Microbial gas is very dry (i.e., it consists almost entirely of methane). In contrast, thermogenic gas can be dry, or can contain significant concentrations of "wet gas" components (ethane, propane, butanes) and condensate (C5+ hydrocarbons). Although thermogenic gas is typically the dominant type of natural gas in coal and petroleum reservoirs, those reservoirs often contain a component of microbial gas. Gas origin or source correlations are determined, in part, from geochemical analysis of the gas, consideration of potential gas migration pathways, and evaluation of migration mechanisms from potential sources (Baldassare, et.al., 2015).

Geochemical analysis of a gas includes both molecular composition (relative abundance in the gas sample of C1 to C6 hydrocarbons and non-hydrocarbon gases such as He, H2, CO2, and N2) and isotopic composition. The isotopic composition of a gas includes characteristics such as the stable carbon and hydrogen isotope ratios of the C1 – C5 hydrocarbon gas species, stable isotope composition of non-hydrocarbon gases, and radioactive carbon and hydrogen isotopes of C1. Data for molecular and isotopic characteristics of a gas can be used to constrain the origin of a gas (bacterial and/or thermogenic) as well as the type and maturity of the organic matter that generated the thermogenic hydrocarbons in a sample. (Coleman et al., 1977; Schoell, 1980; Whiticar and Faber, 1986; Jenden et al., 1993; Laughrey and Baldassare, 1998; Rowe and Muehlenbachs, 1999; Revesz et al., 2010; and Osborn and McIntosh, 2010, Baldassare, et.al., 2015). These data can also identify any secondary processes, such as gas oxidation, that may have affected a sample. For baseline and incident response groundwater quality programs, analysis of dissolved inorganic carbon (DIC) and other water geochemistry parameters provide additional evidence of methane oxidation or degradation.

ECHELON has a proven expertise in the evaluation and interpretation of gas and groundwater geochemistry. We have developed a comprehensive approach for stray gas incident response and investigations where gas and groundwater geochemistry data are synthesized with other investigative information. The geochemistry data are evaluated and interpreted to determine gas origin, thermal maturity, and secondary processes including mixing, oxidation and fractionation. ECHELON's methodology has proven consistently successful in investigations to identify the origin and specific source(s) of stray gas. ECHELON's approach, investigative and remediation techniques have been widely adopted and applied by environmental consulting firms, and governmental agencies.

In regions with shale gas drilling, Sr isotopes have been shown to differentiate deep gas-related fluids from shallower fluids (e.g., shallow brines and coal mine drainage; Chapman et al., Geochemical and Strontium Isotope Characterization of Produced Waters from Marcellus Shale Natural Gas Extraction , Environmental Science & Technology, 2012). Additionally, ECHELON applies Sr isotopes to reveal subsurface water-rock interactions (Chapman et al., Strontium isotope quantification of siderite, brine, and acid mine drainage contributions to abandoned gas well discharges in the Appalachian Plateau, Applied Geochemistry, 2013).