Understanding the Akchagyl and Apsheron in the Greater Caspian Sea Region: new biostratigraphic insights

The Akchagylian (Akchagyl) and Apsheronian (Apsheron) Regional Stages are well known stratigraphic intervals that were widely deposited in and around the Greater Caspian Sea Region during the latest Pliocene and early Pleistocene. Both these stratigraphic units were deposited as a result of water levels in the Caspian Sea that were significantly higher than at the present time. The Akchagyl is important as it provides the regional seal overlying the oil and gas bearing Productive Series, whereas the Apsheron forms a major portion of the ‘overburden’ sediments in this over-pressured petroleum system. Detailed biostratigraphic investigations reveal that both the Akchagyl and Apsheron units were deposited primarily under brackish rather than fully marine conditions. However, marine influences are widely evident in the lower part of the Akchagyl, marked by the presence of dinoflagellate cysts and small cassidulinid foraminifera which typically occur in deep and/or cold marine waters. The marine microfossils are likely to have originated from the Arctic Ocean, and not the Black Sea or Mediterranean Sea as is widely believed. Microfossils with affinity to Central Paratethys (Pannonian and Dacian basins) occur sporadically in the Akchagyl and frequently in the Apsheron intervals. Accurate dating and subdivision of the Akchagyl and Apsheron units is possible using a combination of Ar/Ar dating of ash layers, magnetostratigraphy and biostratigraphy. This is demonstrated in outcrop, core and well studies from Azerbaijan and Russia.