How is Petroleum Formed?

Oil and natural gas were formed from the remains of prehistoric plants and animals—that’s why they’re called fossil fuels. Hundreds of millions of years ago, prehistoric plant and animal remains settled into the seas along with sand, silt and rocks. As the rocks and silt settled, layer upon layer piled up in rivers, along coastlines and on the sea bottom trapping the organic material. Without air, the organic layers could not rot away. Over time, increasing pressure and temperature changed the mud, sand and silt into rock (known as source rock) and slowly “cooked” the organic matter into petroleum. Petroleum is held inside the rock formation, similar to how a sponge holds water.

Over millions of years, the oil and gas that formed in the source rock deep within the Earth moved upward through tiny, connected pore spaces in the rocks. Some seeped out at the Earth’s surface, but most of the petroleum hydrocarbons were trapped by nonporous rocks or other barriers. These underground traps of oil and gas are called reservoirs. Contrary to popular misconception, reservoirs are not underground “lakes” of oil; they are made up of porous and permeable rocks that can hold significant amounts of oil and gas within their pore spaces. Some reservoirs are hundreds of feet below the surface, while others are thousands of feet underground.


Credits: energy4me.org