Natural gas and oil occur in places in the Earth's crust that share the same three basic requirements: source rock, a permeable reservoir and a geologic trap.

The process of creating combustible elements takes thousands of years and begins when dead algae, plankton or other organic plant and animal material sink to the bottom of a body of water, such as a deep sea bed or coastal lagoon. Over the years, sediment covers the material, forming thick deposits that eventually turn into stone. High heat and pressure transform the ancient organisms in the source rock into oil and natural gas.

Oil and natural gas are made up of the elements hydrogen and carbon. Different kinds of hydrocarbon molecules make different kinds of fuel. The more heat the material is exposed to, the smaller the molecules, the lighter the hydrocarbons are. Asphalt is the heavy end of the spectrum, natural gas at the light end.

Because oil and gas are lighter than the saline water found in surrounding rocks, they flow upward. To make an economic deposit, the hydrocarbons must be contained in an underground reservoir of porous rock, usually sandstone or limestone.

If the upward path of oil and gas is not blocked, it continues to flow to the surface, and the volatile hydrocarbons evaporate. This forms "seeps" at the surface. Famous examples of these are the La Brea Tar Pits in Southern California.

In the Middle East, King Nebuchadnezzar of Babylon had seeps mined so his builders could mix asphalt with stones to make roads.

The third requirement for an economic oil deposit is a geologic trap. This is some kind of impediment in the rock that prevents the hydrocarbons from rising to the surface. The first oil well in the United States, in the mid-19th century in Pennsylvania, was drilled into the top of an anticline, an arch of stratified rock that bends downward in opposite directions from the crest. The oil had been trapped at the top of a fold of rock, beneath an impervious layer.

Voids in buried fossil reefs, lava flows and faults — cracks in the rock along which there has been movement — can also trap oil.

The first oil deposits to be exploited were discovered when explorers found seeps at the surface. Later, drillers looked for anticlines in promising sedimentary rocks.

Today, geologists can search for oil using seismic or gravity tests. Seismic tests use small earthquake waves. These can be set off by dynamite or by mechanical "thumpers" towed by trucks. Waves travel at different speeds through different kinds of rock, are picked up by "geophones," and can tell geologists what the subsurface formations and structures are likely to be.

Studies of the magnetic properties of rocks, obtained in airplane flyovers, can give clues about subsurface occurrences.

The specific gravity of rocks also gives an idea as to what is happening under the surface. Sensitive gravity meters can predict the thickness of the sedimentary section.

When oil is removed from the ground, it contains many kinds of hydrocarbons mixed together. At the refinery, it undergoes a process called fractional distillation, where lighter molecules are separated from heavier ones. Similarly, gas is "stripped" to separate propane and methane.

Contact Robin Martin at rmartin@sfnewmexican.com.