Petroleum or crude oil is a substance made from hydrocarbons—chains of hydrogen and carbon atoms— that provide energy and can be made into a variety of petrochemicals. Much of the world’s oil began as organic material from ancient seas, wetlands and forests that was buried under sediments faster than it could decay.At depths between 7,650 and 15,000 feet, heat and pressure slowly cooked it into oil. The oil then collected in porous sandstone or limestone with impermeable cap such as shale or salt that kept it from escaping. Oil that does escape is often downgraded into tar by bacteria and groundwater. Oil is a finite resource. A considerable amount of it has already been used up and no one knows for sure how much is left. The consensus is that there is about 500 billion barrels of difficult to find oil still out there that has yet to be discovered—perhaps half of that is below the oceans.