APPLICATION OF METHANE CLUMPED ISOTOPOLOGUE MEASUREMENTS FOR TRACING THE SUBSURFACE HISTORY OF HYDROCARBON GASES

Measurements of the ¹³C/¹²C and D/H stable isotope ratios of methane have been used to infer pathways of methane generation in hydrocarbon systems. Recent analytical innovations in laser spectroscopy [1] and mass spectrometry [2] have enabled determination of the abundance of “clumped” isotopologues (i.e., molecules containing two or more rare isotopes) of methane (e.g., ¹³CH₃D) in samples of natural gases. The new dimension of stable isotope data accessed by such techniques have enabled the estimation of temperatures of methane formation [3], degree of mixing [4], or extent of hydrogen exchange [5], and have confirmed the identification of microbial methane in a range of hydrocarbon reservoirs, including coalbeds in the Powder River Basin, USA [5] and ~2.5 km beneath the seafloor offshore of the Shimokita Peninsula, Japan [6], and gas hydrate in sediments of the northern Cascadia margin, offshore Vancouver Is., Canada [5]. Experimental studies to investigate clumped isotopologue signatures associated with methanogenesis and methane oxidation are in progress.

We will present approaches to integrating methane clumped isotopologue abundance measurements with associated hydrologic and geochemical data to make inferences about the histories and rates of methane generation, migration, and oxidation in subsurface environments. In particular, we will suggest potential insights into temperature limits of and nutrient availability constraints on subsurface microbial metabolisms, and assess the utility & limitations of stable isotopologue measurements for distinguishing among methane-generating mechanisms in natural settings.

[1] Ono et al., 2014, Anal. Chem.; [2] Stolper et al., 2014, Geochim. Cosmochim. Acta; [3] Stolper et al., 2014, Science; [4] Stolper et al., 2015, Geochim. Cosmochim. Acta; [5] Wang et al., 2015, Science; [6] Inagaki et al., 2015, Science.