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Where does petroleum come from?

The fossil fuels—petroleum, natural gas, and coal—are just that, fossils. Coal was formed from the remains of trees and other woody plants, covered by soil and then buried deeper and deeper and subjected to heat and pressure, which converted their remains to mostly carbon, but with a fair amount of other elements that were part of the plants and the surrounding soil (for more on this, see Chapter 3, "Coal"). Petroleum and natural gas are believed to be the fossil remains of marine organisms.

All fossil fuels that we take out of the ground today were produced eons ago from the growth of photosynthetic organisms—algae, certain bacteria, and green land plants, organisms that can convert the energy in sunlight into energy stored in organic compounds, and do so by removing carbon dioxide from the atmosphere and releasing pure oxygen. The energy that fossil fuels contain is thus a form of solar energy, in most cases provided over many millions of years and stored since then.

Over time, much of the carbon from the carbon dioxide that algae, green plants, and some bacteria removed from the atmosphere was then sequestered—stored in the soils, rocks, and marine deposits, and prevented by various physical and chemical processes from returning to the atmosphere. When fossil fuels are burned, the sequestered carbon is released into the atmosphere as carbon dioxide (CO2), which acts as one of the primary greenhouse gases.

Since petroleum and natural gas are not solids and thus are lighter than the rocks that surround them deep in the Earth (Figure 1.2), they tend to rise under pressure from the rocks and get trapped in geological pockets, like the one shown in Figure 1.3—although in some rarer situations the oil makes it to the surface, as it does in Southern California. Thus, the search for oil and gas is not random; petroleum geologists know which kinds of rock formations they are likely to occur in.

Figure 1.2

Figure 1.2 A typical location of oil and gas. Oil or gas rarely gets pushed right up to the surface, as it does at the La Brea pits in Los Angeles, famous for having trapped many ancient and extinct mammals whose fossils have become familiar.

(Source: D. B. Botkin, and E. A. Keller, Environmental Science: Earth as a Living Planet. New York, John Wiley, 2009)

Figure 1.3

Figure 1.3 A natural oil seep along the California shore at Santa Barbara. Pressure from surrounding rocks has pushed petroleum up to the surface, where it flows into the Pacific Ocean, revealing itself by its bright reflection of sunlight.

(Courtesy of the University of California, Santa Barbara, UCSB Map & Image Laboratory, from the research collection of Prof. Jack E. Estes)3

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