An uncontrolled flow of reservoir fluids into the wellbore, and sometimes catastrophically to the surface. A blowout may consist of salt water, oil, gas or a mixture of these. Blowouts occur in all types of exploration and production operations, not just during drilling operations. If reservoir fluids flow into another formation and do not flow to the surface, the result is called an underground blowout. A blowout can occur when formation pressure exceeds the pressure applied to it by the column of drilling fluid.

Most blow-outs are due to gas, not to oil, but it is the oil associated in the gas that causes the pollution. The gas will just be emitted unseen into the air. In a drilling for natural gas, one needs to remember that gas is rarely found without oil