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Tengiz is an isolated Paleozoic carbonate build-up located in the Pricaspian Basin, Republic of Kazakhstan. The reservoir contains over 26 Billion Barrels OOIP and is one of the world’s deepest supergiant fields. The introduction of petrophysical rock types (PRT) and pore type classification has significantly improved oil in place estimation and reservoir characterization of this complex carbonate field. The Tengiz platform is a cyclic alternation of grainstone and packstone with thin volcanic ash layers and later strongly modified by diagenesis. The Tengiz flank areas are dominated by microbial boundstone deposition in an upper slope environment and detrital sediment in the lower slope. Carbonate facies in the Tengiz flanks are generally tight with bitumen cement and are dissected by fracture systems which act as fluid conduits. The classification of pore types and development of algorithms to predict pore types are based on a data set of more than 550 mercury injection pressure measurements. The Tengiz pore classification is scale-driven and has a direct impact on permeability and flow performance. The character and distribution of pore geometries in Tengiz buildup is determined by a variety of depositional and diagenetic processes, including early differential cementation, mechanical compaction and failure, and phases of dissolution and calcite and bitumen cementation. PRTs were developed to characterize various depositional and diagenetic regions of the Tengiz Field. Several distinct groupings of petrophysical rock types are recognized in the Tengiz platform and flank. These groupings include Volcanic Dominated, Bitumen Rich, Tight/Cemented, and reservoir facies of varying quality. A set of SRT (Slope Rock Types) were developed to improve characterization of the Tengiz slope facies. The slope largely consists of tight, algal boundstone and breccia facies which have undergone extensive corrosive diagenesis. Permeability is significantly enhanced in the Tengiz slope through these diagenetic processes. In 2008, a new reservoir model was constructed for Tengiz which implements the concepts of PRT and SRT to improve characterization of pore sizes and pore throats across the field. These distributions of pore size and pore throat geometry are a primary control on oil recovery in carbonate reservoirs. Improved characterization of pore geometry and key reservoir properties in the Tengiz Field has been achieved using petrophysical rock types. The introduction of PRT and SRT at Tengiz has contributed to improved estimates of volumes of oil in place and improved reservoir management. |
