The structure and genesis of limestones at the boundary between the Abalak and Bazhenov formations in Central West Siberia

In the central West-Siberian basin, fractured and cavernous carbonate rocks that are often oilbearing, which are referred to as correlation layer 1 (CL1), are frequently present at the top of the Abalak formation and/or at the bottom of the Bazhenov formation. They are sporadically distributed over the profile and the area; their genesis is still not completely clear. The structural features and oil-bearing capacity of carbonate rocks have been studied, as well at the distributions of carbon and oxygen stable isotopes from bulk rocks and calcite filling fractures in the CL1 layer that was penetrated by six wells. The spherolitic microstructure of limestones together with the carbon and oxygen isotope distributions (δ¹³C =–14 to–26‰ VPDB; δ¹⁸O = 0 to–5‰ VPDB) indicate the precipitation of carbonate material due to microbial activity on the surface and/or in the upper part of sediments at high methane concentrations. The fractures and caverns in limestones are frequently oil-bearing; they contain coarse crystals of calcite, pyrite, quartz, and, more rarely, barite. This degree of mineralization and the isotope composition of calcite oxygen (up to–18‰ VPDB) indicate that calcite precipitates at elevated temperature (up to 120°C) from the hydrothermal fluids that could migrate from underlying strata.