1. 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

2. 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)

4. 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

7. 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

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

9. Variations in measured gas content
(raw basis)
Recently desorbed

10. Degree of saturation %

11. 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
bituminous
Maximum Thermogenic
gas potential
[m3/t]

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

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

14. 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)

15. RESULTS: Molecular composition of
RESULTS: Molecular composition of coalbed gases from the Illinois Basin
150m
300m
Depth
Ro [%]
Coal maturity (vitrinite reflectance)
* N2 not included

16. 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

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

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

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

20. Estimation of biogenic CH4 content: Compound-specific C-isotopic dataPr 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%

21. Examples of gas variations Sullivan County, Indiana

22. 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)

23. Gas play concepts in Indiana
Indiana – biogenic methane prevailing
Ro at treshold of thermogenic gas generation ( %)
<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

24. 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

25. 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

26. Gas is
beautiful!