Late Miocene and Early Pliocene High-Frequency Lake Level Cycles in Lacustrine Hydrocarbon Reservoir Strata, South Caspian Basin: Insights for Subseismic-scale Lithofacies Variations

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 sedimentology 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 mudstones. 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 backstepping 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.