Reservoir rock characterization in Edremit geothermal field: Geochemical implications for possible fault zones

Abstract

Reservoir lithology of Edremit geothermal field from western Anatolia is investigated by means of mineralogic-petrographic and geochemical analyses. Drill cuttings that belong to reservoir levels are analyzed macroscopically and microscopically, followed by the techniques of X-Ray Diffraction (XRD), Confocal Raman Spectroscopy (CRS) and Electron Probe Micro Analysis (EPMA) for detailed mineral characterizations. In the samples, dominant rock fragments are identified as granitic rocks while quartz, feldspars (plagioclase, orthoclase), micas (biotite, muscovite), carbonates (calcite, dolomite, ankerite) and amphiboles (actinolite, richterite, pargasite, hornblende, edenite) constitute the main mineral phases. CRS and EPMA studies reveal that the composition of plagioclase feldspars ranges from Ca-rich (bytownite) to Na-rich (oligoclase). The effects of cataclasis and alteration which are the indications of faulting and hydrothermal fluid activity, respectively, are detected from textural characteristics of the samples. These textures include cataclastic, mylonitic and mortar textures. The observed alterations are metasomatism and secondary uralitization. The changes in major and trace element concentrations along the well bore are deduced from the results of X-Ray Fluorescence (XRF) analyses. It was observed that changes in textural features which are associated with various grain sizes from coarse to fine, closely coincide with chemical variations with depth, leading to the identification of three distinctive zones at depth intervals of 900–928 m, 930–974 m and 976–1038 m. The correlation of geochemical findings regarding hydrothermal fluid effect (such as pyrite abundance, alteration features) with the information of a probable water leakage around 930 m depth, suggests that the middle part of these three zones is the potential passageway for the fluid. The lithogeochemical results also suggest that the fluids are likely to be Si-rich and (in deeper levels) Ca-rich which are in accordance with the hydrogeochemistry of the system that is previously investigated. This study points out the importance and practicality of geochemical studies as significant indicators utilized in geothermal exploration.