Late Visean to Bashkirian Platform Cyclicity in the Central Tengiz Buildup, Precaspian Basin, Kazakhstan<subtitle>Depositional Evolution and Reservoir Development</subtitle>

Abstract

The Tengiz buildup, an intensely cored and studied isolated carbonate platform in the Precaspian Basin, contains a succession of shallow-water deposits ranging from Famennian to Bashkirian in age. From a reservoir perspective, Tengiz can be subdivided into platform (central and outer) and rim-slope (flank) regions. The upper Visean, Serpukhovian, and Bashkirian form the main hydrocarbon-bearing interval in the platform. Depositional cycles (high-frequency sequences) in this interval are several to tens of meters thick for the Visean and Serpukhovian, and decimeter to meter scale for the Bashkirian. Cycles are made up of a succession of lithofacies overlying a sharp base that locally shows erosion, calcretes, meteoric diagenesis, and other evidence for subaerial exposure. At the base of the succession, tight peloidal mudstone and ash beds are associated with sequence boundaries and are thought to reflect low-energy conditions developed in deeper platform areas at lowstand and during initial flooding. Above this, beds with in-situ articulated brachiopods signal initial open-marine but still low-energy conditions. Succeeding crinoid-dominated intervals represent maximum marine flooding and overlying skeletal-peloidal grainstones highstand shoaling phases. Visean and Serpukhovian cycles are generally easy to correlate from well to well over several kilometers distance. Volcanic ash beds are identified by gamma-ray spikes, and flooding intervals show as low-porosity zones. In contrast, Bashkirian cycles are thinner and incomplete, dominated by thin, peloidal mudstone intervals alternating with high-energy coated-grain and ooid grainstone, and are more difficult to correlate. High-frequency icehouse sea level fluctuations exposed the platform during each fall of sea level, and rapid flooding resulted in incomplete cycles and complex lateral facies changes that may explain relatively poor lateral continuity of log character. The distribution of reservoir rock types in the central platform is determined by burial diagenetic modification of an earlier reservoir system that includes meteoric alteration and porosity enhancement below major sequence boundaries and reduced dissolution along higher order sequence boundaries associated with the presence of volcanic ash. The lateral continuity of tight layers at sequence boundaries probably greatly affected later fluid flow as well as the ultimate distribution of cements, dissolution, and bitumen in the central platform reservoir. The burial diagenetic overprint included two major phases of reservoir modification. First, a corrosion and cementation phase significantly enhanced existing matrix porosity in the interior central platform while reducing porosity in the exterior central and outer platform by pore-filling equant calcite cement. This was followed by bitumen emplacement and associated corrosion. These processes not only exerted an overall porosity-reducing effect prior to and associated with bitumen invasion toward the exterior central platform, but also dampened or flattened the initial cyclic porosity variations and obscured relationships between pore types and permeability. The bitumen overprint is nearly absent in the innermost platform wells; bitumen concentrations are highest near the bases of the cycles, which may imply that the first fill of hydrocarbons migrated through the flanks laterally into the platform cycles.