Sedimentology and depositional setting of the Neogene productive series, Aspheron peninsula, Azerbaijan
The largely Pliocene productive Series and its regional equivalents form the major hydrocarbon reservoir unit of the South Caspian Basin. Examination of outcrops and subsurface data from across the Apsheron Peninsula, Azerbaijan, results in the re-interpretation of component depositional environments. Previous studies over-emphasised the role of deltaic processes and lake-level fluctuation in shaping the straitographic architecture of the succession. The repeated juxtaposition of reservoir sandstone and sealing mudstone instead represents the expansion and contraction of terminal fluvial systems primarily due to climatic control of discharge and sediment supply. Arid phase climates result in coarse-grained sediment starvation, lacustrine contraction, and deposition of low net to gross portions of the succession in alluvial and alluvial/lacustrine plain settings. During humid climatic phases, discharge and coarse-grained sediment supply increase, resulting in expansion of high net’to gross braided fluvial systems.
The fining-upward trend of the Productive Series is viewed as representing a long-term trend towards increasing aridity, which resulted in an extended period of coarse-grained sediment starvation with deposition in alluvial and alluvial/lacustrine plain settings, it does not represent regional transgression. The continued tectonic evolution of the Greater Causcasus is suggested as a cause of increasing aridity by the early establishment of a rain shadow over the region. The mountain belt also provided a source of sediment via the tributaries of the palaeo-Volga and an increasing structural control on the position of the tributaries themselves. Previous interpretations underestimated the role played by tectonic controls on Productive Series depositional systems. The revised depositional models present implications for hydrocarbon exploration by predicting the gradual down-dip termination of reservoir facies. Subsurface data suggest only the Pereriva Suite and Balakhnay Subunit VIII remain sand-prone in more basinal areas. Reservoir intervals also display considerable strike-oriented facies variability. Risk on reservoir sandstone distribution is thus increased.