Carboniferous coals of the Zonguldak basin (northwest Turkey): Implications for coalbed methane potential

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We investigated the Carboniferous coals in the Zonguldak basin (northwest Turkey) using a multidisciplinary approach, including petrographical and geochemical analyses, gas adsorption and gas content measurements, artificial maturation experiments on selected coals, and one-dimensional (1-D) modeling of the basin evolution and hydrocarbon generation. The main emphasis was set on the definition of coalbed methane (CBM) generation capability and adsorption capacity of the coals. Considering these properties of coals and the amount and composition of gas within the coal seams, we carried out a comprehensive evaluation of the CBM potential of the basin. Three areas in the basin, namely, Armutcuk, Zonguldak, and Amasra, were investigated separately for their CBM potential. From the standpoint of petroleum (oil and/or gas) generation, the coals in the Armutcuk and Amasra areas are located within the oil generation window. The coals of the Zonguldak area are located within the oil and gas generation window. Computer-aided basin modeling, using a basin-specific kinetic data set of hydrocarbon generation from coals, suggests that significant quantities of gas have been generated only in the Zonguldak area. Modeling results also indicate that major gas generation from the base of the Westphalian A Kozlu Formation in the Zonguldak area occurred between 80 and 42 Ma (during the Late Cretaceous-early Eocene), prior to basin inversion that started at 42 Ma and continues at present. The predicted amount of generated coalbed gas from coal seams in the gas generation zone is several times higher than the adsorption capacity of the respective seams. A certain amount of this gas, controlled by the adsorption capacity, is stored within the coal seams where it is generated. Gas generated in excess of storage capacity obviously would have been expelled from the coal. Part of the expelled gas is accumulated in coal seams located above the gas generation zone. However, the coal seams within and above the gas generation zone are undersaturated with respect to their adsorption capacity. The undersaturation is probably caused by remigration of gas that is desorbed because of pressure decrease during basin inversion of more than 2500 m (8203 ft) over the last 42 m.y. The rest of the gas generated within the coal seams and expelled first because of the overflow of adsorption capacity and then because of the desorption during basin inversion has either migrated to source conventional gas reservoirs or has been lost to the surface during basin inversion. Because the basin has not been explored for conventional gas reservoirs, a conclusive mass balance cannot yet be performed. Despite basin inversion and remigration of gas, some coal seams still contain gas amounts of up to 10 cm 3 /g of coal, indicating a CBM potential.