As soon as commercial production of extra-heavy oil reserves began, understanding of hydrogeological conditions of heavy oil reservoirs came to the fore. In Tatarstan, heavy oil accumulations are confined to the western slope of the South-Tatarian Arch and the eastern flank of the Melekess Depression. Most of the heavy oil reservoir has been found in the terrigenous Sheshminskian formation dated to the Ufimian age; few reservoirs have been found in the carbonate and in the carbonate-sandstone Kazanian, Sakmarian, and Asselian formations.
Hydrogeological conditions of heavy oil fields in different tectonic regions are discussed in this paper. Conditions of forming of subsurface waters are discussed, results of chemical analysis of waters of the so far most explored fields in the carbonate-sandstone Kazanian formation, the Ufimian sandstone Sheshminskian formation, and the Lower Permian carbonate Sakmarian and Asselian formations are presented. Hydrogeological conditions were studied using geological evidence and information collected during pilot production of heavy oil. We arrived at the conclusion that neither type of waters, nor mineralization depend on the stratigraphic confinement of aquifers; they are rather determined by the distance to heavy oil accumulations and the drainage relief. It is noteworthy that there is a marked difference between chemical compositions of waters of the fields in the western slope of the South-Tatarian Arch and the eastern flank of the Melekess Depression. Because of good connectivity, waters of the Carboniferous and Permian formations mix with each other. When water of a certain type interacts with hydrocarbons in presence of sulfate-reducing bacteria, waters of a quite different type appear in the internal waters of a heavy oil reservoir. To achieve optimal reservoir performance and to control environmental impact caused by the Company’s upstream activities it is very important to understand what causes changes of the chemical composition of waters.
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