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COALBED GAS OF BIOGENIC ORIGIN IN THE MIOCENE SOMA BASIN
TAK Marmara Research Center, Earth and Marine Sciences Institute, Gebze-Kocaeli, 41470, Turkey:
Phone: + (90) 262 677 2850, + (90) 262 641 2309,
The Neogene Basins of Turkey contain as much as 9 billion tons of lignite-rank coal
et al., 2002). The Miocene Soma Basin, a rift basin trending NE-SW
in the Aegean Extensional Province (EAP) of Western Turkey, is estimated to contain at the
least one billion tons of lignite and about half of this reserve is present at depths greater than
600 m (Turkish Coal Enterprises, 2006). Miocene marl/limestone units and Pliocene clastics
and volcanic tuffs overlie the Miocene coals of the Soma basin. In the Soma Basin, Turkish
) has conducted open cut coal mining and underground coal mining
activities for several decades in the Northern and Central part of the basin, respectively.
Recently, coal exploration activities have been extended to the Southern part of the basin by
means of exploratory drillings. In this context, two boreholes encountering a coal seam (M2)
up to 20 m thick were evaluated. The M2 coal seam was encountered between 900 and 940 m
depth in two boreholes drilled approximately 1 km apart. Wellhead gas content was measured
on coal cores following the USBM method (Diamond and Levine, 1981). Additionally, coal
was placed in hermetically sealed canister and desorbed gas was analyzed in laboratory for
chemical composition (by FTIR gas analyzer) and 13C isotope (by GC-IRMS). Coal
characterization was completed by means of Rock Eval (RE) Pyrolysis, Proximate and
Ultimate analyses, as well as microscopic analyses for typing of macerals and vitrinite
The wellhead gas content measurements (six core measurements from two boreholes)
indicate that as much as 4 m3 gas / ton coal is present in the coal recovered from 900 to 940 m
below the surface. The rank of coal based on vitrinite reflectance measurements is lignite to
sub-bituminous (0.40 to 0.45 % Ro); supported by RE Tmax values of 420 0C. TOC content
of the coal samples vary between 53 to 73 %. The composition of the gas is dominantly
methane (more than 99.4 %) and the 13C/12C isotope ratio of methane is 61 to 65 per mil.
Considering the chemical composition of the gas and the del 13C isotope of the methane, the
source of the coal gas is biogenic probably generated by bacteria that are introduced to the
coal seam by fresh water following mainly the normal faults bordering the graben structure.
The maceral analyses show that coal samples on average contain more than 60 %
huminite/vitrinite. Hydrogen Index values derived from RE analyses range from 220 to 360;
supporting the results of microscopic observations suggesting dominant huminite/vitrinit
occurrence. Adsorption on the internal coal surface is considered as the primary mechanism
of gas storage in coals and the surface area, which controls the gas adsorption capacity, is in
general a function of the micro-pore volume (Levy et al., 1997; Crosdale et al., 1998).
The vitrinite/huminite maseral content has positive correlation with gas sorption
capacity (Levy et al., 1997); meaning that at a given pressure, the higher percentage of micro-
pore dominated huminite/vitrinite the more gas adsorption capacity. In this context, the
Miocene Soma lignites have good micro-pore properties in respect to gas adsorption.
Coalbed gas potential of Zonguldak hardcoal basin in Turkey was extensively studied
n et al., 2002 for a review). In this study, however, we report
the first findings on occurrence of coalbed gas of biogenic origin in Turkey. Preliminary
evaluation, based on limited analyses and results summarized above, on gas potential of the
Miocene Soma Basin is encouraging, yet further investigations are underway. In case, further
research support the economic feasibility for biogenic coal gas, a good justification and
ground will have been established for conducting similar studies for other coal-bearing
Crosdale, P. J., Beamish, B. B., and Valix, M. 1998. Coalbed methane sorption related to coal
composition. Int. J. of Coal Geology 35:147–158.
Diamond, W. P., and J. R. Levine, 1981, Direct method for determination of the gas content
of coal: procedures and results: United States Bureau of Mines (USBM), RI 8515, 36 p.
Levy, J., Day, H., and Killingley, J. S. 1997. Methane capacities of Bowen basin coals related
to coal properties. Fuel 76(9):813–819.
., 2001, Lignite and thermal power plants for sustainable development in Turkey,
World Energy Council 18th Congress, Buenos Aires, October 2001
Tuncalıet al., 2002, Chemical and Technological Properties of Turkish Tertiary Coals.
General Directorate of Mineral and Research Exploration of Turkey (MTA) SpecialPublication. ISBN: 6595-47-7, 400 p.
nan, S., Gürdal, G., Mann, U., and Schaefer, G. R. 2002. Carboniferous coals
of the Zonguldak basin (northwest Turkey): Implications for coalbed methane potential.
American Association of Petroleum Geologists Bulletin 86(7):1305–1328.
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