Formation conditions and prospects of the Visean (Bobrikovian) terrigenous deposits of the Volga-Ural anteclise

The paper considers the formation conditions of the Visean (Bobrikovian) terrigenous deposits of the Volga-Ural anteclise and proposes a tool for rapid assessment of their oil potential within licensed fields and unallocated subsoil areas. The productivity of the Bobrikovian horizon is largely controlled by depositional environments, facies affiliation and lithological variability of the rocks; therefore, a forecast based only on the structural position of an object is insufficient. The objective of the study is to develop a tool for rapid assessment of the oil potential of the Bobrikovian deposits that takes into account their formation conditions depending on the depositional environment. Stratigraphic schemes, paleogeographic maps, archive reports, well data and materials of ongoing projects were integrated into a single digital database; paleogeographic reconstructions were digitized and facies zoning of the horizon was performed. Six facies zones were identified: crystalline basement highs, alluvial plain, alluvial-deltaic plain, deltaic plain, delta front and open shelf. For each zone, the formation conditions and reservoir parameters were characterized: net-to-gross ratio, shale content, porosity and permeability. It has been established that the most favorable conditions are associated with the deltaic (average net-to- gross ratio 0.81) and alluvial-deltaic (0.67) plains, which are classified as highly prospective zones; the alluvial plain and delta front are classified as moderately prospective, and the open shelf and basement highs as low-prospective. An algorithm of rapid assessment and a scheme for ranking areas are proposed, including comparison of the facies map with unallocated subsoil areas. A SWOT analysis was performed, and the limitations of the approach and directions for its further development were identified.

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