Miocene to Quaternary Sequence Stratigraphy of the South and Central Caspian Basins

Abstract

The oil industry has been active in Azerbaijan for centuries, and the Apsheron Peninsula, Apsheron sill, onshore, and the shelf margin of Azerbaijan are considered mature areas for exploration. However, large areas of the offshore Caspian, including the deep-water South Caspian, Turkmenistan shelf, and Central Caspian are still exploration frontiers. An understanding of the stratigraphy of reservoir rocks and seals in these areas could significantly reduce exploration risk. The interplay of the paleo-Volga, paleo-Amu Darya, and paleo-Kura deltas, since the late Miocene, provides the first-order controls on prospect distribution. A continuous trend of coastal onlap on the western margin exists for the South Caspian and in the Central Caspian basins from the upper Miocene to lower Pliocene, with onlap of these units over Miocene and Cretaceous rocks. These coastal onlap trends are associated with an overall rise in lake level from the lower to the upper productive series. The three delta systems exhibit significant differences in depositional style and timing, reacting in different ways to the rising lake level. Strong progradation of the paleo-Amu Darya delta occurred on the Turkmenistan shelf, on the eastern margin of the South Caspian Basin. This progradation is related to the Pliocene deposition of the Red series in Turkmenistan (equivalent to the Pereryva to Surakhany suites in Azerbaijan). Thus, the paleo-Amu Darya delta prograded during rising lake level, controlled primarily by sediment supply. During the deposition of the Pereryva suite, the paleo-Volga delta aggraded in the Apsheron region (northern margin of the South Caspian). A transgressive trend marks the Central Caspian Basin from the upper Balakhany to the Surakhany suites and may indicate backstepping of the paleo-Volga delta at that time. In the paleo-Kura system, on the southwest margin of the South Caspian Basin, a backstepping trend occurred during the deposition of the upper Balakhany and Sabunchi suites (lower Pliocene). A downlap surface developed at the base of the paleo-Kura delta in the middle Surakhany suite. This downlap surface along the western Caspian margin correlates to the upper part of the progradational phase of the paleo-Amu Darya delta on the eastern margin of the basin. A paleo-Kura prograding wedge developed concurrent with the deposition of the upper Surakhany and Akchagylian (upper Pliocene). The impact of sediment supply from the Alborz Mountains in Iran could not be evaluated because of lack of data. Climatic fluctuations did exert a dominant control on the style of sedimentation in the South Caspian Basin through their direct impact both on lake levels and on sediment supply. The entire Productive Series reflects the Pliocene golden climate, when the Earth, overall, was much warmer than today. In addition, on shorter time scales, the stratal pattern is controlled by high-frequency climatic cycles. Late lowstand deposits are dominated by aggradational braided streams and braid deltas. Transgressive and highstand deposits consist of extensive lake shales interbedded with silts and sands. The transgressive shales can act as pervasive seals and permeability barriers and baffles in the reservoirs. Very little sand appears to have entered the lake during periods of falling lake level.