T276 Unconventional Energy Resources

Wednesday, 28 September 2016
8:00 AM - 12:00 PM
Mile High Ballroom 2B

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

Paper number: T276-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287715

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