Hydrogeochemical and rare earth element properties of geothermal waters and the use of abandoned oil wells in Diyarbakır

Abstract Türkiye is one of the most tectonically active regions in the world. Diyarbakır, which was chosen as the study area, is located in the south-eastern Anatolian region of Türkiye and also in the south of the Bitlis-Zagros Suture Zone (BZSZ). Some of the geothermal resources in Diyarbakır are actively exploited. The temperature of Çermik geothermal waters was recorded at 40–51 °C by MTA (General Directorate of Mineral Research and Exploration) between 2010 and 2014. However, subsequent measurements conducted following the Kahramanmaraş earthquake (Mw: 7.7 on 6 February 2023) revealed that the surface temperature of geothermal fluids had increased to between 52 and 56 °C. Previous studies have indicated that there are also many abandoned oil wells in the region where the wellhead temperatures are around 107 °C. As a result of hydrogeochemical analyses, deep circulating geothermal waters such as Çermik have Na-HCO3 composition, while shallow circulating and mixed cold surface waters such as Çınar, Bismil and Sur have Na-Ca-Cl-HCO 3 composition. According to the chalcedony geothermometer results, the reservoir temperatures in the study area vary between 90 °C and 173 °C. REY (Rare Earth Elements and Yttrium) results show that geothermal fluids in the study area have negative cerium (Ce), europium (Eu) and yttrium (Y) anomalies. The negative Ce anomaly indicates that the colder aquifers are close to oxygen-rich surface waters and thermal fluids interact with Karacadağ volcanic rocks. Eu anomalies in groundwater are controlled by preferential mobilisation of Eu 2+ during water-rock interaction compared to Eu 3+ , and negative Eu anomalies indicate leakage of geothermal fluids from granites. The Y/Ho values of the thermal fluids in the study area are between 30 and 40. This value is due to the active contribution of fluorine during the mobilisation of REY as a fluoride or fluorocarbonate ligand complex, causing Y to behave as a heavy pseudolanthanide. δ 18 O and δ 2 H isotope values show that the geothermal waters in the study area are of meteoric origin. The obtained results reveal that Diyarbakir province has a geothermal potential and usage potential can be increased by using the abandoned oil wells.