Geochemical Characteristics of Geothermal Hot Water Resources on Territory of Vietnam

Abstract

Main results in geothermal study in Vietnam until today is the archive of manifestation and chemical compositions, temperature of the water samples in over 200 outcrop hot water sites and in drill holes has a temperature >30°C distributing in the entire territory. They are collected during the geological and hydro-geological, oil and gas exploration since 1954 (mainly from 1975) today. The paper presents results of some approaches and applies common worldwide methods for identifying the characteristics of hot water resources. By using series of chart SO4-HCO3-Cl, Na-K-Mg, correlations between log (SiO2) with log (K2/Mg), between the levels of pH and temperature-dissolved silica, between the concentration of dissolved silica and chlorine, for the available data allowed classify nature of each hot water sample such as in every region on territory. The sources distributing near the sea coast with high chlorine indicate sea water. Most sources in Northern territory characterized a high HCO3 - SO4 and low Cl indicating heated and mixed with the near surface water-rich HCO3. Conversely, in the central and Southern parts of territory, almost sources water distributing along the HCO3 - Cl axis with low SO4 indicating water rising from periphery of hot granitic source. Eight formulas of geochemical thermometer as the silica - quartz, silica-chalcedony, the cation thermometers Na-K, Na-K-Ca, mixing model silica – enthalpy were applied at available 50 sources entire territory giving values of reservoir temperature, that concentrated around 80-200°C, a few values >200°C to 400°C at a small number of sources received from the Na-K thermometers may not ensure reliability. The average values of the reservoir temperature from the all applied thermometers concentrated in range 120°C to 160°C. In comparison between regions, the reservoir temperature in Central part of territory has higher, than in the Northern and the Southern one.