Implications for Possible Fault Zones Deduced from Lithogeochemical Characterization of Reservoir Levels of a Geothermal Field: Edremit Example

Abstract

Investigation of the reservoir lithology of Edremit geothermal field from western Anatolia is performed using the drill cuttings that belong to the reservoir levels. The samples are analysed in an attempt to determine the mineralogical and petrographical features, mineral compositions and whole rock geochemistry. The examinations include macroscopic and microscopic analyses, followed by the techniques of X-Ray Diffraction (XRD), Confocal Raman Spectroscopy (CRS), Electron Probe Micro Analysis (EPMA) and X-Ray Fluorescence (XRF). Dominant rock fragments and mineral phases are identified as granitic rocks and quartz, feldspars, micas, carbonates and amphiboles, respectively. Textural characteristics of the samples display the effects of cataclasm and alteration, indicating faulting and hydrothermal fluid activity. The changes in major and trace element concentrations along the well bore reveal three different concentration levels at depth intervals of 900-928 m, 930-974 m and 976-1038 m as well as an inverse relationship between SiO2 and CaO. These three distinctive zones correlate well with the variations in grains sizes (coarse to fine) and textural features (cataclastic to mylonitic) with depth. Taking into account the reported water leakage zone around 930 m depth and the evidences for hydrothermal fluid effect (such as pyrite abundance, alterations, sulphur presence) observed along the well bore, possible fault zone and as a result, the likely pathway for the hydrothermal fluid is inferred to be in the middle section (930-974 m) of the identified three zones. It is also deduced from the lithogeochemical results that the fluid is potentially Si-rich in this zone and Ca-rich in deeper levels which are in line with the previously investigated hydrogeochemistry of the system. This study points out that mineralogical-petrographical studies integrated with geochemical analyses can potentially serve as significant indicators in determination of lithological variations that can be correlated with fault zones of deep systems for prospective exploration sites. This study has been published in Applied Geochemistry in 2022 and complete work can be accessed from https://doi.org/10.1016/j.apgeochem.2022.105388