Hydrogen Isotopes as Tracers of Methane Sources

A. Townsend-Small

School of Environment and Sustainability, University of Cincinnati; 949-614-6250, E-mail: townseay@ucmail.uc.edu

Scientists investigating rising CH₄ concentrations regionally and globally have relied on a variety of approaches to estimate sources of excess methane, including inventories of sources, carbon isotopes of CH₄, and methane:ethane ratios. In this presentation, I will give examples from several studies that have measured the hydrogen stable isotopic composition (δD) of CH₄ in both sources (i.e., at the "bottom up" level) and in well mixed air masses (for "top down" source apportionment), and argue that this tracer is a valuable potential tracer of global methane sources. Advantages of hydrogen isotopes include 1), consistent δD ratios of CH₄ within oil and gas basins as compared to δ¹³C and CH₄:C₂H₆; 2), most sources have a distinct δD-CH₄ from atmospheric background, which makes it easier to distinguish small enhancements in CH₄, unlike δ¹³C, where some oil and gas sources have similar signatures to background air; and 3), the ability to use a two-endmember mixing model for source apportionment rather than a one-endmember mixing model, which is the case with CH₄:C₂H₆ (because biogenic sources do not have C₂H₆). Some disadvantages of using δD vs the others include 1), there are no in situ instruments available for measuring δD, as there are for δ¹³C and C₂H₆, and fewer laboratories measuring this isotope in CH₄; and 2), currently there are somewhat larger sample volume requirements for δD than δ¹³C, although still much smaller than in the recent past.  

Figure 1. Composition of methane from natural gas sources in the Barnett Shale region. (a) Keeling plot of δ¹³C-CH₄ vs 1/[CH₄]; (b) δD-CH₄ vs 1/[CH₄]; (c) [C₂H₆] vs [CH₄]; (d) [C₃H₈] vs [CH₄]; (e) [n-C₄H₁₀] vs [CH₄]; and (f) [n-C₅H₁₂] vs [CH₄].