Petroleum is a name for a broad group that includes many mixtures of liquid hydrocarbons. According to the U.S. Geological Survey, "[p]etroleum is a naturally occurring substance consisting of organic compounds." Petroleum can be found in semisolid, liquid, or gas form. Crude oil and products created from refined natural gas are included in this group.
Petroleum is formed in sedimentary basins by the organic matter left behind by receding oceans. In order for petroleum to form in these basins deceased plant or other microorganisms must be present. Over the earth's lifespan oceans have expanded and receded. As oceans and swamps receded they left water, plants and plankton behind in basins. These plants and plankton eventually decayed as the basins became more oxygen deficient, creating layers of sediment. Over time these sedimentary layers were compressed into rock, either coal, black shale, or bituminous limestone. Living in these rocks were microorganisms that consumed the decaying, organic, sediment, and produced methane from their consumption. At times this methane would be trapped under layers of impermeable sediment creating seals that in turn created methane reservoirs. These reservoirs are the natural gas pockets being mined today for energy production. An early example of this process can be seen in the picture to the right.