Weathering Crust of the Basement in Parametric Wells 50 Novournyak and 2000 Tuimazy in the South-Tatar Arch

The paper deals with allocation of the vertical profile of the weathering crust based on well logging in the Precambrian basement of the South-Tatar Arch in the Republic of Bashkortostan. Within the most elevated part of the basement in the region, crystalline rocks lie at depths of 1650-3000 m. Despite the fact that the weathered rocks are potential reservoirs, they were not objects of special studies in the Republic of Bashkortostan. One of the reasons is in poor study of core section. The exceptions are parameter wells ‘2000 Tuimazy’ and ‘50 Novournyak’, in which the core was taken out in several intervals of the weathering crust. Based on the comparison of logging data and rock material in these wells the areas were allocated of subsequent change of crystalline rocks under action of supergene factors from the initial disintegration of the initial substrate to the final products of its decomposition. In the well 50 Novournyak rocks alternated in the surface conditions are blocked by the most ancient sediments – Tyuryushevskian suite of the Lower Riphean. The authors believe that the basic pack of the suite presented with quartz-feldspathic sandstones and quartzites and visibly separated from the overlying sandstones according to logging completes a full profile of the weathering crust. In the well 2000 Tuimazy the weathering crust is overlapped by deposits of Koyvenkovskian horizon and is characterized by a greater degree of alternation and fragmentation of the initial substrate. Its formation in the gap between the Archean-Proterozoic and Lower Devonian was a multi-staged: with periods of complete or partial erosion, with the imposition of supergene processes in the previously formed areas. Despite some differences, the vertical profile of the weathering crust in both wells has similar logging characteristics. It is recommended to check signs identified on more wells drilled within the South-Tatar arch, and in the case of confirmation, they can be used to study the structure of the weathering crust in low coring or its absence.