Research Paper

Split Viewer

Econ. Environ. Geol. 2022; 55(3): 261-271

Published online June 30, 2022

https://doi.org/10.9719/EEG.2022.55.3.261

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Interpretation of Origin and Methanogenic Pathways of Coalbed Gases from the Asem-Asem Basin, Southeast Kalimantan, Indonesia

Jong-Hwa Chun*, In Gul Hwang, Wonsuk Lee, Taehun Lee, Yuri Kim

Marine Geology & Energy Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea

Received: April 25, 2022; Revised: June 22, 2022; Accepted: June 23, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Six gas samples were collected from coal and coaly shale from core AA-1, which was acquired from the Asem-Asem Basin, southeast Kalimantan, Indonesia. These coalbed gas samples were analyzed for the molecular composition, carbon isotope (δ13CCH4, δ13CC2, and δ13CCO2), hydrogen isotope (δDCH4), hydrocarbon index (CHC), and carbon dioxide-methane index (CDMI) to document their origin and methanogenic pathways. Core AA-1 successively consists of lower clastic sedimentary rocks (Sedimentary Unit-1, SU-1) containing coal and coaly shale, and upper limestone (Sedimentary Unit-2, SU-2), unconformably underlain by serpentinized basement interpreted as part of the Cretaceous Meratus subduction complex (MSC). The coal and coaly shale (SU-1) were deposited in a marshes nearby a small-scale river. Compositions of coalbed gases show that methane ranges from 87.35 to 95.29% and ethane ranges from 3.65 to 9.97%. Carbon isotope of coalbed methane (δ13CCH4) ranges from –60.3 to –58.8‰, while hydrogen isotope (δDCH4) ranges from –252.9 to –252.1‰. Carbon isotope of coalbed ethane (δ13CC2) ranges from –32.8 to –31.2‰, carbon isotope of coalbed carbon dioxide (δ13CCO2) ranges from –8.6 to –6.2‰. The coalbed CO2 is interpreted to be an abiogenic origin based on a combination of δ13CCO2 and CDMI and could have been transported from underlying CO2 bearing MSC through faults. The methanogenic pathways of coalbed gases are interpreted to have originated from primary methyl-type fermentation and mixed with CO2 reduction, affecting thermogenic non-marine coal-type gases based on analyses of isotopic ratios and various indexes.

Keywords Indonesia, Asem-Asem Basin, coalbed gases, origin, methanogenic pathways

인도네시아 칼리만탄 남동측에 위치하는 아셈-아셈분지 석탄층 가스의 기원과 메탄생성경로 해석

천종화* · 황인걸 · 이원석 · 이태훈 · 김유리

한국지질자원연구원 해저지질에너지연구본부, 대전광역시 유성구 과학로 124, 34132

요 약

인도네시아 칼리만탄 남동측에 위치하는 아셈-아셈분지(Asem-Asem Basin)에서 길이 540.3 m의 AA-1 시추코어를 획득하였고, 이 시추코어에 포함된 석탄층과 석탄질 셰일에서 6개의 석탄층 가스 시료를 채취하였다. 아셈-아셈분지에서 채취된 석탄층 가스의 성분, 탄소동위원소(δ13CCH4, δ13CC2, δ13CCO2), 수소동위원소(δDCH4), 탄화수소지표(CHC), 이산화탄소-메탄지표(CDMI)의 분석을 통하여 이들의 기원과 메탄생성경로를 해석하였다. AA-1 시추코어는 최하부에 메라투스섭입복합체(Meratus subduction complex)의 일부로 해석되는 사문암 기반암 상부에 석탄층과 석탄질 셰일을 포함하는 쇄설성 퇴적암(SU-1)과 석회암(SU-2)이 순차적으로 놓인다. 석탄층과 석탄질 셰일(SU-1)은 소규모 하천 주변의 습지에서 형성된 것으로 해석된다. SU-1에서 채취된 석탄층 가스의 탄화수소가스를 100%로 환산한 메탄 함량은 87.35∼95.29% 범위이고, 에탄 함량은 3.65∼9.97% 범위이다. 석탄층 메탄의 탄소동위원소(δ13CCH4) 값은 –60.3∼–58.8‰ 범위이고 수소동위원소(δDCH4) 값은 –252.9∼–252.1‰ 범위이다. 석탄층 에탄의 탄소동위원소(δ13CC2) 값은 –32.8∼–31.2‰ 범위이고, 석탄층 이산화탄소의 탄소동위원소(δ13CCO2) 값은 –8.6∼–6.2‰ 범위이다. 석탄층 이산화탄소는 탄소동위원소값과 이산화탄소-메탄지표 도표에서 비생물기원(abiogenic origin)에 도시되었고, 기반암인 메라투스섭입복합체에 포함된 이산화탄소가 단층 통하여 이동된 것으로 해석된다. AA-1 시추코어 석탄층 가스의 메탄생성경로는 동위원소, 탄화수소지표, 이산화탄소-메탄지표 분석을 바탕으로 일차적인 미생물 메틸발효작용과 이산화탄소환원작용의 혼합으로 해석되며, 열기원 비해성 석탄형 가스의 영향을 받은 것으로 해석된다.

주요어 인도네시아, 아셈-아셈분지, 석탄층 가스, 기원, 메탄생성경로

Article

Research Paper

Econ. Environ. Geol. 2022; 55(3): 261-271

Published online June 30, 2022 https://doi.org/10.9719/EEG.2022.55.3.261

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Interpretation of Origin and Methanogenic Pathways of Coalbed Gases from the Asem-Asem Basin, Southeast Kalimantan, Indonesia

Jong-Hwa Chun*, In Gul Hwang, Wonsuk Lee, Taehun Lee, Yuri Kim

Marine Geology & Energy Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea

Received: April 25, 2022; Revised: June 22, 2022; Accepted: June 23, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Six gas samples were collected from coal and coaly shale from core AA-1, which was acquired from the Asem-Asem Basin, southeast Kalimantan, Indonesia. These coalbed gas samples were analyzed for the molecular composition, carbon isotope (δ13CCH4, δ13CC2, and δ13CCO2), hydrogen isotope (δDCH4), hydrocarbon index (CHC), and carbon dioxide-methane index (CDMI) to document their origin and methanogenic pathways. Core AA-1 successively consists of lower clastic sedimentary rocks (Sedimentary Unit-1, SU-1) containing coal and coaly shale, and upper limestone (Sedimentary Unit-2, SU-2), unconformably underlain by serpentinized basement interpreted as part of the Cretaceous Meratus subduction complex (MSC). The coal and coaly shale (SU-1) were deposited in a marshes nearby a small-scale river. Compositions of coalbed gases show that methane ranges from 87.35 to 95.29% and ethane ranges from 3.65 to 9.97%. Carbon isotope of coalbed methane (δ13CCH4) ranges from –60.3 to –58.8‰, while hydrogen isotope (δDCH4) ranges from –252.9 to –252.1‰. Carbon isotope of coalbed ethane (δ13CC2) ranges from –32.8 to –31.2‰, carbon isotope of coalbed carbon dioxide (δ13CCO2) ranges from –8.6 to –6.2‰. The coalbed CO2 is interpreted to be an abiogenic origin based on a combination of δ13CCO2 and CDMI and could have been transported from underlying CO2 bearing MSC through faults. The methanogenic pathways of coalbed gases are interpreted to have originated from primary methyl-type fermentation and mixed with CO2 reduction, affecting thermogenic non-marine coal-type gases based on analyses of isotopic ratios and various indexes.

Keywords Indonesia, Asem-Asem Basin, coalbed gases, origin, methanogenic pathways

인도네시아 칼리만탄 남동측에 위치하는 아셈-아셈분지 석탄층 가스의 기원과 메탄생성경로 해석

천종화* · 황인걸 · 이원석 · 이태훈 · 김유리

한국지질자원연구원 해저지질에너지연구본부, 대전광역시 유성구 과학로 124, 34132

Received: April 25, 2022; Revised: June 22, 2022; Accepted: June 23, 2022

요 약

인도네시아 칼리만탄 남동측에 위치하는 아셈-아셈분지(Asem-Asem Basin)에서 길이 540.3 m의 AA-1 시추코어를 획득하였고, 이 시추코어에 포함된 석탄층과 석탄질 셰일에서 6개의 석탄층 가스 시료를 채취하였다. 아셈-아셈분지에서 채취된 석탄층 가스의 성분, 탄소동위원소(δ13CCH4, δ13CC2, δ13CCO2), 수소동위원소(δDCH4), 탄화수소지표(CHC), 이산화탄소-메탄지표(CDMI)의 분석을 통하여 이들의 기원과 메탄생성경로를 해석하였다. AA-1 시추코어는 최하부에 메라투스섭입복합체(Meratus subduction complex)의 일부로 해석되는 사문암 기반암 상부에 석탄층과 석탄질 셰일을 포함하는 쇄설성 퇴적암(SU-1)과 석회암(SU-2)이 순차적으로 놓인다. 석탄층과 석탄질 셰일(SU-1)은 소규모 하천 주변의 습지에서 형성된 것으로 해석된다. SU-1에서 채취된 석탄층 가스의 탄화수소가스를 100%로 환산한 메탄 함량은 87.35∼95.29% 범위이고, 에탄 함량은 3.65∼9.97% 범위이다. 석탄층 메탄의 탄소동위원소(δ13CCH4) 값은 –60.3∼–58.8‰ 범위이고 수소동위원소(δDCH4) 값은 –252.9∼–252.1‰ 범위이다. 석탄층 에탄의 탄소동위원소(δ13CC2) 값은 –32.8∼–31.2‰ 범위이고, 석탄층 이산화탄소의 탄소동위원소(δ13CCO2) 값은 –8.6∼–6.2‰ 범위이다. 석탄층 이산화탄소는 탄소동위원소값과 이산화탄소-메탄지표 도표에서 비생물기원(abiogenic origin)에 도시되었고, 기반암인 메라투스섭입복합체에 포함된 이산화탄소가 단층 통하여 이동된 것으로 해석된다. AA-1 시추코어 석탄층 가스의 메탄생성경로는 동위원소, 탄화수소지표, 이산화탄소-메탄지표 분석을 바탕으로 일차적인 미생물 메틸발효작용과 이산화탄소환원작용의 혼합으로 해석되며, 열기원 비해성 석탄형 가스의 영향을 받은 것으로 해석된다.

주요어 인도네시아, 아셈-아셈분지, 석탄층 가스, 기원, 메탄생성경로

    Fig 1.

    Figure 1.Map showing the location of the core AA-1 recovered from the Asem-Asem Basin, southeast Kalimantan, Indonesia. (a) Five major coal-bearing basins (yellow color) are developed in the eastern part of Kalimantan (modified from Stevens and Hadiyanto, 2004; Singh et al., 2010; Witts et al., 2012; Friederich et al., 2016). Meratus subduction complex is located at the boundary between the Asem-Asem Basin and the Barito Basin (Wakita et al., 1998). The inferred boundary (black dot line) of the southern Sundaland is shown in the inset map (Wakita et al., 1998). MS: Makassar Strait, (b) Cenozoic geology and core site in the Asem-Asem Basin.
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Fig 2.

    Figure 2.A columnar section showing lithologic variations from core AA-1 in the Asem-Asem Basin, Southeast, Indonesia. Core sediments were divided into basement, clastic sedimentary rocks (Sedimentary Unit-1; SU-1), and limestone (Sedimentary Unit-2; SU-2). A prominent unconformity was developed between the basement and SU-1. Coal and coaly shale were only identified from SU-1a, and the six coalbed gas samples were collected from the lower part of SU-1a from core AA-1.
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Fig 3.

    Figure 3.Photographs showing various lithologies of the lower part of clastic sedimentary rocks (SU-1a) in core AA-1 from the Asem-Asem Basin. (a) Serpentine (Sp) basement and overlying breccia (Bc) are separated by an unconformity reveal at the 525~528 meter below surface (mbs) below which weathered serpentine (wSp) was formed. (b) Various lithologies of coaly shale (Cs), gravelstone (G), sandstone (Sd), and mudstone (M) were deposited at 519.5~522 mbs. (c) The thick-bedded coaly shale appear at 513.4~517 mbs. (d) Alteration of mudstone (M) and siltstone (St), coaly shale, and coal were deposited at 457.1~463.8 mbs. Enlarged view of (e) massive gravelstone, (f) bioturbated mudstone, (g) wavy-laminated siltstone, and (h) volcanic ash-rich coaly shale.
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Fig 4.

    Figure 4.Diagram showing a combination of δ13CCH4 and δDCH4 to classify the microbial and thermogenic origins of coalbed methane in AA-1 core samples from the Asem-Asem Basin, Indonesia. The coalbed methane of AA-1 core samples (red solid circles) is plotted to a mixture of methyl-type fermentation and CO2 reduction, and transition between microbial and thermogenic origins in the gas genetic field of Whiticar (1999). Coalbed methane from the Ordos Basin, China (open circles, Zhang et al., 2019), and the Powder River Basin, USA (open triangles, Flores et al., 2008). Headspace gases from marine sediments in the East Sea (open rectangular, Chun et al., 2011).
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Fig 5.

    Figure 5.Diagram showing combined δ13CCH4 and hydrocarbon index (CHC=C1/C2+C3, Kotarba and Rice, 2001) of coalbed gases in AA-1 core samples. The coalbed gases of AA-1 core samples (red solid circles) are plotted to early mature thermogenic gas or mixing of microbial and thermogenic origins in the genetic fields (Milkov and Etiope, 2018). CR: CO2 reduction, EMT: early mature thermogenic gas, LMT: late mature thermogenic gas, MF: methyl-type fermentation, OA: oil-associated thermogenic gas, SM: secondary microbial. Bohai Bay Basin, China (blue solid circles, Liu et al., 2019), Qaidam Basin, China (green solid circles, Liu et al., 2019).
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Fig 6.

    Figure 6.Diagrams showing δ13CC2 versus δ13DCH4 (a) and δ13CCH4 (b) of coalbed gases in AA-1 core samples. The coalbed gases of AA-1 core samples (red solid circles) were plotted to non-marine coal-type gas or a mixture of microbial and coal-type gases in the genetic fields (Liu et al., 2019). Coal-type gases in the Bohai Basin samples (blue solid circles) were distinguished from microbial gases in the Qaidam Basin (green solid circles), China (Liu et al., 2019).
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Fig 7.

    Figure 7.Diagram showing a combination of δ13CCO2 and δ13CCH4 to analyze carbon isotope separation factor (Ɛc) of coalbed gases in core AA-1 samples. The coalbed gases of core AA-1 samples (red solid circles) were mainly plotted at methyl-type fermentation and mixed CCO2 reduction in the genetic fields (Moore et al., 2014). The secondary microbial methane (SM; blue dot line) was clarified in different methanogenic pathways with primary microbial methane in coalbed gases from the Ordos Basin, China (open circles, Zhang et al., 2019) and Powder River Basin, USA (open triangles, Flores et al., 2008).
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Fig 8.

    Figure 8.Diagram showing a combination of δ13CCO2 and CDMI ([CO2/(CH4+CO2)]*100) to identify carbon dioxide origin of coalbed gases in AA-1 core samples of the Asem-Asem Basin, Indonesia. The coalbed gases of AA-1 core samples (red solid circles) were plotted abiogenic CO2 source (magmatic or mantle origin) in the genetic fields (Kotarba and Rice, 2001). The secondary CO2 (SM; blue dot line) is associated with a microbial methanogenic pathway in coalbed gases from the Ordos Basin, China (open circles, Zhang et al., 2019).
    Economic and Environmental Geology 2022; 55: 261-271https://doi.org/10.9719/EEG.2022.55.3.261

    Table 1 . Hydrocarbon gas compositions and carbon (δ13CCH4) and hydrogen (δDCH4) isotope ratios of coalbed methane in AA-1 core samples from the Asem-Asem Basin, Indonesia.

    Depth (mbs)LithologyHydrocarbon gas composition (100%)CHCStable isotopes (‰)
    C1C2C3C4C5C1/C2+C3δ13CCH4δDCH4
    505.90∼506.60coal92.335.312.240.120.0112.2ndnd
    507.10∼507.40coaly shale87.359.972.480.100.107.0-58.8-252.1
    508.40∼508.50coaly shale94.244.511.210.040.0116.5-59.8-252.7
    514.35∼514.67coaly shale94.224.641.090.040.0116.4-59.7-252.4
    516.80∼517.10coaly shale93.415.381.160.03nd14.3-59.1-252.9
    517.10∼517.37coaly shale95.293.651.010.050.0110.1-60.3-252.3

    Hydrocarbon gases were normalized to 100%. CHC; hydrocarbon index (C1/C2+C3, Kotarba and Rice, 2001). mbs; meter below surface. nd; no detection..


    Table 2 . Molecular compositions and carbon isotope ratios of ethane (δ13CC2) and carbon dioxide (δ13CCO2) of coalbed gases in AA-1 core samples from the Asem-Asem Basin, southeast Kalimantan, Indonesia.

    Depth (mbs)Molecular composition (mol %)Stable isotopes (‰)CDMI (%)
    C1C2C3C4+5CO2N2O2COδ13CC2δ13CCO2
    505.90∼506.601.040.05980.02520.00146.7184.256.860.067-32.8-7.386.6
    507.10∼507.400.9720.1110.02760.00222.3791.443.910.100-31.2-8.270.9
    508.40∼508.506.120.2930.07840.002918.3172.042.27nd-31.5-6.274.9
    514.35∼514.674.950.2440.05730.002616.3475.412.08nd-31.8-6.276.7
    516.80∼517.103.700.2130.04590.00225.0184.575.46ndnd-7.057.5
    517.10∼517.370.9390.07180.02140.00132.2591.124.500.049nd-8.670.6

    CDMI; carbon dioxide-methane index ([CO2/(CH4+CO2)]*100(%), Kotarba and Rice, 2001). mbs; meter below surface..


    KSEEG
    Jun 30, 2024 Vol.57 No.3, pp. 281~352

    Stats or Metrics

    Share this article on

    • kakao talk
    • line

    Related articles in KSEEG

    Economic and Environmental Geology

    pISSN 1225-7281
    eISSN 2288-7962
    qr-code Download