HIGH FREQUENCY CYCLICITY OF SEDIMENTATION IN THE SOUTH CASPIAN EARLY PLIOCENE: IMPLICATION TO PRODUCTIVE SERIES RESERVOIRS ARCHITECTURE

E. Aliyeva¹, D. Nummedal², D. Huseynov¹, G. Riley³, J. Friedmann^4
1 Geological Institute of Azerbaijan National Academy of Sciences, Azerbaijan
² Institute of Energy Research, University of Wyoming, USA
³ BP Azerbaijan Resource Development Team, UK
⁴ University of Maryland, USA

The Caspian Sea being the unique largest isolated basin in the world with hyper unstable sea level regime represents the beautiful model area for studying of dynamics of sedimentary basins' development in short -term intervals. Large rate of sedimentation reached to 2,5 mm/per year and high frequency sea level fluctuations played a decisive role in formation of Early Pliocene Productive Series (PS) consisting of rhythmically bedded fluvial-deltaic sediments deposited in the isolated South Caspian Sea by several large river systems and containing about 90% hydrocarbon accumulations in the South Caspian basin.

Involving of detailed sedimentological and geochemical data set (¹⁸O/¹⁶O, ¹³C/¹²C, Sr/Ba, Ca/Mg ratios in microfauna and ³⁴S/³² S in sediments) from PS outcrop along a pipeline trench crossing the Kirmaki valley (Apsheron peninsula) provided us by unique opportunity for characterizations of short-term cyclicity in Early Pliocene. Tens of climatic controlled high order depositional cycles occurred during Productive Series sedimentation.

Within the established cycles one can distinguish the different types of paleosettings, which were formed as a result of high-frequency sea level fluctuations and changes in sediment supply - from fluvial to deltaic and lacustrine environment.

Sea level fall was accompanied by progradation of PaleoVolga delta and development in Apsheron peninsula and Baku archipelago the braided fluvial system with accumulation of stacked good laterally and vertically connected sand bodies. Probably, those stages of basin evolution were characterized by the largest rate of sedimentation led to formation of thick sand beds representing the good reservoirs. The following then sea level rise led to retrogradation of delta and facial shifts to more distal delta front –lacustrine environment characterized by sedimentation of laterally extended mudstones representing the seals. Sometimes during sea high stand highly sediment loaded flow provided the accumulation of sheet like laterally extended, but vertically restricted sand bodies in delta front successions (Kirmaki suite).

The results of conducted research are an important for understanding of climatically forced sedimentary model and stratigraphic architecture of depositional successions and hydrocarbon reservoirs in closed basins.