H. Ben Dhia

Application of chemical geothermometers to some Tunisian hot springs

Abstract Forty-five Tunisian hot springs are studied as geothermal indicators. Several chemical geothermometers are applied in this sedimentary geological environment. Hot springs are selected among 70 existing within the country. Selection is based on complete water analyses with a good ionic balance. Measured temperature ranges from 21 to 73°C and discharge rates range from 0.1 to 40 l/s. Studied springs show mainly chloride type waters. Geothermometers applied are: silica, Na/K, NaKCa, NaKCaMg, Na/Li and Mg/Li. Temperatures estimated by those geothermometers are plotted against the measured values to evaluate the applicability of the geothermometers used. The Na/K and Na/Li geothermometers appear to give unreliable results, as water composition seems to be greatly affected by interaction with the evaporitic and dolomitic rocks that are ubiquitous in Tunisia. The NaKCaMg, Mg/Li and silica geothermometers seem to give plausible values, emphasizing the role played by Mg and its importance for Tunisian hot waters.

Shallow geothermal studies in Tunisia: Comparison with deep subsurface information

Abstract Shallow geothermal prospecting has been undertaken in three zones in Tunisia for which few deep thermal data are available. Five areas have been included in this work, and temperature data from depths between 35 and 335 m from 46 wells in these areas have been analyzed. In addition, thermal measurements have been made on 112 limestones, dolomites, and sandstones to add to the data base. Temperature profiles from the different areas vary substantially from well to well, and these variations depend on both the local and regional geological and hydrological conditions. The shallow thermal gradients vary from 11 mkm -1 to 132 mkm -1 , compared with the maximum deep gradient so far observed in Tunisia of 52 mkm -1 (Ben Dhia, 1988). The discrepancies between shallow and deep gradients in the various regions appear to strongly depend on the geological continuity between the shallow and deeper layers, especially in zones where substantial tectonism has occurred. It is concluded that although near surface geothermal data are useful in areas where few deep data are available, great care must be taken in their interpretation, and both regional and local conditions must be considered. Furthermore, predicting the temperature conditions at depth from these observations can be uncertain, and so combining results from such studies with those from deeper data must be done with care. The Zazghouan area appears as an interesting geothermal prospect, while the others need more investigation to be conclusive.

Hydrodynamic framework of Saharan Triassic aquifers in South Tunisia and Algeria

Abstract The main characteristics of the lower Triassic in the Saharan part of Tunisia are presented. This first study of the aquifer is made possible because of data available from numerous petroleum wells that exist in the region. The results show that the reservoir is of importance for either geothermal energy recovering or human water needs; especially since its salinity lies in the range 2 g/l to 60 g/l. Along the Tunisian-Llibyan frontier, because of its pressure and salinity ( The study also shows that the salinity gradient (SE-NW) increases orthogonally to the runoff direction (SW-NE). This phenomenon was unexpected and it is necessary to consider the aquifer in its regional North African framework and to include its Algerian part to understand it; when the salinity and potentiometric maps include both countries, a regional pattern is evident. Furthermore, a correspondence is noted between the salinity variations and the percentage of detritic elements in the reservoir. Salinity increases toward the NW, while the detritic elements decrease in that direction. Zones with salt content lower than 5 g/l seem to be related to good reservoirs and shales, that are rich in sands, and carbonates. The aquifer water supply is primarily linked to gravity flow and secondarily to compaction flow.