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 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.

Single-grain, laser-ablation inductively coupled plasma–mass spectrometry analyses of detrital apatites from Pliocene sandstones in the South Caspian Basin (Azerbaijan) and Devonian-Carboniferous sandstones from Clair oil field, west of Shetland (UK), demonstrate that apatite geochemistry has significant potential in provenance analysis. Apatites in Pliocene sandstones deposited by the paleo-Kura River system, which drained the Lesser Caucasus region, were derived largely from mafic to intermediate and alkaline rocks. Apatite populations in Pliocene sediments transported by the paleo-Volga River system, which drained the Russian Platform, show greater compositional diversity and indicate supply from granitoids or other acidic rocks together with subordinate mafic to intermediate and alkaline rocks. Apatites in the Devonian-Carboniferous succession west of Britain were derived predominantly from acidic rocks, either directly from Archean gneisses or indirectly from metasedimentary rocks. In the two case studies, the most useful discriminators of apatite provenance proved to be La/Nd and La + Ce/ΣREE. Since apatite is stable during burial in sedimentary basins, apatite geochemistry can be used to determine provenance of sandstones from the full range of diagenetic environments, although the instability of apatite during weathering means that the method will be difficult to apply to sandstones with prolonged weathering history. At present, identification of provenance using apatite geochemistry is limited by the lack of a comprehensive database on apatite compositions in some of the potential source rocks, particularly those of metamorphic origin. The role played by sediment recycling is another factor that requires consideration when reconstructing source areas on the basis of apatite compositions.

Abstract The Caspian Basin is one of the largest and most attractive petroleum basins in the world, even after more than 100 yr of production. Numerous rich oil and gas fields in the Caspian are present, and the potential for further exploration and development exists. However, the petroleum system in the northern part of the Apsheron anticlinorium is poorly understood. This study focuses on South Caspian reservoir components of the petroleum systems in the northern part of the postdepositional Apsheron Ridge. Two sets of three-dimensional (3-D) seismic data, acquired in the 1990s, were provided for study. The research focuses on the seismic expression of the Productive Series deposited in a lake or lacustrine environment. It was determined that tectonic control on sedimentation during deposition of the Middle Productive Series is the principal control on reservoir development. The documentation of relative lake level change is important because it impacts reservoir facies architecture and potential reservoir quality in the study area. It was further concluded that in some locations, seals may have been breached by reactivation of basement faults. Seismic attributes including amplitude and spectral decomposition were applied to the 3-D seismic data for interpretation. Valleys and channels that have been interpreted through these seismic attributes may represent feeder systems to downslope depositional lobes with better down-structure reservoir quality. High-amplitude anomalies off the structures are indicative of these depositional lobes. Lobes that are faulted and isolated in the flank areas of the structures may exhibit better seal capacity.

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.