Overview of Prograding Slope Deposits Impact on Reservoir Dynamic Behaviour, Karachaganak Field, Kazakhstan
Authors: Cozzi Mauro , Jenkins Steve , Kassenov Alibek , Yakhiyayev Nursultan , Mutaliyev Galymzhan 
Published: October 2017

Karachaganak Field contains hydrocarbon reservoirs of Carboniferous age with depositional patterns which are important for hydrocarbon recovery. The current phase of development drilling activity at this giant carbonate field is focused in reservoirs from a slope depositional environment having distinct clinoform progradational geometry. Production history has shown that the geometry and reservoir characteristics of these deposits significantly impact hydrocarbon recovery. Development wells are designed and completed to maximize recovery from this progradational system. The current phase of development at Karachaganak consists of drilling sub-horizontal wells to access reserves on the flanks of the carbonate build-up.

Clinoforms have been previously recognized at Karachaganak, and we have integrated new subsurface data to gain insights into the nature of these slope deposits. A recent 3D pre-stack depth migration (PSDM) of seismic data has been integrated with new log and dynamic data to provide an improved image of the internal geometry of the reservoir. Wireline logs and core data from new wells has been integrated with production logging and well pressure data to reveal the presence of distinct pressure compartments separated by tight intervals or baffles. A dedicated campaign of pressure data acquisition in sub-horizontal wells has been ongoing since 2015 to improve the geological understanding and unravel the dynamic behavior of Karachaganak prograding slope deposits. Continuous core has been collected in two recent Karachaganak wells to gain insights into the reservoir facies and slope depositional/diagenetic processes.

Integration of seismic, core, and wireline data along with dynamic data has improved the characterization of the Karachaganak reservoir architecture and has provided a sound basis for field development decisions. Pressure data in wells show the presence of partially connected geological compartments (geo-bodies) in the prograding Carboniferous deposits. These geo-bodies contain slope deposits with volumes of redeposited microbial boundstone breccia and in-situ boundstone. They are separated by tight intervals which create partial pressure barriers during field development. Tight and cemented intervals are formed during a depositional hiatus of the prograding system, and are sometimes associated with the presence of grainy material derived from the platform top. The geometry of the cemented zone is an inclined surface which follows the slope of the progradational depositional system. This sloping depositional surface is referred to as a clinoform surface.

Understanding the geometry of prograding deposits and the associated diagenetic alteration at Karachaganak has led to improved understanding of the reservoir connectivity. Clinoform surfaces have been mapped and related pressure baffles have been included in the current reservoir model. These pressure baffles have improved the calibration and "history match" process and have improved the reliability of predictions from the dynamic model. The development strategy has been optimized with 1200 meter long sub-horizontal wells oriented perpendicular to the paleo-slope direction of the carbonate build-up, allowing the well to access multiple clinoform compartments for improved hydrocarbon recovery. Karachaganak sub-horizontal wells use a multistage completion for selective stimulation. Karachaganak Petroleum Operating b.v. (KPO b.v.) joint-venture is acquiring new reservoir data to provide further insights into the complex dynamic behavior of the field and further optimize reservoir management activities.

World-wide many carbonate reservoirs have known or suspected prograding geometry. These may benefit from the reservoir characterization and field development experience matured in Karachaganak Field. The Permian Capitan Reef of Texas and New Mexico, USA, the Triassic Dolomites in the Southern Alps of Italy, and the Devonian reefs of the Canning Basin in Northern Western Australia represent examples of outcrop analogues in terms of sedimentology and depositional geometries.