Abstract: Forecast of Phase State and Composition of Hydrocarbon Systems of Subsalt Formations of the North Caspian Region

Nikolay N. Nemchenko, Alla Rovenskaya

The Pricaspian oil and gas province is a region where the increase in commercial hydrocarbon reserves is primarily due to the development of deep hydrocarbon reservoirs. At present, three giant fields have been discovered within this region, characterized by differing geology and hydrocarbon content -- oil (Tengiz), oil condensate (Karachaganak), and gas condensate (Astrakhan). The main hydrocarbon reservoir consists of Lower to Middle carboniferous carbonates (Astrakhan, Tengiz) and Lower Permian carbonates (Karachaganak). Seals are subsalt Lower Permian shales and carbonates and Kungurian salt-bearing sections. common characteristics include a high degree of productivity, high formation pressure and temperature, and complex composition of associated gas (H₂S, CO₂, nitrogen, C5+).

The high concentration of acid components, characteristic of the Pricaspian hydrocarbon system, strongly influences the phase transition parameters and the amount of gas and gas condensate due to high hydrocarbon solubility in these acid components. This also influences the pressure of the phase transition which is depressed (lower temperatures and pressures) and causes a relatively low condensate content.

In the Pricaspian oil and gas province, deep Paleozoic reservoirs will be the main exploration and development targets for the future. It is possible to identify different hydrocarbon types, controlled by the relative solubility of liquid and gaseous hydrocarbons under varying PVT conditions. They can be defined as "primary" and "secondary" gas condensate systems (the formation of the latter is connected with oil systems), with gradational phase transition boundaries.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France