NOBLE GAS, HYDROCARBON, AND NITROGEN ISOTOPIC COMPOSITIONS OF COALBED METHANE RESERVOIRS FROM THE ILLINOIS BASIN

Coalbed methane (CBM) currently makes up ~9.3% of the natural gas production in the US. Despite this economic importance and increasing scientific interest, there are still many unknowns regarding the source (biogenic vs. thermogenic), residence time, and gas-water interactions that determine the occurrence and composition of natural gas within CBM reservoirs. The majority of research on CBM fields explore the molecular (C₁/C₂+) and isotopic (δ¹³C-CH₄) composition of hydrocarbons in relation to water chemistry (e.g., [DIC] and δ¹³C-DIC) to conclude CBM gas composition is dominated by biogenic methane production. These conclusions are related to the very dry (C₁/C₂+= >2,000), light δ¹³C-CH₄ <-50⁰/₀₀, and correlations between δ¹³C-CH₄ and δ¹³C-DIC, amongst other lines of evidence. Nonetheless, in some basins (e.g., Illinois, San Juan, and Black Warrior Basins), heavier δ¹³C-CH₄ >50⁰/₀₀ suggest the possibility for a mixture of biogenic and thermogenic contributions.

In addition to the ambiguous stable isotopic compositions in some CBM reservoirs, many systems display marked excesses in He and N₂ content relative to air-saturated water (i.e., the dominant source of N₂ in crustal fluids). Excess He may relate to prolonged residence times or migration of exogenous thermogenic natural gas. Although excess N₂ has largely been dismissed as air contamination or the product of gas-water interactions, significant N₂ excesses suggest the potential for the addition of a non-atmospheric source of N₂, such as denitrification, thermal maturation of coals, or another unidentified exogenous source.

To evaluate the source of excess helium and nitrogen in CBM reservoirs, we have conducted a comprehensive analysis of the molecular gas composition (CH₄, C₂H₆+, CO₂, N₂, and H₂), noble gas molecular and isotopic composition (He, Ne, Ar, Kr, and Xe), and isotopic composition of carbon and nitrogen (δ¹³C-CH₄, δ¹³C-CO₂, and δ¹⁵N-N₂) in the Illinois Basin. Our preliminary data confirms the presence of dry natural gas dominated by methanogenesis, but also suggests significant and quantifiable contributions from migrated thermogenic gas and non-atmospheric nitrogen excesses.