Chemical geothermometry and fluid/mineral equilibria of waters in the NW-Bourgas synclinorium, Bulgaria

Abstract

The report focuses on the interpretation of water chemistry in the northwestern part of the Bourgas synclinorium with the aim of predicting reservoir temperature. The types of waters in this region are, a) dilute neutral pH cold springs, and b) HCO3-Na, Cl-HCO3-Na and Cl-Na well waters, some of them with relatively high SO4. Silica and cation geothermometers are applied to predict possible subsurface temperatures using 75 chemical analysis from wells and springs calibrated by chemical equilibrium calculation with the chemical speciation programmes WATCH and SOLVEQ. The equilibrium state of thermal waters was studied by means of log Q/K diagrams, Na-K-Mg triangular diagram and the equilibrium state of cation/proton ratios at the measured chalcedony and quartz reference temperatures. In this way, it was shown which minerals are possibly in equilibrium with the fluid constituents at certain temperature. This approach facilitates the choice of geothermometers and provides a better understanding of the system. Furthermore, the silica-enthalpy mixing model was used to estimate underground temperature of the hot unmixed geothermal water. Finally, the possible subsurface temperatures were predicted. It was found that quartz solubility controls reservoir temperature in the Straldja field and water with temperatures of 100-120°C, which is 20-40°C higher than presently found, could be obtained by deep drilling. Chalcedony solubility controls reservoir temperature in the Aitos field and water with temperature 80-100°C could be obtained by drilling. The geothermal water in this region could be used for balneology, swimming pools, bathing, recreation and heating systems.