5:15 PM - 6:45 PM
[SVC28-P10] Origin of hydrocarbon gases in the Hatchobaru geothermal field, Kyushu
Keywords:Hatchobaru geothermal field, Hydrocarbon gas, Carbon isotopic composition
Exploration of geothermal resources using a combination of various techniques is an essential process to successful developing geothermal energy, which has become attractive as a renewable and sustainable resource. Geochemical exploration provides direct information on the properties and behaviors of geothermal fluid, based on chemical analysis of hot spring waters, fumaroles collected at geothermal areas, and high-temperature fluids obtained from the heads of drilled wells. Although geochemical exploration using liquid fluids has been conducted in the Hatchobaru geothermal area, the gas chemistry of geothermal fluids has rarely been reported.
This study aims to elucidate the origin of hydrocarbon gases in the steam at the Hatchobaru geothermal field through the analysis of stable carbon isotopic compositions.
The steam samples were collected from production wells in the Hatchobaru geothermal power plant, as well as from fumaroles in the Hatchobaru geothermal field surrounding the power plant. Concentrations and stable carbon isotopic compositions of methane (δ13C-CH4), ethane (δ13C-C2H6), propane (δ13C-C3H8), and carbon dioxide (δ13C-CO2) in the steam were analyzed.
The δ13C-CH4 values and the methane/ethane concentration ratios (C1/C2) in the the Hatchobaru geothermal power plant ranged from -29 to -26‰ (average -28‰, n=4), and from 56 to 479 (average 235, n=4), respectively. The δ13C-CH4 values and the C1/C2 ratios ranged from -30 to -23‰ (average -25‰, n=20), and from 53 to 1256 (average 374, n=20). These samples showed higher δ13C-CH4 and C1/C2 values compared to typical thermogenic methane (δ13C-CH4= -50 -35‰,C1/C2=1-50). There are two possible origins of methane with higher δ13C-CH4 and C1/C2 than typical thermogenic methane: 1) thermogenic methane produced by matured organic matters, and 2) mixture of thermogenic and abiotic methane.
The above two possibilities are discussed using δ13C-CH4, -C2H6, and -C3H8. The average carbon isotopic compositions of each hydrocarbon gas were δ13C-CH4 = -26‰ (n=34), δ13C-C2H6 = -23‰ (n=22), and δ13C-C3H8 = -21‰ (n=5). This isotope distribution is a characteristic pattern for thermogenic hydrocarbon gases, whereby the C1-C3 alkanes become more enriched in 13C with increasing molecular mass, suggesting that the hydrocarbon gases in the Hatchobaru geothermal field are likely to be of thermogenic rather than abiotic origin.
The apparent equilibrium temperature estimated by carbon isotopic fractionation between CO2 and CH4 at the power plant and fumaroles averaged 386℃(n=6), and 418℃ (n=24), respectively, which are higher than those of the previously reported geothermal reservoirs (250~300℃) estimated by geothermometers using liquid fluid chemistry.
This study aims to elucidate the origin of hydrocarbon gases in the steam at the Hatchobaru geothermal field through the analysis of stable carbon isotopic compositions.
The steam samples were collected from production wells in the Hatchobaru geothermal power plant, as well as from fumaroles in the Hatchobaru geothermal field surrounding the power plant. Concentrations and stable carbon isotopic compositions of methane (δ13C-CH4), ethane (δ13C-C2H6), propane (δ13C-C3H8), and carbon dioxide (δ13C-CO2) in the steam were analyzed.
The δ13C-CH4 values and the methane/ethane concentration ratios (C1/C2) in the the Hatchobaru geothermal power plant ranged from -29 to -26‰ (average -28‰, n=4), and from 56 to 479 (average 235, n=4), respectively. The δ13C-CH4 values and the C1/C2 ratios ranged from -30 to -23‰ (average -25‰, n=20), and from 53 to 1256 (average 374, n=20). These samples showed higher δ13C-CH4 and C1/C2 values compared to typical thermogenic methane (δ13C-CH4= -50 -35‰,C1/C2=1-50). There are two possible origins of methane with higher δ13C-CH4 and C1/C2 than typical thermogenic methane: 1) thermogenic methane produced by matured organic matters, and 2) mixture of thermogenic and abiotic methane.
The above two possibilities are discussed using δ13C-CH4, -C2H6, and -C3H8. The average carbon isotopic compositions of each hydrocarbon gas were δ13C-CH4 = -26‰ (n=34), δ13C-C2H6 = -23‰ (n=22), and δ13C-C3H8 = -21‰ (n=5). This isotope distribution is a characteristic pattern for thermogenic hydrocarbon gases, whereby the C1-C3 alkanes become more enriched in 13C with increasing molecular mass, suggesting that the hydrocarbon gases in the Hatchobaru geothermal field are likely to be of thermogenic rather than abiotic origin.
The apparent equilibrium temperature estimated by carbon isotopic fractionation between CO2 and CH4 at the power plant and fumaroles averaged 386℃(n=6), and 418℃ (n=24), respectively, which are higher than those of the previously reported geothermal reservoirs (250~300℃) estimated by geothermometers using liquid fluid chemistry.