Abstract
This article presents the results of study of fluid inclusions in calcite and quartz of the Devonian, Carboniferous and Permian strata of the eastern part of the Precaspian syneclise and the Aktobe Preuralie of Pre-Ural trough. Were determined by microthermometry method the temperatures of melting and homogenization of fluids in crystals of rock and the types of hydrocarbons in inclusions. Interpretation of the measurement results made it possible to determine the most specific values of pressure and temperature, equal, respectively, 350-500 atm and 85°-130°C increasing up to 650 atm / 350°C and more towards the Urals. Accordingly, the geothermal gradient increases from west to east from 24°C/km to 40°C/km in the Aktobe Preuralie. The mineralization of fluid inclusions varies in the same direction from 10-12% to 3-6% of the weight equivalents of NaCl. These results demonstrate three types of gradients: the lowest in the western inner and platform parts of the Precaspian basin (24°C/km), the average in the Pre-Ural trough and the highest in the eastern suture with the Ural folded system. Accordingly, there is an increase in the geothermal background in the Aktobe Preuralie of Pre-Ural trough to 40°C/km.