№ 5 (2024)

The modern structure of the Eurasian Margin combines supra-subduction Upper Cretaceous volcanogenic formations – the marginal continental Okhotsk-Chukchi volcanogenic belt and the accreted Nemuro‒Olyutor intra-oceanic arc formed during the subduction of the oceanic crust of the Pacific. We consider the formation of the Okhotsk-Chukchi volcanic belt, separated by the Nemuro-Olyutor intraoceanic arc from the Pacific, in the conditions of a paleo-oceanic basin that existed for ~20 Ma. As a result of the conducted paleo-geodynamic analysis, it was found that in the Late Cretaceous in northeastern Eurasia, between the Eurasian continent and the tectonic blocks of the Sea of Okhotsk, Western Kamchatka, and Northern Koryak located to the south and southeast of it, a small oceanic basin of the pull-apart type developed in the conditions of transtension, which can be called the Okhotsk‒Penzhina‒Anadyr. This basin was formed as a result of a major strike slip fault when terranes tectonic escaped out of the eastern part of the closing Mongol‒Okhotsk Ocean. The progressive opening of the Okhotsk‒Penzhina‒Anadyr spreading basin subducted from the southwest to the northeast under the Eurasian continent, which caused a corresponding delay in the onset of supra-subduction volcanism in the Okhotsk‒Chukchi Belt. The geodynamic evolution of the southwestern and northeastern parts of the Okhotsk‒Penzhina‒Anadyr basin differed in that the Iruney (West Kamchatka) island arc arose and developed in the southwestern part of it, earlier in time of formation, near the West Kamchatka continental block. In the northeast, the arc was bounded by a transform fault that stretched to the southeast, delimiting not only the Kamchatka terrane from the North Koryak and Uchkhichkhil, but also the Valaginsky (East Kamchatka) and Olyutor intra-oceanic island arcs. In the Cenozoic stage of development, this fault was transformed into the Palano‒Ozernovskaya trans-litospheric zone, which is the boundary between the accreted Olyutor and East Kamchatka terranes of the Nemuro-Olyutor island arc system. A geodynamic model of the pre-arc, the Alb‒Turonian stage of intra-oceanic subduction, which lasted until the beginning of the formation of the early Nemuro arc in the coniacian, was applied the Olyutor island arc system. The duration of the pre-arc stage of intra-oceanic subduction corresponds to the time of the same stage of the geodynamic evolution of Izu‒Bonin‒Mariana Island arc system. This allows us to confirm the expansion of Pacific since the Albian time due to subduction in Nemuro‒Olyutor island-arc system.

The article summarizes the results of the authors’ work on the comprehensive substantiation of the geological, tectonic, and geofluid-dynamic conditions for generating and accumulation of hydrocarbons in the subsalt floor of the Astrakhan Arch inside the Caspian oil and gas Province.

We analyzed a significant amount of archival geological and geophysical data and materials such as (i) an array of regional seismic profiles; (ii) gravity and magnetic exploration data of different scales; (iii) results of deep drilling.

Specific seismic geological bodies that are capable of serving as oil and gas reservoirs have been identified. For the first time in creating the geological model of the Astrakhan Arch, its location within the autoclave hydrocarbon system of the subsalt floor of the Central subprovince of the Caspian oil and gas Province (within the Astrakhan-Jambay oil and gas accumulation zone) has been taken into account. This allowed the authors to substantiate the geofluid dynamic nature of the number of large non-structural-type oil and gas reservoirs boundaries. It is opening up new possibilities for predicting oil and gas fields, including those with low sulfur content. Several generation-accumulation mini-hotbeds have been identified on the Astrakhan Arch, and in three of them - reservoirs with increased probability of discovering oil and gas fields. The main result of the study is the refinement of the initial total geological resources of hydrocarbons of the Astrakhan Arch and adjacent areas.