Riphean-Vendian deposits in Yuryuzano-Aisky depression of Urals foreland basin: features of geological structure and petroleum potential

Basing on an integrated analysis of data on composition, structural and textural features of rocks, results of core laboratory experiments and thin section studies, interpretation of well logs and 2D CDP seismic data, the authors identified the features of geological structure of Riphean and Vendian deposits in the present-day Yuryuzano-Aisky depression. They revealed the complicated block structure of the sequence in the western part of the area, and presence of folded structures of reverse fault and thrust type in the eastern part. Geological map of Riphean-Vendian deposits coming out to the surface of pre-Devonian erosion is created. Zones of occurrence and absence of Upper Proterozoic deposits are revealed. The most complete Riphean column with maximum thickness of 13-15 km is identified in the southern part of the study area. Minimum thickness of Vendian deposits (200-300 m) are observed in the south-west. Paleo-arch is revealed in the northern part. Formation analysis of the deposits is conducted. Red-coloured continental rudaceous formations of the early rifting stage, grey-coloured and variegated sandy-argillaceous, argillaceous-carbonate, and carbonate coastal- and shallow-marine formations of the mature rifting stage, and delta-front formations of the later stage of rifting are identified in the Riphean interval. Typical platform siltstone-sandy-argillaceous formations are identified in Vendian series. Lithologic and paleogeographic maps are created. Zones of occurrence of coastal-marine deposits of tidal plains, shallow-marine shelf, moderately deepwater deposits of open shelf are revealed in the Early Riphean Kaltasinsky time. Zones of coastal-marine and shallow-marine sediment accumulation are identified for Middle Riphean deposits. Zones of terrigenous coastal-marine and shallow-marine, lagoonal and shallow-marine carbonate deposits are revealed for Later Riphean time. Zones of coastal-marine and shallow-marine deposits are identified in Later Devonian series. Studies of rock reservoir properties showed the lack of porous reservoirs and predominance of those fractured. In the course of geochemical studies, it is found that exploration maturity of Organic Matter is insufficient for complete and unbiased estimate of oil and gas generation potential of the rocks. That is why the issue of Riphean petroleum potential remains undecided and requires further consideration. Vendian deposits are a part of a promising underexplored sequence