The Middle East has a very large oil reserve mainly due to the special geological conditions formed hundreds of millions of years ago. Most of this region was once a shallow sea rich in organisms such as algae and microorganisms. When they died, their remains accumulated on the seabed and were buried by layers of mud, sand, and rock over a long period. Under high pressure and temperature deep within the earth, these organic materials gradually transformed into oil and natural gas. Additionally, the geological structure of the Middle East has many porous rock layers containing oil beneath impermeable rock layers, creating natural "oil traps." As a result, oil is retained and accumulated into very large fields.