Origin of Coalbed Gas in Indiana: Insights from Isotopic Ratios Maria Mastalerz Indiana Geological Survey , IU Dariusz Strapoc Arndt Schimmelmann Department of Geological Sciences, IU, Bloomington

INTRODUCTION What is coalbed gas? Natural gas formed and stored in coals (mainly adsorbed on micropores <2nm) Consists mainly of CH4 + some C2, C3, C4, CO2, N2 Generated by: 1) thermal cracking of OM from coal, if Ro > 55% - the treshold value (thermogenic gases – C1-C4 hydrocarbons) 2) microbial activity (biogenic methane)

INTRODUCTION Microbially generated methane Microbial consortia (consuming the organic matter from coal, mainly long chain aliphatics): acetogens, acetotrophs, with CO2 reducing methanogens as a terminal step CO2 + 4H2 = CH4 + 2H2O Content of biogenic CH4 can be up to 100% in shallow coals’ gas and with Ro<55% (<200m) In deeper and/or more mature coals it is mixed with thermogenic gas and their ratio can be assessed using molecular and isotopic analyses of the gases

OBJECTIVES OF THIS STUDY CBM Origin Establish method of representative coalbed gas sampling from desorption canisters Generate compositional and isotopic data of coalbed gases (C1-C4 hydrocarbons, CO2) in Indiana Evaluate thermogenic and biogenic contribution to the total gas Determine controls on gas distribution

Gas content of Indiana coals >2.8m3/t >3.4m3/t Gas content of Indiana coals

Variations in measured gas content (raw basis) Recently desorbed

Degree of saturation %

RESULTS Coal maturity (Ro) and maximum Modified from Meissner, 1984 Coal maturity (Ro) and maximum thermogenic gas potential of coal vs coalbed gas content in Illinois Basin coals Coal Rank 0.4 Illinois Basin subbituminous high volatile bituminous C 0.6 Indiana coals high volatile bituminous B 0.8 KY high volatile In addition to thermogenic gas, Indiana coals have another source of gas (biogenic methane ??) bituminous A 1.0 medium volatile 3 9 15 bituminous Maximum Thermogenic gas potential [m3/t]

Study area SE Illinois Basin Core samples of Pennsylvanian coals: Herrin Springfield (V) Houchin Creek (IVa) Survant (IV) Seelyville (III) Range of vitrinite reflectance 0.55-0.62% (high volatile bituminous C coals) 100km fault zone

Sampling coalbed gas sampling from desorption canisters with N2 purging of tubing

Analytical procedures Continuous flow molecular and C-isotopic analyses of C1-C4, CO2 using on-line GCC/MS (syringe injection module with variable sample size 0.05ul to 50ul; GC; oxidative reactor Cu, Ni, Pl at 850C; MS Finnigan MAT 252 or Delta Plus) Off-line H,C-isotopic analyses of CH4 (oxidation of CH4 in quartz tube with CuO at 800C; cryogenic separation of CO2 and H20; reduction of H2O to H2 in Uranium oven at 850C; dual inlet on Finnigan MAT 252 or Delta Plus)

RESULTS: Molecular composition of RESULTS: Molecular composition of coalbed gases from the Illinois Basin 150m 300m Depth Ro [%] 0.57 0.56 0.55 0.75 0.54 0.54 0.62 Coal maturity (vitrinite reflectance) * N2 not included

CBM origin – genetic classifications: C vs H isotopes in CH4 Biogenic field - CO2 Reduction Biogenic field – acetate fermentation Mixing zone Thermogenic field CO2 reduction reaction used by methanogenic bacteria: CO2 + 4H2 = CH4 + 2H2O

CBM origin: C1/[C2+C3] gas wetness indicator vs C-isotopes in CH4 Microbial field Transition zone Mixed Thermogenic field

CBM origin: C-isotopes in CH4 and CO2 Thermogenic gases Laboratory pyrolysis gases Microbial CO2-reduction

Estimation of biogenic CH4 content: Compound-specific C-isotopic data Bu Pr Et Extrapolated pure T-genic methane Assumed pure biogenic CH4 -70 ‰ 57% biogenic CH4 Me 0.25 0.33 0.5 method developed by Chung et al. 1987

Estimation of biogenic CH4 content: Compound-specific C-isotopic data Pr Et Bu Me Extrapolated pure T-genic methane Assumed pure biogenic -70 ‰ Danville 96% Herrin, IL 89% Springfield, IN 57% Springfield, KY -12% (~0) Seelyville 59%

Examples of gas variations Sullivan County, Indiana

Biogenic vs thermogenic CH4 content Seelyville coal Sullivan County ~constant Thermogenic gas content - almost constant, low and corresponding to Ro Biogenic gas content – varies within coalbed (controls unknown, pore size distribution ~permability?, nutrients availability?), important contribution to total CBM (62% on average)

Gas play concepts in Indiana Indiana – biogenic methane prevailing Ro at treshold of thermogenic gas generation (0.55-0.62%) <50% thermogenic gas content coal rarely reached T>80C (upper limit of bacterial activity), therefore… bacteria can be synsedimantary or …transported with fresh water after erosional uplift and/or …transported with water from melting ice sheets after last ice age into the coal after relaxation of the cleats and fractures, or …transported with modern meteoric waters Illinois Indiana Kentucky Max range of ice

Gas play concepts in Indiana Indiana – thermogenic gas present in low quantities tectonic and volcanic activity (higher geothermal gradient) in the Western Kentucky fault zone Ro up to 0.8% corresponding to ~140C (thermogenic gas generation treshold ~0.55%) no secondary biogenic methane generation thus after erosional uplift ice melt or meteoritic water hadn’t transported methanogens to the coalbed environment (ice didn’t cover this area – higher elevation) fault zone Illinois Indiana Kentucky Max range of ice

CONCLUSIONS C isotopic data of CO2 and C1-C4 hydrocarbons plus H isotopic data of C1 provide powerful tools for assessing the coalbed gas origin, evolution and alteration; Biogenic gas is the dominant component in CBM in Indiana (62% on average), whereas thermogenic gas occurs in lower but variable quantities; The highest volumes of gas are expected in areas where there is a substantial amount of thermogenic gas and also there were (are?) favorable conditions for biogenic gas generation CBM in Indiana – renewable resource?? meteoric prod water

Gas is beautiful!