Rosneft has been conducting geological exploration in the Northern Caspian since 2005. Mesozoic deposits - the conventional objects of hydrocarbon prospecting in the region - are characterized by high success ratio, relatively small size and low profitability. Unique fields such as Tengiz, Kashagan, Tsentralno-Astrakhanskoye have been discovered in the Upper Paleozoic complex in the north part of Northern Caspian along the basin boundary. Until recently, these subsalt deposits were not considered as a promising direction for hydrocarbon exploration in Rosneft license blocks, which are located south of the Precaspian syneclise, within the Northern Caspian fold-thrust zone. To evaluate prospectivity of the Paleozoic complex, the Company carried out regional geological studies. Vintage geological and geophysical data of the adjacent onshore and offshore areas were analyzed, reference wells and 2D seismic profiles were acquired. Part of vintage 2D seismic lines was reprocessed with unified graph, significant increase in information was obtained.
Based on the results of structural interpretation, uplifts were localized within the Northern Caspian fold-thrust zone, which can be traps for oil and gas accumulations. In addition, all elements of the oil and gas system were described. In the Caspian syneclise, the reservoirs in the Paleozoic complex are carbonate formations of Late Devonian - Early Permian age. In the Northern Caspian fold-thrust zone, several meters of shallow-water Middle Carboniferous limestones were penetrated by drilling. An interval with decreased frequency and constant thickness is distinguished in the wave field, which is identified as carbonate platform similar to the platform established in Astrakhanskoye field. The Kungurian salt-bearing strata and Artinskian shale deposits act as a seal for discovered Paleozoic fields. In the Northern Caspian fold-thrust zone Kungurian stratum is eroded over most of the uplifts, while the Lower Permian terrigenous complex is characterized by lithological heterogeneity in the region. Thus, sealing is key risk for the area. According to the existing models, accumulations in Mesozoic strata formed due to HC leakage from Paleozoic deposits. Thus, the discovered fields in the Jurassic section allow us to conclude that in the Northern Caspian fold-thrust zone the risks of source rock presence and migration pathways presence are minimal, but there are substantial risks related to the preservation of pre-salt hydrocarbon accumulations. As a result of regional work, prospectivity of new for this area subsalt Paleozoic complex was identified, the resource potential was increased, and the territory was ranked according to the geological risks.
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