LATE VISEAN TO BASHKIRIAN PLATFORM CYCLICITY IN THE CENTRAL TENGIZ BUILDUP, PRECASPIAN BASIN, KAZAKHSTAN: DEPOSITIONAL EVOLUTION AND RESERVOIR DEVELOPMENT

Abstract

he 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