Gas cap – accumulation of free gas in the most raised part of oil-bearing reservoir above the oil deposit. The gas from the cap is connected with oil – both dimensionally and genetically. Heavy hydrocarbons content in gas gap significantly exceeds their content in gas deposits; their total amount may reach 35–40 per cent. With the purpose to preserve the gas deposit gas from the gas cap is withdrawn, usually it is done after oil is recovered. In the process of recovering oil from the deposit the gas gap expands, which contributes to oil displacement. Gas caps also may be formed in the process of developing the upper layer of oil reservoir due to emission of gas dissolved in oil when reservoir pressure is decreased significantly.