Bibliographical review about Na/Li geo-thermometry and lithium isotopes applied to worldwide geothermal waters. Final report

This study is performed within the framework of the FP6 European project HITI (High Temperature Instruments for supercritical geothermal reservoir characterization and exploitation). This research project, co-funded by EU and the different partners, aims to provide geophysical and geochemical sensors and methods to evaluate deep geothermal wells up to supercritical conditions (T > 370 deg. C), which are more cost-effective than those of the conventional wells. A deep geothermal well is currently being drilled for this purpose into the Krafla area, Iceland, as part of the IDDP ('Iceland Deep Drilling Project') and with joint funding from Icelandic industry and science Institutes. Another deep well will be drilled in the Reykjanes peninsula, Iceland, within the framework of the same project. This study, a bibliographical review about the Na/Li geo-thermometer and lithium isotopes applied on the world geothermal waters, is the first step of the task envisaged by BRGM to use and validate the sodium-lithium (Na-Li) chemical geo-thermometer on Icelandic geothermal waters at temperatures ranging from 25 to 500 deg. C. In this study, more than 120 temperature and chemical data from world geothermal and oil-fields, sedimentary basins, oceanic ridges, emerged rifts and island arcs have been collected and investigated. These additional data have allowed to confirm and refine the three existing Na/Li thermometric relationships. Moreover, a new Na/Li thermometric relationship relative to the processes of seawater or dilute seawater-basalt interaction occurring in the oceanic ridges and emerged rifts is proposed. Even if the running of Na/Li is still poorly understood, the existence of a new thermometric relationship confirms that the Na/Li ratios not only depend on the temperature but also on other parameters such as the fluid salinity and origin, or the nature of the reservoir rocks in contact with the geothermal fluids. For most of the geothermal waters in contact with volcanic rocks or granite in the deep reservoir, rock leaching and hydrothermal alteration (illite or smectite precipitation) are probably the main processes controlling the Li concentrations. Additional field and laboratory studies as well as a thermodynamic approach based on mineral dissolution-precipitation equilibrium reactions could help to support and confirm this assumption. The scarcity of the δ7Li values analysed in the geothermal and oil-field fluids and the presently available data indicate that it is necessary to acquire additional data in both fluids and rocks to better understand the Li behaviour in the processes of water-rock interaction. However, as the dissolved Li during these processes seems to be controlled by several influent factors, it is recommended to study the Li behaviour at different temperatures for a same type of process of water-rock interactions. Among possible applications, the determination of δ7Li values in geothermal waters shows that these data can be used to determine the nature of the reservoir rocks in contact with these waters and indirectly estimate the depth of these reservoirs. The Na/Li geo-thermometer, which is often less accurate than other classical geo-thermometers but can be more reliable in many cases, is then very useful for geothermal exploration. In order to improve the use of this geo-thermometer, additional developments in different environments and regions are necessary and it is essential to well define the environment in which it will be applied before its use. Experimental works in laboratory relative to water-rock interaction processes as a function of temperature integrating chemical, isotopic and mineralogical analyses should allow to improve the knowledge and to understand the running of this geo-thermometer. (authors)