Sedimentation in a discharge dominated fluvial-lacustrine system: the Neogene Productive Series of the South Caspian Basin, Azerbaijan

The largely lower Pliocene Productive Series and its regional equivalents contain the major hydrocarbon reservoirs of the South Caspian Basin. Examination of outcrops in the Apsheron region, Azerbaijan, has resulted in re-interpretation of the depositional environments of the Productive Series. The lower part of the Productive Series consists of sandstones and mudstones interpreted as channelised and sheetflood fluvial deposits, intercalated with mudstones that represent short-lived lacustrine transgressions of a regional Caspian lake. The upper Productive Series (Pereriva and younger suites) comprises sandstone-prone intervals, interpreted as deposition during periods of increased fluvial discharge and sediment supply, and mudstone-prone intervals, interpreted as deposition during periods of decreased discharge and therefore coarse-grained sediment starvation. Sand-prone reservoir intervals in the Pereriva and overlying Balakhany suites mainly comprise amalgamated low sinuosity, braided fluvial sheet sandstones. Mudstone-rich intervals form inter- and intra -suite seals, comprising alluvial plain and lacustrine facies. The succession displays an overall fining-up trend, which is controlled by long-term climatic cooling, decreased fluvial discharge and reduced coarse clastic input. The modern Volga/Caspian system is a partial analogue for the lower Productive Series in the Apsheron region, where fluvial and lacustrine conditions alternate on a low-gradient ramp, without evidence for a shelf-break. Lake Eyre, Australia, may be a modern analogue for the upper Productive Series, where there are fewer lacustrine influences discernible in the sedimentology and the overall setting resembles a dryland river/terminal fan.