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 Su-rakhany 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 Ba-lakhany 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.

Abstract The Pliocene Productive Series of the South Caspian Basin records a major lowstand relative to pre-existing Miocene and subsequent Pleistocene and Holocene shelf margins. The Productive Series records approximate 2.6 m.y. of relatively continuous deposition of lacustrine sediments and reaches more than 6 km (0.62 mi) in thickness. Regional-scale seismic mapping has allowed an interpretation of the basin evolution during the deposition of the Productive Series. The Productive Series is subdivided into four phases of deposition. Differences between these phases are interpreted to reflect changes in the balance between sediment and water input and the evolving basin morphology. Basin morphology inherited from a previous depositional phase strongly influence successive phases of deposition. Phase 1. Messinian: Base-level fall, roughly coincident with the Messinian Salinity Crisis, resulted in isolation of the Caspian Sea from the global oceans. Reconstruction of the basin profile immediately after this event indicates that the South Caspian Basins base-level fell by approximately 1.5 km (0.93 mi). Phase 2. Integration of drainage systems to this dramatically lowered base level resulted in the delivery of large volumes of sediment and water from the Russian Platform, Caucasus mountains, and Kopet-Dagh mountains to the South Caspian Basin through the Paleo Volga, Kura, and Amu darya/Uzboy rivers. Within the South Caspian Basin, fluvial and fluvio-lacustrine facies were deposited in preexisting structural depressions, forming the Lower Productive Series. Phase 3. Infill of paleotopographic depressions by the Lower Productive Series resulted in the formation of a low gradient ramp over much of the South Caspian Basin. Middle Productive Series strata were deposited on this low-gradient ramp as aerially extensive fluvial, deltaic, and lacustrine facies. Reconstructed basin profiles show a reduction of the depositional gradient as the result of sediment infill. Phase 4. During deposition of the Upper Productive Series, the ratio of sediment supply to water supply decreased as much of the coarse-grained sediment supplied from the Paleo-Volga was deposited updip. Thus, the updip depositional profile became increasingly low relief. This change led to the creation of a relatively deep-water lake in the center of the South Caspian Basin surrounded by lacustrine shelf margins. Evaporites were deposited in the center of this lake, suggesting that it was subject to episodic phases of desiccation. Overlying the Productive Series is a regionally extensive marine condensed section, the Akchagyl Suite. The Akchagyl Suite records a major regional transgression and a return to marine conditions as the South Caspian Basin was reconnected to the global oceans. Pleis-tocene shelf margin complexes downlap onto the Akchagyl Suite and record progradation of shorelines into a deep-water brackish lake (~1000 m [3280 ft]). This Pleistocene depositional system was similar to the present-day depositional setting of the Caspian Sea.

Abstract Discuss alternative models for deposition. 1. Parasequence model. Deposits represent normal progradation and transgression of lacustrine deltas, interrupted by occasional base level fall and basinward shift in facies. Base level driven, and the vertical facies succession in Kirmaky Valley is a result of lateral shifts in facies belts. a. Kirmaky Suite - sublacustrine distributary channels in background of lacustrine mudstones. b. Lack of well developed paleosols. Only brief exposure periods. c. Interpreted distributary mouth bar facies in lower net to gross intervals in cores. d. Coarsening and shallowing up of the upper portion of Kirmaky Suite into overlying Nad Kirmaky Sandstone (NKP) sandstones. 2. Climatically driven lake model. Sandstones primarily represent humid phases and abundant sediment supply whereas mudstone intervals primarily represent lowered inputs of both sediment and water. Sediment and water supply driven, and much of the vertical facies succession results from different depositional environments associated with different climatic settings. a. Nad Kirmaky Shale (NKG) Suite — One of the most continuous mudstone intervals in basin—regional pressure seal, yet abundant mud cracks, and palynology indicates arid climate (terminal fan?). b. Shales within Fasila and Balakhany at Kir-maky Valley similar to NKG. c. Extensive flooding shales at top of Balakhany and Sabunchy in offshore contain thin evaporites. d. Laterally extensive sand sheets. e. Lack of large-scale progradational cycles on well logs or seismic. f. Apparent, near-horizontal time lines.

Abstract The Neogene Productive Series, the main reservoir unit of the prolific hydrocarbon province in the South Caspian Basin, and the modern Volga delta in the northern Caspian Sea, are deltas deposited by the same river into the same closed sea. Both deltas are low- gradient, mud-dominated, river-dominated, multichannel, ramp deltas without a shelf break, and show the impact of rapid changes in sea level, climate-driven discharge, and sediment input. But there are also prominent differences. The Productive Series forms the lowstand wedge of the most dramatic sea-level fall the Caspian has ever experienced. It consists of a succession, up to 7 km thick, of fluviodeltaic sediments, deposited at extremely high sedimentation rates (2-4 mm/y) by a paleo-Volga River in the narrow, rapidly subsiding South Caspian basin. Simultaneously, the paleo-Volga carved a canyon 2000 km long and up to 600 m deep far upstream into the Russian plain. The smaller Kura and Amu Darya rivers also contributed to the Productive Series. The sedimentary succession in the proximal part of the Productive Series shows the transition from an alternation of sheetflood sandstones and floodplain mudstones with great lateral continuity to finer-grained packages in which coarsening-upwards facies successions are common and there is evidence of repeated emergence and desiccation. A coarser-grained interval reflects increasing uplift in the adjacent Greater Caucasus mountains. The upsection increase in mud-dominated deposition in the Productive Series is thought to reflect a trend towards more humid climates. The modern Volga delta is not more than 20 m thick and has been deposited during the last 6000 years on a wide stable continental platform at a level halfway between a major Last Glacial highstand and a deep Early Holocene lowstand. It shows rapid lateral and vertical facies changes at the delta front, and it is characterized by many small radial sand bodies with low connectivity, coarsening-upwards mouthbar and levee deposits overlying clayey prodelta deposits, and fining-upwards channel fills. There is evidence of frequent emergence and submergence due to rapid sea-level changes. Average sedimentation rates are lower than in the Productive Series (0.7-1 mm/y in uncompacted muds). The reasons for the differences are threefold. Sedimentation in the Productive Series spanned two million years, but in the modern Volga delta less than 6,000 years, so the latter cannot be more than a partial analogue of the former. The outcropping sediments of the Productive Series were deposited in a more proximal position than the studied sediments of the modern Volga delta front, which may partly explain the difference in lateral continuity of the sandy successions. But above all the paleo-Volga shed its load in a narrow, rapidly subsiding basin, whereas the present Volga spreads its sediment across a wide and shallow stable continental platform. The differences in basin geometry and dynamics explain part of the differences in 3-D architecture and sedimentation rates.