A novel isotopic approach to distinguish primary microbial and thermogenic gases in shallow subsurface environments

• Published in: Applied geochemistry 2021-08, Vol.131, p.105048, Article 105048
• Main Authors: Cesar, Jaime, Mayer, Bernhard, Humez, Pauline
• Format: Article
• Language: English
• Subjects: Carbon isotopes; Ethane; Methane; Microbial; Thermogenic
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2021.105048

The stable carbon isotopic compositions of methane and ethane have been used for several decades to characterize petroleum gas in the subsurface. However, difficulties have frequently remained in determining the exact origin of natural gas samples derived from microbial activity, thermal degradation of organic matter (e.g. thermogenic gas), and mixtures of both components. To further refine this approach, we represent the carbon isotope difference between methane and ethane, or Δ13CC1–C2, plotted versus the individual δ13C values of methane and ethane as a new graphical tool for a refined assessment of natural gas origins and their mixtures. In addition, a new mathematical notation using the δ13C values of methane and ethane is also proposed for discrimination of the origin of natural gas samples. This parameter has been termed isotope factor O(C1/C2) and is defined as the ratio [(δ13CC1*1000)/(δ13CC2)3]. When this factor is represented against the δ13C values of methane and ethane, an improved distinction between primary microbial and thermogenic gases is achieved. Our study does not propose mechanisms for microbial generation of ethane, but the data suggest that the isotopic composition of ethane should be taken into account for the characterization of the origin of natural gas samples from shallow subsurface environments. For primary microbial and thermogenic gases with similar δ13C values of methane, the primary microbial gas will be characterized by lower O(C1/C2) values (2), and will have a Δ13CC1–C2 value 10–20‰ more positive than that of thermogenic gas. Gas mixtures have intermediate O(C1/C2) and Δ13CC1–C2 values and are graphically identifiable using the here presented classification tools. The effectiveness of the new graphical tools is demonstrated using natural gas samples from Canadian groundwater wells, low-permeability petroleum reservoirs (including coalbed methane plays) in Canada and the USA, Quaternary primary microbial gas accumulations in China, and secondary microbial gases from Northern Africa and China. •The δ13C of ethane improves discrimination between microbial and thermogenic gas.•The new notation O(C1/C2) discriminates microbial and thermogenic gas and mixtures.•The Δ13CC1–C2 discriminates microbial and thermogenic gas and their mixtures.•Early mature gas can be distinguished from primary microbial gas.