TM] samples and EV samples of this study (Tables S2, S3, and S4). (Supplementary Materials)

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Kazuya Miyakawa (1) and Fumiaki Okumura (2)

(1) Horonobe Underground Research Center, Japan Atomic Energy Agency, Hokushin 432-2, Horonobe-cho, Hokkaido 098-3224, Japan

(2) JAPEX Research Center, Japan Petroleum Exploration Co., Ltd., 1-2-1 Hamada, Mihama-ku, Chiba 261-0025, Japan

Correspondence should be addressed to Kazuya Miyakawa; miyakawa.kazuya@d.nagoya-u.jp

Received 11 April 2018; Accepted 29 July 2018; Published 1 October 2018

Academic Editor: Andri Stefansson

Caption: Figure 1: Plot of [delta][sup.13][C.sub.CH4] vs. [delta][sup.13][C.sub.CO2] in coexisting gases, showing fields relating to different gas sources and isotopic shifts resulting from production and oxidation (adapted from [50]). The data are from Funaki et al. [14] and Miyakawa et al. [28].

Caption: Figure 2: Maps showing the location of the Horonobe URL site and the boreholes: (a) location map and (b) geological map (after [51]). The plate boundaries and the direction of plate movement in (a) are from Wei and Seno [52].

Caption: Figure 3: Layout of the Horonobe URL, locations of boreholes, and surrounding geology. Red color-labelled "borehole 350-Fz-01" indicates core sampling depths (Table S2).

Caption: Figure 4: Temporal variations in (a) [delta][sup.13][C.sub.CO2] and (b) [delta][sup.13][C.sub.CH4] after sampling. Solid arrow indicates the trend for samples with 350 m groundwater; dashed arrow indicates the trend for samples with 50 m groundwater.

Caption: Figure 5: Temporal variations in the apparent carbon isotopic fractionation factor, [alpha], between C[O.sub.2] and C[H.sub.4].

Caption: Figure 6: Plot of [delta][sup.13][C.sub.CH4] vs. [delta][sup.13][C.sub.CO2] in coexisting gases. (a) Grey areas show fields relating to different gas sources and isotopic shifts resulting from production and oxidation (adapted from [50]). The area indicated by the dashed rectangle is enlarged in (b). (b) Solid arrow indicates a methane oxidation trend for samples with 350 m groundwater; dashed arrow indicates the trend for samples with 50 m groundwater.

Caption: Figure 7: Plot of [delta][sup.13][C.sub.CH4] values vs. (a) amount of microbial suppressant used and (b) concentration of additives.
Table 1: Information on IsoJar[TM] samples and analytical
results of headspace gases.

            Amount
Sample      of core     Amount of      Depth of        Type of
no.       sample (g)    water (g)    groundwater       additive

IJ1           220          350          350 m            None
IJ2           369          262          350 m            None
IJ3           342          304          350 m            None
IJ4           360          269           50 m            None
IJ5           239          333           50 m            None
IJ6           411          247           50 m            None
IJ7           339          277           50 m            None
IJ8           338          279           50 m            None
IJ9           302          274          350 m        Hg[Cl.sub.2]
IJ10          263          328          350 m        Hg[Cl.sub.2]
IJ11          380          274          350 m        Hg[Cl.sub.2]
IJ12          383          241          350 m        Hg[Cl.sub.2]
IJ13          358          262           50 m        Hg[Cl.sub.2]
IJ14          225          349           50 m        Hg[Cl.sub.2]
IJ15          420          252           50 m        Hg[Cl.sub.2]
IJ16          338          284           50 m        Hg[Cl.sub.2]
IJ17          341          229          350 m            BKC
IJ18          264          281          350 m            BKC
IJ19          388          253          350 m            BKC
IJ20          333          302          350 m            BKC
IJ21          320          290           50 m            BKC
IJ22          345          256           50 m            BKC
IJ23          392          258           50 m            BKC
IJ24          368          291           50 m            BKC
IJ25          360          266           50 m            BKC
IJ26          388          242          350 m            None
IJ27          379          272          350 m            None
IJ28          333          316          350 m            None

          Storage
Sample     period     C[O.sub.2]    [O.sub.2]    [N.sub.2]
no.        (days)         (%)          (%)          (%)

IJ1          67           6.5          6.9           83
IJ2          41           8.9          2.8           79
IJ3          79           3.1          11.0          80
IJ4          17          0.63          14.6          80
IJ5          68           2.3          9.4           86
IJ6          42           6.7          0.01          79
IJ7          79           2.5          5.4           80
IJ8          79          0.36          18.1          79
IJ9          19           3.9          7.9           80
IJ10         67           5.0          2.4           86
IJ11         41           4.7          5.1           76
IJ12         92          1.49          14.1          80
IJ13         19           2.6          8.1           75
IJ14         68          0.52          16.8          81
IJ15         42           4.0          3.6           83
IJ16         79          0.57          14.2          80
IJ17         19           6.2          6.9           82
IJ18         67           8.9          2.2           80
IJ19         41           7.4          0.02          76
IJ20         79           5.0          4.4           79
IJ21         19           4.7          1.82          85
IJ22         68           3.7          8.2           84
IJ23         42           4.4          3.2           80
IJ24         79           2.2          4.1           80
IJ25         79           2.6          3.0           79
IJ26          5           2.3          14.2          79
IJ27          5           3.0          11.6          78
IJ28          5           2.1          12.7          80

                         [delta][sup.13]      [delta][sup.13]
                            [C.sub.CH4]           [C.sub.CO2]
Sample    C[H.sub.4]     ([per thousand]       ([per thousand]
no.           (%)              VPDB)                 VPDB)

IJ1          0.030             -45.4                 +13.6
IJ2           6.7              -35.5                 +13.2
IJ3           2.8              -30.8                 +13.2
IJ4           2.2              -49.3                 +5.0
IJ5          0.014             -51.4                 +2.1
IJ6          12.5              -42.5                 +3.5
IJ7           9.6              -36.8                 -1.0
IJ8          0.45              -22.3                 -1.4
IJ9           5.8              -57.7                 +15.2
IJ10          4.8              -40.7                 +14.5
IJ11         12.3              -48.8                 +15.7
IJ12         0.88              -26.8                 +12.9
IJ13         11.9              -55.3                 +6.0
IJ14         0.059             +4.7                  +2.7
IJ15          7.5              -45.0                 +4.7
IJ16          2.0              -38.2                 +2.5
IJ17         1.89              -50.3                 +14.5
IJ18          5.2              -44.6                 +13.5
IJ19         15.5              -48.6                 +14.3
IJ20          7.6              -46.2                 +14.4
IJ21          5.5              -46.2                 +3.7
IJ22          2.3              -26.7                 +2.8
IJ23         10.2              -42.7                 +4.3
IJ24         10.2              -41.8                 +2.2
IJ25         13.4              -43.5                 +3.2
IJ26          3.6              -55.4                 +14.5
IJ27          6.1              -53.8                 +14.8
IJ28          3.5              -55.5                 +15.1

Table 2: Information on IsoJar[TM] samples and analytical
results of headspace gases for Section 3.1.2.

          Amount of       Amount
Sample       core        of water        Type of         Amount of
no.       sample (g)        (g)          additive      additive (ml)

IJ29          311           310            None              0
IJ30          299           282            None              0
IJ31          284           324            BKC               1
IJ32          323           297            BKC               1
IJ33          332           312            BKC               3
IJ34          352           279            BKC               3
IJ35          368           290            BKC               6
IJ36          382           278            BKC               6
IJ37          309           299            BKC               10
IJ38          310           280            BKC               10
IJ39          340           285            BKC               20
IJ40          336           309            BKC               20
IJ41          272           319        Hg[Cl.sub.2]          1
IJ42          323           296        Hg[Cl.sub.2]          1
IJ43          348           284        Hg[Cl.sub.2]          5
IJ44          319           314        Hg[Cl.sub.2]          5
IJ45          293           322        Hg[Cl.sub.2]          10
IJ46          362           293        Hg[Cl.sub.2]          10
IJ47          285           308        Hg[Cl.sub.2]          20
IJ48          362           295        Hg[Cl.sub.2]          20

          Concentration     Storage
Sample     of additives      period
no.            (wt%)         (days)     C[O.sub.2] (%)

IJ29             0             27            12.8
IJ30             0             27            13.7
IJ31           0.03            27            12.5
IJ32           0.03            27            12.4
IJ33           0.10            27            11.6
IJ34           0.11            28            13.1
IJ35           0.20            28            10.2
IJ36           0.21            28            10.2
IJ37           0.32            28            12.8
IJ38           0.34            28            12.8
IJ39           0.66            29            10.9
IJ40           0.61            29             9.0
IJ41           0.02            29             8.7
IJ42           0.02            29            10.2
IJ43           0.13            29            10.8
IJ44           0.12            29            11.0
IJ45           0.22            30            12.7
IJ46           0.24            30            13.3
IJ47           0.45            30             21
IJ48           0.47            30             23

Sample                                      C[H.sub.4]
no.       [O.sub.2] (%)    [N.sub.2] (%)        (%)

IJ29           1.0               86            0.039
IJ30           1.0               85            0.029
IJ31           1.0               86            0.036
IJ32           1.0               87            0.040
IJ33           4.2               84            0.053
IJ34           2.0               85            0.046
IJ35           4.1               86            0.067
IJ36           2.9               87            0.067
IJ37           7.3               80            0.054
IJ38           6.6               80            0.052
IJ39           6.2               83            0.068
IJ40           3.7               87            0.105
IJ41           10.9              80            0.042
IJ42           9.9               80            0.041
IJ43           9.2               80            0.037
IJ44           9.0               80            0.044
IJ45           8.4               79            0.043
IJ46           6.9               80            0.046
IJ47           7.0               72            0.033
IJ48           6.0               71            0.045

           [delta][sup.13]      [delta][sup.13]
              [C.sub.CH4]           [C.sub.CO2]
Sample     ([per thousand]       ([per thousand]
no.              VPDB)                 VPDB)

IJ29             -48.3                 +14.2
IJ30             -74.6                 +15.2
IJ31             -74.2                 +14.0
IJ32             -84.1                 +13.8
IJ33             -88.5                 +13.9
IJ34             -75.7                 +13.0
IJ35             -58.1                 +12.9
IJ36             -90.4                 +12.5
IJ37             -56.2                 +14.8
IJ38             -57.9                 +14.6
IJ39             -54.5                 +13.6
IJ40             -57.9                 +13.4
IJ41             -59.0                 +15.0
IJ42             -63.1                 +14.9
IJ43             -65.4                 +15.0
IJ44             -60.1                 +14.8
IJ45             -62.6                 +15.2
IJ46             -61.9                 +15.3
IJ47             -62.2                 +16.6
IJ48             -62.1                 +16.1

Table 3: [delta][sup.13][C.sub.CH4], [delta][sup.13][C.sub.CO2],
methane, and TIC contents in EV samples.

                      Sampling depth     [delta][sup.13]
          Storage     of groundwater        [C.sub.CH4]
Sample     period        (ground         ([per thousand]
no.        (days)       level - m)             VPDB)

EV1           6            248.5               -56.7
EV2          55            248.5               -54.0
EV3          98            248.5               -53.7
EV4           6            248.5               -56.8
EV5          55            248.5               -54.5
EV6          98            248.5               -52.7

           [delta][sup.13]        Concentration
              [C.sub.CO2]          of dissolved
Sample     ([per thousand]          C[H.sub.4]
no.              VPDB)           (mmol [kg.sup.-1])

EV1             + 17.1                   --
EV2              +17.8                  7.8
EV3              +17.3                  8.5
EV4              +16.9                  8.7
EV5              +16.2                  6.7
EV6              +16.7                  6.7

           Concentration of
            total inorganic
Sample       carbon (TIC)
no.        (mmol [kg.sup.-1])

EV1                --
EV2                --
EV3                --
EV4                42
EV5                37
EV6                35
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Title Annotation:Research Article
Author:Miyakawa, Kazuya; Okumura, Fumiaki
Publication:Geofluids
Geographic Code:9JAPA
Date:Jan 1, 2018
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