T.K. Tsaregorodtseva, V.A. Bykadorov, Yu.A. Volozh
Pliocene (Balakhansky) deposits of South Caspian Depression: features of geological structure and oil and gas occurrence
Key words: South Caspian Depression; North Apsheron Trough; Turkmenian shelf; Crimea-Kopet-Dagh strike-slip fault; Lower Pliocene; red-coloured and productive series; Dagadzhiksky horizon; seismic stratigraphy; petroleum potential.
For citation: Tsaregorodtseva T.K., Bykadorov V.A., Volozh Yu.A. Pliocene (Balakhansky) deposits of South Caspian Depression: features of geological structure and oil and gas occurrence. Geologiya nefti i gaza. 2022;(4):27–38. DOI: 10.31087/0016-7894-2022-4-27-38. In Russ.
Funding: The paper was prepared within the framework of the State Assignment (subject “Accumulation settings and features of post-sedimentary transformations of sedimentary sequences in geochemically different paleobasins of the Russian Plate, Crimean-Caucasian region and adjacent regions”, No. АААА20-120030490100-6). This work was supported by RNF grant No. 22-27-00827 (“Paleogeography of regressive stages of the Paratethis basin evolution in Early Oligocene”).
South Caspian Depression contains considerable unexplored hydrocarbon resources; and new discoveries in the Azerbaijan and Iran sectors of the sea confirm this fact. Although, exploration activities on the Turkmenian shelf were unsuccessful. One reason is insufficiently substantiated knowledge of structure of deep sedimentary cover horizons. In this work, the authors use seismic stratigraphic analysis to consider the underexplored structural features of oil and gas bearing Balakhansky (Lower Pliocene) deposits within the depression. They expand the boundaries of the South Caspian Depression. Seismic data analysis shows that North Apsheron Trough filled with thick Pliocene-Quaternary sequence should also be assigned to the depression. Tectono-sedimentary bench near the Yalama-Peschanomyssky high is a northern boundary of the depression in the Caspian waters. The Apsheron Sill is a part of the zone of regional Crimea-Kopet-Dagh strike-slip fault. Active movements along the strike-slip fault started in middle Pliocene and continue to this day, as evidenced by frequent earthquakes. Some researchers of the Apsheron Sill, unreasonably associate it with the ongoing subduction of the South Caspian. The authors clarify a concept of productive and red-coloured series correlation and oil and gas occurrence in them. It is shown that the Dagadzhiksky Formation in the east of the depression correspond to the lower part of pay zone in the west. The authors substantiate argillaceous-salt-bearing composition of the lower part of the red-coloured series. Absence of faults and argillaceous-salt-bearing composition prevent vertical migration of hydrocarbons from the Maikopsky oil and gas source formation and formation of accumulations in the red-coloured series within the Turkmenian shelf. Therefore, the authors concluded that there is a high probability of oil and gas discovery in the Turkmenian Bench below the argillaceous- salt-bearing bedset of red coloured series, i.e. in its basal part. In this case, depth of exploratory wells should exceed the depth of Pliocene Bottom (6,5–7,5 km).