Overview of Depositional and Diagenetic Impacts on Reservoir Characterization, Karachaganak Field, Kazakhstan

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Abstract Karachaganak Field is a giant gas-condensate reservoir located in north-western Kazakhstan. The reservoir is composed of an isolated carbonate build-up of Carboniferous - Early Permian age, located on the northern edge of the Pri-Caspian Basin. The reservoir is overlain by three thick salt domes which were formed by the movement of an evaporitic sequence deposited at the end of Early-Permian. This salt tectonics has locally affected the seismic image of the reservoir and consequently makes seismic interpretation challenging. A high fold and wide-azimuth 3D seismic volume has been acquired over Karachaganak. Preliminary migrations of this new seismic volume have improved resolution of the reservoir geometry and facies. The Karachaganak carbonate reservoir is characterized by both slope and platform depositional systems, and the reservoir has undergone significant diagenetic alteration. Diagenesis has altered the original depositional rock fabric, pore and pore throat geometry. Critical reservoir properties such as porosity, permeability and irreducible water saturation at Karachaganak are influenced by of both depositional and diagenetic processes. The degree of diagenetic alteration of Karachaganak is greater than some other analogous carbonate reservoirs in the Pri-Caspian basin. This paper discusses the current reservoir model for the post-Tula Carboniferous deposits, which is based on a synthesis of understanding of the depositional and diagenetic processes at Karachaganak. The current reservoir model incorporates the results from recently drilled wells, recent interpretations of 3D seismic data and well tests. The resulting dynamic model has a good history match to observed well performance, and provides a basis for field development decisions. Karachaganak Petroleum Operating (KPO) joint-venture is conducting additional studies of the field deposition and diagenesis to provide new insights into the complex geological history of the field to improve reservoir characterization and reservoir management activities.