Associated petroleum gas is gas dissolved in oil under in-situ conditions; it is recovered during oil deposits development as a result of the formation pressure decrease to a point lower than oil bubble-point pressure. Petroleum gas content in crude oil (gas factor) ranges from 3–5 cubic meters per ton in the shallowest horizons to 200–250 and more in deep formations in case of fine survival of deposits. The associated petroleum gas composition depends on the composition of oil, in which it is dissolved, the deposits occurrence and accumulation conditions determining stability of natural oil and gas systems, and the possibility of their natural degassing.

According to their composition, petroleum gases are divided into:

• essentially hydrocarbon gases (95 to 100 per cent of hydrocarbons);
• hydrocarbon gases with carbon dioxide content (4 to 20 per cent of carbon dioxide);
• hydrocarbon gases with nitrogen content (3 to 15 per cent of nitrogen);
• hydrocarbon-nitrogen gases (up to 50 per cent of nitrogen).