Architecture variability in the Pereriva and Balakhany suites of the Neogene Productive Series, Azerbaijan: implications for reservoir quality

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 siltstone 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.