Components and isotope fractionation of deep coalbed methane and their geological implications in the Daning-Jixian block, Ordos Basin

Authors

• WANG Chengwang, Petrochina Coalbed Methane Company Limited, Beijing 100020, China; China United Coalbed Methane National Engineering Research Center Co., Ltd，Beijing 100095 ,ChinaFollow
• ZHANG Yun, Petrochina Coalbed Methane Company Limited, Beijing 100020, China
• WANG Hongna, CNPC Exploration Software Co. Ltd., Beijing 100080, China
• CHEN Yiyu, Petrochina Coalbed Methane Company Limited, Beijing 100020, China; China United Coalbed Methane National Engineering Research Center Co., Ltd，Beijing 100095 ,China
• CHEN Gaojie, Petrochina Coalbed Methane Company Limited, Beijing 100020, China; China United Coalbed Methane National Engineering Research Center Co., Ltd，Beijing 100095 ,ChinaFollow
• WANG Jun, School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China; Sanya Offshore Oil & Gas Research Institute of Northeast Petroleum University, Sanya 572025, China
• LIU Lingqi, Sanya Offshore Oil & Gas Research Institute of Northeast Petroleum University, Sanya 572025, China
• LI Panxue, Sanya Offshore Oil & Gas Research Institute of Northeast Petroleum University, Sanya 572025, China
• HE Hongcheng, Sanya Offshore Oil & Gas Research Institute of Northeast Petroleum University, Sanya 572025, China
• LI Wenbiao, Sanya Offshore Oil & Gas Research Institute of Northeast Petroleum University, Sanya 572025, China; State Key Laboratory of Continental Shale Oil, Northeast Petroleum University, Daqing 163712, China; Key Laboratory of the Ministry of Education for Terrestrial Shale Oil and Gas Reservoir Formation and Efficient Development, Northeast Petroleum University, Daqing 163318, China

Abstract

Objective The components of deep coalbed methane (CBM), along with its isotopic composition and fractionation patterns, hold great significance for revealing the genetic types, occurrence characteristics, flow, and production patterns of CBM. Methods This study investigated the Shanxi and Benxi formations in the Daning-Jixian block, Ordos Basin. A range of analyses and tests were conducted, including field CBM desorption experiments on coal samples from the deep parts of both formations, continuous monitoring of wellhead gas from two production wells, physical property tests on the samples, organic petrological experiments, and geochemical analysis. Furthermore, this study analyzed the components and isotopes of both gas samples from natural desorption at the wellfield and produced gas from the production wells. In combination with the reported isotope data and gas-bearing property parameters of CBM worldwide, this study systematically delved into the carbon isotope fractionation patterns of methane during the desorption and production of deep CBM, as well as their geological implications. Results and Conclusions Coals in the Daning-Jixian block have entered the highly mature to overmature stage, characterized by a high fixed carbon content (average: 77.3%), high porosity (average: 7.4%), a strong adsorption capacity (average Langmuir volume: 25.5 m³/t), and a high measured desorbed gas content (average: 16.5 m³/t). During the field desorption of coal samples, the δ¹³C₁ values of methane generally decreased initially and then increased, with isotope fractionation amplitudes ranging from 14.1‰ to 23.9‰. These results indicate the phased differences between the early-stage free gas release and the late-stage adsorbed gas desorption. In contrast, the δ¹³C₁ values of methane in the wellhead gas of two production wells kept decreasing over time, suggesting that the produced gas is dominated by free gas presently. The isotope fractionation patterns of CBM during field desorption can be generally classified into types A, B, C, and D, with the δ¹³C₁ values continuously increasing, decreasing initially and then increasing, increasing initially and then decreasing, and decreasing, increasing, and decreasing sequentially, respectively. These distinct types are primarily attributed to variations in the gas loss ratio and observation stage. Compared to shallow CBM, deep CBM generally exhibits higher carbon isotope fractionation amplitudes of methane (average: 21.1‰), higher δ¹³C₁ values (average: −36.7‰), greater gas-in-place content (average: 21.8 m³/t), and higher free gas proportions (average: 19.9%). These characteristics are intimately associated with the features of deep CBM reservoirs, including their tight nature, high metamorphic grade, and high formation temperature and pressure. The origin identification results demonstrate that the deep CBM in the Daning-Jixian block is dominated by thermogenic gas, with partial gas samples subjected to secondary modifications by hydrothermal fluids or inorganic gas with a deep-seated origin. The results of this study deepen the understanding of mechanisms underlying the enrichment, preservation, flow, and production of deep CBM, serving as a guide for the selection of optimal sweet spots and production system optimization in the Daning-Jixian block and similar deep CBM-bearing zones.

Keywords

Daning-Jixian block, coalbed methane (CBM), components and isotope fractionation, gas-bearing, genetic identification

DOI

10.12363/issn.1001-1986.26.01.0038

Recommended Citation

WANG Chengwang, ZHANG Yun, WANG Hongna, et al. (2026) "Components and isotope fractionation of deep coalbed methane and their geological implications in the Daning-Jixian block, Ordos Basin," Coal Geology & Exploration: Vol. 54: Iss. 9, Article 8.
DOI: 10.12363/issn.1001-1986.26.01.0038
Available at: https://cge.researchcommons.org/journal/vol54/iss9/8

Reference

[1] 李国欣,张水昌,何海清,等. 煤岩气:概念、内涵与分类标准[J]. 石油勘探与开发,2024,51(4):783−795. LI Guoxin,ZHANG Shuichang,HE Haiqing,et al. Coal–rock gas: Concept,connotation and classification criteria[J]. Petroleum Exploration and Development,2024,51(4):783−795.

[2] 周立宏,熊先钺,丁蓉,等. 煤岩气内涵、富集机理及实践意义[J]. 天然气工业,2025,45(3):1−15. ZHOU Lihong,XIONG Xianyue,DING Rong,et al. Connotation,enrichment mechanism and practical significance of coal–rock gas[J]. Natural Gas Industry,2025,45(3):1−15.

[3] 邹才能,赵群,刘翰林,等. 中国煤岩气突破及意义[J]. 天然气工业,2025,45(4):1−18. ZOU Caineng,ZHAO Qun,LIU Hanlin,et al. China's breakthrough in coal–rock gas and its significance[J]. Natural Gas Industry,2025,45(4):1−18.

[4] 秦勇,申建. 论深部煤层气基本地质问题[J]. 石油学报,2016,37(1):125−136. QIN Yong,SHEN Jian. On the fundamental issues of deep coalbed methane geology[J]. Acta Petrolei Sinica,2016,37(1):125−136.

[5] 邢丽茹,张洲,任峻杉,等. 准噶尔盆地深部与浅部煤层气储层物性特征对比分析[J]. 中国煤炭,2024,50(9):9−17. XING Liru,ZHANG Zhou,REN Junshan,et al. Comparative analysis of physical properties of deep and shallow CBM reservoirs in Junggar Basin[J]. China Coal,2024,50(9):9−17.

[6] 李婵,童长兵,杨凡,等. 鄂尔多斯盆地延长组页岩孔隙系统发育特征及影响因素研究[J]. 非常规油气,2025,12(3):22−32. LI Chan,TONG Changbing,YANG Fan,et al. Study on pore system characteristics and controlling factors of Yanchang Formation shale,Ordos Basin[J]. Unconventional Oil & Gas,2025,12(3):22−32.

[7] 牛小兵,范立勇,闫小雄,等. 鄂尔多斯盆地煤岩气富集条件及资源潜力[J]. 石油勘探与开发,2024,51(5):972−985. NIU Xiaobing,FAN Liyong,YAN Xiaoxiong,et al. Enrichment conditions and resource potential of coal–rock gas in Ordos Basin,NW China[J]. Petroleum Exploration and Development,2024,51(5):972−985.

[8] MILKOV A V,ETIOPE G. Revised genetic diagrams for natural gases based on a global dataset of >20,000 samples[J]. Organic Geochemistry,2018,125:109−120.

[9] 王香增,曹红霞,曹军,等. 鄂尔多斯盆地延安地区下古生界天然气气源分析[J]. 非常规油气,2022,9(6):9−13. WANG Xiangzeng,CAO Hongxia,CAO Jun,et al. Analysis of natural gas source of Lower Paleozoic in Yan'an area,Ordos Basin[J]. Unconventional Oil & Gas,2022,9(6):9−13.

[10] SCHOELL M. The hydrogen and carbon isotopic composition of methane from natural gases of various origins[J]. Geochimica et Cosmochimica Acta,1980,44(5):649−661.

[11] 陈建平,王绪龙,陈践发,等. 甲烷碳同位素判识天然气及其源岩成熟度新公式[J]. 中国科学:地球科学,2021,51(4):560−581. CHEN Jianping,WANG Xulong,CHEN Jianfa,et al. New equation to decipher the relationship between carbon isotopic composition of methane and maturity of gas source rocks[J]. Science China: Earth Sciences,2021,51(4):560−581.

[12] 李文镖,卢双舫,李俊乾,等. 页岩气/煤层气运移过程中的同位素分馏研究进展[J]. 石油勘探与开发,2022,49(5):929−942. LI Wenbiao,LU Shuangfang,LI Junqian,et al. Research progress on isotopic fractionation in the process of shale gas/coalbed methane migration[J]. Petroleum Exploration and Development,2022,49(5):929−942.

[13] 李文镖,卢双舫,李俊乾,等. 页岩气运移过程中的碳同位素分馏:机理、表征及意义[J]. 中国科学:地球科学,2020,50(4):553−569. LI Wenbiao,LU Shuangfang,LI Junqian,et al. Carbon isotope fractionation during shale gas transport: Mechanism,characterization and significance[J]. Science China: Earth Sciences,2020,50(4):553−569.

[14] LI Wenbiao,LU Shuangfang,LI Junqian,et al. Geochemical modeling of carbon isotope fractionation during methane transport in tight sedimentary rocks[J]. Chemical Geology,2021,566:120033.

[15] WANG Jun,CHEN Fangwen,LI Wenbiao,et al. A multi–scale and multi–mechanism coupled model for carbon isotope fractionation of methane during shale gas production[J]. Petroleum Science,2025,22(7):2719−2746.

[16] WANG Jun,WANG Feng,CHEN Fangwen,et al. Dynamic fractionation of methane carbon isotope during mass transport in coal with bidisperse pore structures: Experiments,numerical modeling,and applications[J]. Chemical Engineering Journal,2024,498:154942.

[17] LI Wenbiao,LI Junqian,LU Shuangfang,et al. Evaluation of gas–in–place content and gas–adsorbed ratio using carbon isotope fractionation model: A case study from Longmaxi shales in Sichuan Basin,China[J]. International Journal of Coal Geology,2022,249:103881.

[18] LI Wenbiao,LI Xiao,ZHAO Shengxian,et al. Evaluation on carbon isotope fractionation and gas–in–place content based on pressure–holding coring technique[J]. Fuel,2022,315:123243.

[19] LI Xiao,LU Shuangfang,ZHAO Shengxian,et al. Gas–in–place content and occurrence state of deep shale gas in the Luzhou area,Sichuan Basin,China[J]. Marine and Petroleum Geology,2024,160:106662.

[20] ZHAO Shengxian,LU Shuangfang,WU Jianfa,et al. Comparison and verification of gas–bearing parameter evaluation methods for deep shale based on the pressure coring technique[J]. Energy & Fuels,2023,37(3):2066−2077.

[21] 端祥刚,李文镖,胡志明,等. 深部煤岩气产出过程碳同位素分馏与吸附气/游离气产出规律[J]. 石油勘探与开发,2025,52(5):1166−1179. DUAN Xianggang,LI Wenbiao,HU Zhiming,et al. Carbon isotope fractionation and production patterns of adsorbed and free gas during deep coal–rock gas production[J]. Petroleum Exploration and Development,2025,52(5):1166−1179.

[22] WANG Jun,GAO Shusheng,DUAN Xianggang,et al. Experimental and numerical simulation of dynamic fractionation for methane carbon isotope during shale gas depletion development[J]. Fuel,2026,404:136354.

[23] 赵圣贤,杨学锋,刘永旸,等. 深层页岩气产出过程碳同位素分馏特征及其地质意义:以川南地区泸州区块奥陶系五峰组–志留系龙马溪组为例[J]. 天然气工业,2024,44(8):72−84. ZHAO Shengxian,YANG Xuefeng,LIU Yongyang,et al. Characteristics and geological implications of carbon isotope fractionation during deep shale gas production: A case study of the Ordovician Wufeng Formation–Silurian Longmaxi Formation in the Luzhou Block of southern Sichuan Basin[J]. Natural Gas Industry,2024,44(8):72−84.

[24] 赵喆,徐旺林,赵振宇,等. 鄂尔多斯盆地石炭系本溪组煤岩气地质特征与勘探突破[J]. 石油勘探与开发,2024,51(2):234−247. ZHAO Zhe,XU Wanglin,ZHAO Zhenyu,et al. Geological characteristics and exploration breakthroughs of coal rock gas in Carboniferous Benxi Formation,Ordos Basin,NW China[J]. Petroleum Exploration and Development,2024,51(2):234−247.

[25] 赵海峰,王成旺,席悦,等. 深层煤层水平井压裂动态应力场研究:以鄂尔多斯盆地大宁–吉县区块为例[J]. 油气藏评价与开发,2025,15(2):310−323. ZHAO Haifeng,WANG Chengwang,XI Yue,et al. Study on dynamic stress field of fracturing in horizontal wells of deep coal seams: A case study of Daning–Jixian Block in Ordos Basin[J]. Petroleum Reservoir Evaluation and Development,2025,15(2):310−323.

[26] 张道锋,孟康,黄有根,等. 鄂尔多斯盆地石炭系本溪组煤岩解析气甲烷碳同位素特征及其地质意义[J]. 天然气地球科学,2025,36(9):1728−1740. ZHANG Daofeng,MENG Kang,HUANG Yougen,et al. Characteristics of methane carbon isotopes in desorbed gas from the Carboniferous Benxi coal rock in Ordos Basin and their geological significance[J]. Natural Gas Geoscience,2025,36(9):1728−1740.

[27] 段利江,唐书恒,刘洪林,等. 煤储层物性对甲烷碳同位素分馏的影响[J]. 地质学报,2008,82(10):1330−1334. DUAN Lijiang,TANG Shuheng,LIU Honglin,et al. The impact of coal reservoir physical properties on carbon isotope fractionation of coalbed methane[J]. Acta Geologica Sinica,2008,82(10):1330−1334.

[28] WANG Bo,SUN Fenjin,XU Fengyin,et al. High rank coalbed methane desorption characteristic and its application in production in Qinshui Basin[J]. Journal of Coal Science and Engineering (China),2013,19(3):321−324.

[29] 刘洪林,王红岩,赵国良,等. 稳定碳同位素δ13C1在煤层气田勘探中的应用[J]. 西安科技大学学报,2004,24(4):442−446. LIU Honglin,WANG Hongyan,ZHAO Guoliang,et al. Application of coal bed methane stable carbon isotope in exploration of coal bed methane field[J]. Journal of Xi'an University of Science and Technology,2004,24(4):442−446.

[30] WANG Feng,LI Yongzhou,CHEN Mo,et al. Evaluation of gas–in–place content and free gas ratio in deep CBM of the Daning–Jixian Block: An isotope fractionation method[J]. Energy & Fuels,2025,39(39):18845−18856.

[31] CHEN Yilin,ZHANG Bo,QIN Yong,et al. Differences in CH4 and C2H6 carbon isotopic compositions from open and closed pores in coal: Implications for understanding the two–stage δ13C shift during canister desorption[J]. International Journal of Coal Geology,2020,230:103586.

[32] NIEMANN M. Stable isotope systematics of coalbed methane[D]. Hannover: Universität Hannover, 2006.

[33] 黄道军,周国晓,杨兆彪,等. 鄂尔多斯盆地深部煤岩气井产出气–水地球化学特征及其地质响应[J]. 石油与天然气地质,2024,45(6):1617−1627. HUANG Daojun,ZHOU Guoxiao,YANG Zhaobiao,et al. Geochemical characterization of gas–water output from deep coalrock methane wells in the Ordos Basin and its geological responses[J]. Oil & Gas Geology,2024,45(6):1617−1627.

[34] 刘季松,肖正辉,曹运江,等. 黔西有益煤矿煤层气含气性及控气因素研究[J]. 内蒙古科技与经济,2018(4):96−97.

[35] 王国玲,秦勇. 煤储层含气性特征及其地质动力学控制因素[J]. 中国煤炭地质,2009,21(2):18−23. WANG Guoling,QIN Yong. Gas–bearing characteristics of coal reservoirs and their geological dynamic controlling factors[J]. Coal Geology of China,2009,21(2):18−23.

[36] 肖本万. 西山煤田马兰矿煤层气储层特征与含气性分析[J]. 中国煤炭,2017,43(6):42−45. XIAO Benwan. Analysis of characteristics and gas–bearing capacity of coalbed methane reservoir in Malan mine of Xishan coalfield[J]. China Coal,2017,43(6):42−45.

[37] 熊孟辉,秦勇. 五轮山矿区煤层含气性与煤层气理论抽采率[J]. 中国煤炭地质,2009,21(5):1−3. XIONG Menghui,QIN Yong. Coal seam gas–bearing properties and theoretical CBM recoverable ratios in Wulunshan mine area,Guizhou[J]. Coal Geology of China,2009,21(5):1−3.

[38] 高丽军,谢英刚,潘新志,等. 临兴深部煤层气含气性及开发地质模式分析[J]. 煤炭学报,2018,43(6):1634−1640. GAO Lijun,XIE Yinggang,PAN Xinzhi,et al. Gas analysis of deep coalbed methane and its geological model for development in Linxing Block[J]. Journal of China Coal Society,2018,43(6):1634−1640.

[39] 孙斌,杨敏芳,杨青,等. 准噶尔盆地深部煤层气赋存状态分析[J]. 煤炭学报,2017,42(增刊1):195−202. SUN Bin,YANG Minfang,YANG Qing,et al. Analysis on occurrence state of deep coalbed methane in Junggar Basin[J]. Journal of China Coal Society,2017,42(Sup.1):195−202.

[40] 李勇,高爽,吴鹏,等. 深部煤层气游离气含量预测模型评价与校正:以鄂尔多斯盆地东缘深部煤层为例[J]. 石油学报,2023,44(11):1892−1902. LI Yong,GAO Shuang,WU Peng,et al. Evaluation and correction of prediction model for free gas content in deep coalbed methane: A case study of deep coal seams in the eastern margin of Ordos Basin[J]. Acta Petrolei Sinica,2023,44(11):1892−1902.

[41] 贾秉义,晋香兰,李建武,等. 低煤级煤储层游离气含量计算:以准噶尔盆地东南缘为例[J]. 煤田地质与勘探,2015,43(2):33−36. JIA Bingyi,JIN Xianglan,LI Jianwu,et al. Calculation of free gas content of low rank coal reservoirs: A case study of the southeast margin of Junggar Basin[J]. Coal Geology & Exploration,2015,43(2):33−36.