Geological and hydrogeological features of the lower Permian carbonate locally bituminous complex of the Tatarstan

© 2020 IOP Publishing Ltd. All rights reserved. The Lower Permian bituminous complex is characterized by complex hydrogeological conditions. The predominant type of reservoir is mixed porous-fractured-cavernous. Water layers lateral prevalence is directly related to the degree of fracturing and cavernous of dolomites and limestones. Regional hydrogeological studies have shown that at the dividing ranges the Lower Perm sedimentary stratum is anhydrous, and in the river valleys it is water-rich. The center parts of such areas are erosive incisions of river valleys. From the periphery to karst regions center, a consistent increase in well flow rates and a decrease in piezometric groundwater levels from the slopes of dividing ranges to the river valleys are observed. In the domed sections of the South Tatar Arch in the Asselian - Sakmarian sediments, calcium-sulphate waters with a total salinity of 1.5 to 3.5-4.0 g/dm3 are prevalent. Sodium sulfate waters appear within the western slope of the arch. Within the eastern part of the Melekess Depression, the Asselian-Sakmarian stage groundwater deposits are already completely sulphate-sodium. The predominance of sodium components over calcium in groundwater is observed during total salinity of 4.5-5.5 g/dm3. In the areas of the Melekess Depression, where the Lower Permian sediments are drowning, groundwater acquires to sodium-chloride composition

The influence of lithofacies features of the Permian natural bitumen-saturated reservoir rocks on groundwater formation conditions

© 2020 IOP Publishing Ltd. All rights reserved. The reservoirs rocks of natural bitumen deposits of the Lower Permian system and the Kazanian stage of the Tatarstan territory are considered. The relationship of groundwater composition in bitumen reservoirs with lithologic features of water-saturated rocks is shown. Rocks compositions contribute to formation of three water classes in oil deposits: hydrocarbonate, sulfate and chloride. Hydrocarbonate waters lie over erosion downcutting of rivers in the most elevated territory part. Their origin is associated with hydrolysis and carbon-dioxide leaching of the Permian terrigenous rocks. Inland waters of hydrocarbonate-sodium (soda) composition of bitumen deposits are located above local drainage basis. They are formed as a result of sulfate reduction processes in deposits. The inland waters of sulfate composition of calcium and sodium groups are characteristic of deposits located below the drainage basis. Their origin is associated with dissolution of gypsum and anhydrites, as well as with ion exchange reactions between the calcium of the solution and the sodium which absorbed by rock complex. Chloride waters associated within the eastern side of the Melekess Depression, where there is an active hydrodynamic relationship between the Permian system aquifers and chloride-calcium salt brines of the Carboniferous rocks. The nature of the distribution and presence of aquifers in deposits is determined by the lithological features of reservoir formation