The Miocene coal seams in the Soma Basin (W. Turkey): Insights from coal petrography, mineralogy and geochemistry

The Neogene Soma Basin, western Turkey, hosts three coal seams (kM2, kM3 and kPl) and several altered tuff layers within the Miocene sequences. This study focuses on the evaluation of coal-petrography, mineralogical and elemental compositions, as well as the Rock-Eval pyrolysis of the three coal seams from Eynez, Isiklar and Denis sectors, and to identify the mineralogy of altered tuff layers within kM2 and kPl seams. The routine coal quality analyses show that coal samples from the kM2 seam are characterized by lower ash yields and total S contents, and higher gross calorific values than kM3 and kPl seams. The mineralogical composition of the bulk coal samples varies between all three seams. The statistical treatment implies that major elements in bulk coal samples such as Al, Fe, K, Mg and Na, and the vast majority of minor and trace elements have inorganic affinity. The trace element contents and elemental enrichments in the coal samples are generally changeable from seam to seam, which could be related to differences in ash yield and mineralogy. Altered tuff layers, or tonsteins were identified in kM2 and kPl seams. The tonsteins are clay rich, and certain trace elements (e.g. As and Zr) are enriched in the coal layers beneath the tuff layers in the kM2 seam. This might be related to penetration of leached pore water into these layers and precipitation of epigenetic minerals. The coal-petrography composition and the coal-facies diagrams show that the precursor peat of the kM2 seam was accumulating under telmatic, mesotrophic, anoxic conditions, whereas the water level was high and stable. The relatively low ash yields and thicker coal beds suggest a lower clastic input from the basin margins, and water table fluctuations resulted in calcareous intercalations in the kM3 and kPl seams. These imply the development of limno-telmatic conditions whereas fluvial activity affected during peat accumulation of both seams. Considering the maceral composition along with the coal thickness, coal seams in the Soma Basin might display oil generation potential; however, the pyrolysis analysis shows that the studied coal seams are mainly gas-prone and only certain coal beds have mixed-hydrocarbon generation capacity. All data presented in this study indicates that lithological features, differences in coal qualities and elemental contents in the Soma Basin were mainly controlled by changes in the depositional conditions and the detrital input during Miocene. (C) 2017 Elsevier B.V. All rights reserved.