Found 400 feet below the Earths surface, crude oil - as petroleum directly out of the ground is called - is a remarkably varied substance, both in its use and composition. It can be a straw-colored liquid or tar-black solid. Red, green and brown hues are not uncommon. By the 1920s, crude oil as an energy source - not just as a curiosity - came into its own. But to many, it's still as mysterious as it was to ancient man. Even in the petroleum industry, most people never see crude oil.

Geologists generally agree that crude oil was formed over millions of years from the remains of tiny aquatic plants and animals that lived in ancient seas. There may be bits of brontosaurus thrown in for good measure, but petroleum owes its existence largely to one-celled marine organisms. As these organisms died, they sank to the seabed. Usually buried with sand and mud, they formed an organic-rich layer that eventually turned to sedimentary rock. The process repeated itself, one layer covering another.

Then, over millions of years, the seas withdrew. In lakes and inland seas, a similar process took place with deposits formed of non-marine vegetation. In some cases, the deposits that formed sedimentary rock didn't contain enough oxygen to completely decompose the organic material. Bacteria broke down the trapped and preserved residue, molecule by molecule, into substances rich in hydrogen and carbon. Increased pressure and heat from the weight of the layers above then caused a partial distillation of the organic remnants, transforming them, ever so slowly, into crude oil and natural gas.

It is the particular crude oil's geologic history that is most important in determining its characteristics. Some crudes from Louisiana and Nigeria are similar because both were formed in similar marine deposits. In parts of the Far East, crude oil generally is waxy, black or brown, and low in sulfur. It is similar to crudes found in central Africa because both were formed from non-marine sources. In the Middle East, crude oil is black but less waxy and higher in sulfur. Crude oil from Western Australia can be a light, honey-colored liquid, while that from the North Sea typically is a waxy, greenish-liquid. Many kinds of crudes are found in the United States because there is great variety in the geologic history of its different regions.

1. For complete article: www.chevron.com/about/learning_center/crudeWHAT IS CRUDE OIL? QUESTIONS2. How many feet below the Earths surface is crude oil?

3. Crude oil was originally what kinds of material millions of years ago?

4. What happened to this material? Where did this material go? What kind of rock did it form?

5. What did the bacteria do to the organic material? What chemical compounds was the result?

6. How did this organic get transformed into crude oil and natural gas?

7. What is most important in determining the characteristics of crude oil?

8. Are all kinds of crude oils found in the United States the same? Why?

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