The variation of deposition is due to the long period over which the Karachaganak structure was formed. From the Late Devonian to the middle Carboniferous the field was an atoll, over which in the Early Permian a system of reefs were built. At its greatest the reservoir is 1.5 km thick. This type of reservoir structure has been seen in analogous fields including Kenkiyak, Zhanazhol, Tengiz, and possibly Astrakhan fields. A west-east cross section through Karachaganak resembles the twin humps of a camel, indicating two separate reef highs.