Dombayova (Dinar, Afyonkarahisar) Linyitlerinin Jeolojik Konumu, Kimyasal, Mineralojik ve Petrografik Özellikleri

Abstract

The Domboyova Graben hosts one of the most significant coalfields recently discovered in Türkiye, with approximately 1 GT of coal resources. In the coalfield, coal beds are located in the Late Miocene (?)–Early Pliocene Güzelyayla Formation-Akgün member. The thickness of the coal-bearing zone ranges from 15-20 to 420 m. In this study, geological settings, chemical, mineralogical, petrographical, and majör and trace elemental analyses of coals were investigated. For these purposes, a total of 131 coal samples were obtained from the D-77 and D-103 wells, and a total of 16 rock samples were collected from the A-160 well. Furthermore, selected outcrop/core samples were subjected to paleontological, meso- and microporosity, micro-FTIR, TOC, and Rock-Eval analyses. The results of proximate analyses of coal samples from both wells show that the weighted averages of moisture, ash yield, volatile matter, and gross calorific values on an air-dried basis are 7.5%, 24.2%, 49.3%, and 3485 kcal/kg, respectively. The results of the ultimate analyses show that the weighted averages of C, H, N, S on an air-dried basis are 40.2%, 4.0%, 1.1%, and 1.5%, respectively. The identified minerals in coal and rock samples using XRD-TK analysis are calcite, aragonite, quartz, feldspars, pyrite, clay minerals, and in a few samples serpantin and opal-CT were also detected. Several accessory minerals along with calcerous fossil shell remains and skeletal remains in siliceous (diatoms and sponge spacules) and Ca-phosphate compositions were identified during the SEM-EDS analyses. The identified clay minerals are illite/mica, kaolinite, and smectite according to XRD-CF analyses of rock samples from the A-160 well. The coal petrography analyses indicate that the weighted averages of maceral and mineral-matter are 80.1% huminite, 2.9% inertinite, 1.3% liptinite, and 15.7% mineral matter. Nevertheless, the measured random huminite reflectance (%Rr) values are around 0.32±0,02%, and these values imply that the studied coal samples are of lignite in rank. Rock-Eval pyrolysis analyses indicate that Type III kerogens are predominant in the studied coal samples, whereas mixed Type III-IV kerogens were more common in the studied rock samples. Additionally, studied coal samples are gas-prone, while rock samples have very limited gas generation potential. The correlations of meso- and microporosity analysis results among ash yields, C, and the mineralogical and maceral compositions of composite coal samples from the D-103 well imply that the mesoporosity characteristics are controlled by mineral matter, and microporosity is controlled by organic matter. The distributions of majör oxides and trace elements are generally related to mineralogical compositions. The SEM-EDS analyses of selected coal polished sections indicate that Cr enrichments are most likely controlled by detritic chromite grains and clay minerals. Besides, the Mo, U, As, and V enrichments could be controlled by anoxic conditions within palaeomires. The coal petrography, coal facies diagrams, palynoflora, and mineralogical and geochemical features indicate that the precursor peats were accumulated under lake-shore conditions, which were open to Ca2+ and HCO3- rich karstic aquifer support and detritic mineral (for example clay minerals, quartz, feldspar, and especially carbonate minerals (more calcite and less dolomite)) inputs. Hence, alkaline conditions were elevated in paleomires, and calcerous fossil shell remains are common. Overall, the preservation of organic matter, mineralogical, majör oxide and trace elements contents are controlled by water chemistry, pH conditions and detritic-influx into palaeomires.