Karachaganak Field - Geology

Geology

The Karachaganak Field is one of the world's largest gas condensate fields. It is located in the Pre-Caspian (or North Caspian) Basin, which extends from the southeastern margin of the Russian Platform down to the northern coast of the Caspian Sea, and includes the offshore Caspian Sea north of about 44° N. The North Caspian is a pericratonic depression of Late Proterozoic-Early Paleozoic age. Sediments in the 500,000-square-kilometre (190,000 sq mi) basin are up to 22 kilometres (14 mi) thick in places. The basin is subdivided into numerous zones by large salt domes, and the primary salt layer, the Permian Kungurian salt, separates strata vertically into subsalt and suprasalt layers. The basin is bounded to the east by the Hercynian Ural Mountains and to the southeast and south by other orogenic belts. In the north, the basin lies on the flank of the Voronezh Massif in the west and the Volga-Ural Platform in the north. Numerous oil and gas fields have been discovered in this region in addition to Karachaganak, such as Astrakhan, Tengiz and Zhanazhol Fields.

The field consists of a heterogeneous carbonate massif with strata from three geologic periods and numerous stages of deposition during these periods. The following have been identified:

• Permian: Kungurian, Artinskian, Sakmarian, Asselian
• Carboniferous: Serpukhovian, Visean, Tournaisian
• Devonian: Famennian

The depositional setting of the field is also varied. On the basis of core sample analysis and seismic studies the following depositional settings have been identified: limestone, talus, normal marine, shallow marine, inner reef lagoon, reef core, relatively deep water, slope, and anhydrite.

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 fileds 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.

Likewise, the large variation of deposition has led to four different types of carbonate cores in the structure: biothermal, biomorphic detrital, organo-clastic, and biochemogenic. Of these biomorphic detrital are the most common followed by biothermal rock types. However, estimates of their volumes range from 30-90% for the former and 10-60% for the latter.