Geological controls on gas content distribution of coal seams in the Kunuk coalfield, Soma Basin, Turkey

The Miocene Soma Coal Basin may be one of the most promising areas for low-rank coalbed methane in Turkey. So far, the coalbed methane potential of the coal seams in the Kunuk Coalfield, Soma Basin has not been adequately studied. A systematic investigation was performed on the gas content distribution of the kP1 and kM2 coal seams and its geological controls, which has been based on the geological data and the experimental results of 84 coal samples taken from 37 coal exploration wells in the Kunuk coalfield, Soma Basin. Samples were analyzed in terms of coal properties and gas content. The gas content was measured by the direct method based on the canister desorption test. The gas content of the kP1 and kM2 coal seams range from 0.90 to 2.61 m(3)/t and 0.51 to 3.86 m(3)/t respectively, on as-received basis, with increasing trend from west to east areas in both seams. Diffusivity values of the kP1 and kM2 coal seams vary from 2.84 x 10(-5) min(-1) to 56.4 x 10(-5) min(-1), and from 0.64 x 10(-5) min(-1) to 54.3 x 10(-5) min(-1), respectively. The gas content of the kP1 seam has not been correlated with the coal properties, whereas the gas content of the kM2 seam has positive correlation with fixed carbon content and gross calorific value, and negative correlation with moisture and ash content. Normal faults are well developed in the study area play an important role at gas migration, causing gas dissipation and reduced gas content of each coal seam. In the northern part, the low gas content may result from the combined effects of the geological structure, thickness of overlying unit, burial depth and coal thickness. In the southern part, the gas content of coal seams is controlled by the combined effects of relatively favorable factors including fewer normal faults, thicker overlying unit, high burial depth and thick coal seam. The variation of the gas content for the coal seams is a comprehensive reflection of geological factors with each coal seam has its own pattern of gas distribution and this should be considered for the future coalbed methane exploration and development studies in Soma Basin.