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 Devonian-Tournasian 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 Radaevskian-Bobrikovian 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.