Geodynamic and geological factors in the formation of large hydrocarbon deposits in subsalt sediments of the North Caspian region

Abstract

Background. The Astrakhan-Primorsk region of oil and gas deposits located in the North Caspian region is one of the world’s largest reserves of hydrocarbons. This region stretches from land in the northwestern part of the Caspian basin, through the northern waters of the Caspian, to the south-eastern land part of the region under consideration.Aim. To identify geodynamic and geological factors in the formation of large hydrocarbon deposits in subsalt sediments in the complex structure of the North Caspian region of the Caspian syneclise. Materials and methods. An analysis of data on the geodynamic and geological evolution of the North Caspian region of the Caspian syneclise revealed specific features of its geological structure and development.Results. The nature of the formation series, as well as the geodynamic and geological conditions (the presens of spreading and subduction zones that caused an intense heat flow, the presense of an isolated sedimentation basin with intensive accumulation of carbonates in the Upper Devonian-lower Permian, a powerful salt-bearing Kungurian cap) were favourable for the generation, migration and accumulation of hydrocarbons in large tectonic-sedimentary Upper Devonian–Lower Permian uplifts characteristic of the Astrakhan-Primorsk oil-and-gas bearing region of the North Caspian. These uplifts were formed by shelf bioherms, barrier reefs and large reeftogenic atolls and emerged through ancient troughs. The large hydrocarbon accumulations, such as Kashagan, Tengiz, Astrakhanskoye and Korolevskoye, with high specific hydrocarbon reserves (more than 100 thousand/km2) are associated with these troughs.Conclusion. During the Upper Paleozoic, the main paleodepressions of the isolated basin of the North Caspian underwent a steady and long-term subsidence process, favourable for the accumulation of a thick stratum of carbonate (mainly reef) formations. This period was also characterized by the geothermal and baric conditions necessary for the transformation and subsequent migration of hydrocarbons from oil- and gas-producing complexes to reservoirs, which were mainly presented by massive carbonate formations of reef genesis. The as-formed deposits were preserved by thick salt-bearing Kungurian sediments. Directions for prospecting and exploration works aimed at discovering new hydrocarbon deposits in the subsalt sediments of the region were outlined.