Oil and gas are formed from organic material mainly deposited as sediments on the seabed and then broken down and transformed over millions of years. If there is a suitable combination of source rock, reservoir rock, cap rock and a trap in an area, recoverable oil and gas deposits may be discovered there.Oil refineries are major polluters, consuming large amounts of energy and water, producing large quantities of wastewaters, releasing hazardous gases into the atmosphere and generating solid waste that are difficult both to treat and to dispose of.