Petroleum - A broad term which includes both crude oil and natural gas

Crude Oil - a liquid mixture of naturally occurring hydrocarbons (compounds containing carbon and oxygen).

Natural Gas - A gaseous mixture of naturally occurring hydrocarbons.

The Origin of Oil and Gas - Oil and natural gas originate from organic matter in marine sediment. Microscopic organisms such as single-celled algae settle to the sea floor and accumulate in marine mud. This organic matter may partially decompose, using up the dissolved oxygen in the sediment. As soon as the oxygen is used up, decay stops and the remaining organic matter is preserved. Continued burial subjects the organic matter to higher temperatures and pressures, which cause physical and chemical changes in the organic compounds. These changes produce tiny droplets of oil and natural gas. As muddy sediments compact, the oil and gas are squeezed out and move into more porous and permeable sandy layers.

Anaerobic - conditions under which oxygen is not present. This is the condition necessary for the formation of oil and gas. Such conditions are found in a highly restrictive environment such as in a closed basin like the bottom of the Black Sea.

Aerobic - conditions under which oxygen is present to oxidize organic material and sulfur.

Occurrence of Oil and Gas - four conditions or requirements are necessary in order for large accumulations of oil and gas to form. The requirements are:

Source Rock - A shale must contain organic material which has not been oxidized and which is converted to petroleum by burial pressures and temperatures.

Reservoir Rock - A sandstone or other relatively porous rock is required to store and transmit the petroleum when wells are drilled into the reservoir.

A Trap - a set of conditions to hold the petroleum in a reservoir and prevent its escape by migration.

Deep enough burial but not too deep - Temperatures and Pressures are required to "cook" the petroleum out of the organic matter. Of course, if the rock gets too hot the petroleum is all "burned" off.