4.3 Petroleum Geology: How Does Oil Form? Formation of Oil and Natural Gas

Petroleum and natural gas are found in similar environments and frequently occur together. These hydrocarbons consist of various compounds consisting of hydrogen and carbon, mixed together. They may also contain small quantities of other elements, such as sulfur, nitrogen, and oxygen. Like coal, petroleum and natural gas are biological products derived from the remains of organisms. However, the environments in which they form are very different, as are the organisms.

Coal is formed mostly from plant material that accumulated in a swampy environment above sea level. Oil and gas are derived from the remains of both plants and animals having a marine origin.

Petroleum formation is complex and not completely understood. Nonetheless, we know that it begins with the accumulation of sediment in ocean areas that are rich in plant and animal remains. These accumulations must occur where biological activity is high, such as in near shore areas.

Most marine environments are oxygen rich, which leads to the decay of organic remains before they can be buried by other sediments. Therefore, accumulations of oil and gas are not as widespread as are the marine environment that support abundant biological activity. This limiting factor notwithstanding, large quantities of organic matter are buried and protected from oxidation in many offshore sedimentary basins. With increasing burial over millions of years, chemical reactions gradually transform some of the original organic matter into the liquid and gaseous hydrocarbons we call petroleum and natural gas.

Unlike the organic matter from which they formed, the newly created petroleum and natural gas are mobile. These fluids are gradually squeezed from the compacting, mud-rich layers where they originated into adjacent porous and permeable beds such as sandstone, where openings between sediment grains are larger. Because this occurs under water, the rock layers containing the oil and gas are saturated with water. But oil and gas are less dense than water, so they migrate upward through the water filled pore spaces of the enclosing rocks. Unless something blocks or traps this upward migration, the fluids will eventually reach the surface, at which point the volatile components will evaporate.

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