Abstract The Pereriva and Balakhany suites of the mainly Pliocene Productive Series are the major reservoir units in the Azerbaijan sector of the South Caspian basin. Facies distribution throughout this succession is interpreted as representing an evolving fluvial system, from one of low sinuosity with highly amalgamated, relatively coarse-grained facies (Pereriva Suite) to one of increased sinuosity, with a lower degree of amalgamation, and relatively fine-grained facies (Balakhany Suite). Four models characterize the architecture and heterogeneity of these strata, with variations related to changing accommodation space/sediment supply ( A/S ) ratio. The lower 55 m (180 ft) of the Pereriva Suite represents the least heterogeneous part of the succession. Well-sorted, sheet sandstones are divided by the laterally continuous erosive horizons of alluvial degradational phases (low A/S ratio). Few permeability barriers to fluid flow exist. Qualitatively, this is the best part of the studied succession for reservoir properties. The upper 50 m (164 ft) of the Pereriva Suite is similar, but erosive lags form laterally discontinuous mud intraclast horizons. These horizons, and localized mudstone and siltstone facies, represent potential baffles and barriers to fluid flow. Most of the lower 70 m (229 ft) of the Balakhany Suite displays low heterogeneity, especially above and below a central interval of amalgamated erosion surfaces. The overlying 80 m (262 ft) of the Balakhany Suite represents the highest A/S ratio conditions of the studied succession. Reservoir heterogeneity is potentially created by contorted sandstones and by the preservation of the finer grained parts of channel fills. Laterally extensive mudstone and silt-stone horizons form potential barriers to fluid flow. Speculatively, the changes in architecture are controlled by climatic fluctuations on several scales, acting on a basin subject to increasing influence of the rising Greater Caucasus.

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 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 This chapter presents a comprehensive analysis of the nature and origin of the observed cyclic patterns in the late Miocene and Pliocene rocks that are the main hydrocarbon reservoirs along the Azerbaijan margin of the Caspian Sea. Data from extensive onshore studies of outcrop sed-imentology were combined with well-log interpretations from the offshore Azeri-Chirag-Deep Water Guneshli oil field to quantify the nested nature of depositional cycles inferred to represent 20 ka, 100 ka, and 400 ka astronomically driven climate cycles. Spectral analysis of gamma-ray logs supports the conclusion that much of the Productive Series in the South Caspian Basin (base KS to top of the Balakhany) records the controlling influence of astronomical changes in insolation that acted in phase across both the Volga drainage basin and the Caspian Sea. We constructed an idealized depositional sequence and its link to the lake level/climate drivers based on evidence for cyclic sedimentation from spectral analysis of the gamma-ray logs, the range of depositional systems interpreted, and the climate signal derived from palynology. These sequences are expressed consistently in strata deposited in fluvial, lake-margin mudflats, shoreline, and lake center settings. In the sandier stratigraphic intervals, the 20 k.y. sequence is expressed as follows. The sequence boundary is an exposure surface within mud-stones. Overlying the sequence boundary is generally a forestepping succession of terminal splay sandstones and mudstones suggesting slowly rising lake level. Above this, a stack of braided stream deposits is present that generally represents the dominant sandstone interval of the entire sequence. We interpret this as a lowstand systems tract (LST). The LST is abruptly truncated by a lacustrine flooding surface, which in turn is capped by a back stepping succession of more terminal splay deposits and density underflow strata. In most sequences, no definite expression of the highstand and falling stage systems tracts exists. This contrasts greatly with shallow marine depositional sequences, where the falling stage systems tract generally contains the best-developed sandstones. The recognition that climate drivers of astronomical origin did fundamentally control sedimentation in the Caspian Sea profoundly affects both petroleum systems modeling and reservoir modeling by reducing the degrees of uncertainty compared with what is commonly the case in other less ordered depositional systems.

Abstract The Azeri, Chirag, and Gunashli (ACG) fields, located offshore Baku in the Caspian Sea, are being developed under a 30 yr Production Sharing Agreement. Contiguous with the ACG accumulation, the shallow-water Gunashli (SWG) field has produced in excess of 800 MMSTB since 1980 from more than 220 wells. SWG provides an opportunity to study the static and dynamic behavior of reservoirs important to the development of ACG. BP, operator of ACG, contracted with the Reservoir Modeling Centre of the State Oil Company of Azerbaijan to perform a study of the Balakhany IX and Balakhany X reservoirs in the SWG development. This study comprised construction of geologic descriptions for both reservoirs and their use in dynamic simulation models. This was achieved using hand-contoured geologic mapping, incorporating depositional trends such as channel architecture. Matching historical performance data from these reservoirs verified the underlying geologic models and provided confidence in predictions derived from them. Results of this study show that channel width was the key to accurate estimation of stock tank oil initially in place, and that to achieve this, as many wells as possible must be incorporated. In addition, vertical communication between sublayers of the reservoir was defined using geologically derived maps referred to as “interface transmissibility.” Thus, a set of deterministic maps based on well data provided an approximate three-dimensional (3-D) representation of the Balakhany IX and Balakhany X stacked channel architecture. This work will assist in the planning and optimization for the development of the ACG field.