Influence of Erosion-karst Processes on Lithological Features of Productive Strata in Bobrikovian-tournasian Oil Reservoir.

Erosion-karst processes on the surface of Tournaisian land, formed as a result of regression of DevonianTournasian Sea, led to both significant morphological transformation of Tournasian paleorelief and formation in the Late Radaevskian-Bobrikovian transgression of sand lenses with secondary clay-carbonate cement with a predominance of carbonate component. On the example of two adjacent wells it is shown that sandstones with abundant clay-carbonate cement during logging interpretation may be taken for carbonate reservoir rocks, if factors are not taken into account such as increased calcium in seawater of RadaevskianBobrikovian basin and carbon dioxide in the atmosphere of Lower Carboniferous. They led to the development of carbonate cement in Visean sandstone and secondary calcitization in Tournaisian rocks.