| Title | Investigation of Major Geothermal Fields of Pakistan Using Isotope and Chemical Techniques |
|---|---|
| Authors | Manzoor Ahmad, Muhammad Rafiq, Naveed Iqbal, Waheed Akram, M. Azam Tasneem, Mubarik Ali |
| Year | 2005 |
| Conference | World Geothermal Congress |
| Keywords | Pakistan, Geothermal waters, Origin, Subsurface history, reservoir temperatures, isotope geothermometers; chemical geothermometers |
| Abstract | Environmental isotopes like 18O, 2H & 3H of water; 13C of dissolved inorganic carbon; 34S & 18O of dissolved sulphates and water chemistry were used to investigate the origin, subsurface history and reservoir temperatures of major geothermal fields of Pakistan. Seven hot springs of the Murtazabad area lye along the Main Karakoram Thrust in Northern Areas and have discharge from 50 to 1200 liters per minute with the surface temperature range of 47 to 92 C. All the thermal waters are NaHCO3 type and their origin is meteoric water recharged from precipitation at higher altitude, which undergoes 18O shift of about 1â due to exchange with rocks. The wide range of tritium concentrations shows that the shallow fresh groundwater is mixing with the thermal water in different proportions but the 18O- 2H plot and other data indicate that the circulation time of thermal end-member is more than 50 years. Different geothermometers give reservoir temperature in the range of 185-225 C. The isochemical-mixing model based on the Na-K and quartz geothermometers estimates 227 C temperature. 18O(SO4-H2O) geothermometer indicates equilibrium temperatures in from 130 to 185 C. The dissolved silica vs. enthalpy plot suggests heat losses mainly through conduction. Tatta Pani and Tato thermal springs along the Main Mantle Thrust have discharge from 30 to 2000 liters per minute with the surface temperature from 48 to 92 C. These are also NaHCO3 type. Their origin is local meteoric water and there is no mixing of shallow fresh water. Tritium data show that the residence time of thermal fluids is more than 50 years. The reservoir temperatures of Tatta Pani springs (100-120 C), determined by Na-K and quartz geothermometers, are in good agreement. The 18O(SO4-H2O) geothermometer indicates equilibrium temperatures around 150 C. For the Tato springs, the isotope and chemical geothermometers (except for the K-Mg) agree on an equilibrium temperature of about 175-200 C, while 18O(SO4-H2O) equilibrium temperatures (170 C) is in good agreement. Geothermal springs of Chagai are related to the youngest volcano (Koh-I-Sultan) of Pakistan. Their 18O is very enriched (up to +4.3â), while 2H is depleted (-22 to -36â) indicating recharge from meteoric water and very high 18O-shift (up to 10â), which is due to rock-water interaction at higher temperatures. Tritium data show the age of thermal waters more than 50 years. Three springs having very high electrical conductivities (25300 to 45600 S/cm) are NaCl type, while three other springs with the EC around 4000 S/cm are MgSO4 type. The chemical geothermometers like Na-K, K-Ca and Na-K-Ca-Mg indicate high reservoir temperature estimates mostly in the range of 200-300 C, while the silica and 18O(SO4-H2O) geothermometers are in good agreement indicating the temperature around 150 C. |