What is Crude Oil

Crude oil 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.

Until the late 19th century, an oil find often was met with disinterest or dismay. Pioneers who settled the American West dug wells to find water or brine, a source of salt; they were disappointed when they struck oil.

Several historical factors changed that. The kerosene lamp, invented in 1854, ultimately created the first large-scale demand for petroleum. (Kerosene first was made from coal, but by the late 1880s most was derived from crude oil.) In 1859, at Titusville, Penn., Col. Edwin Drake drilled the first successful well through rock and produced crude oil. What some called "Drake's Folly" was the birth of the modern petroleum industry. He sold his "black gold" for $20 a barrel.

Petroleum was prized mostly for its yield of kerosene until the turn of the century. Gasoline was burned off, and bitumen and asphalt (the heavier parts of crude oil) were discarded. But gradually rising in importance were the incandescent light and the internal combustion engine. The former relied on oil-fired generating plants; the latter, on gasoline.

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.

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.

Although various types of hydrocarbons - molecules made of hydrogen and carbon atoms - form the basis of all petroleum, they differ in their configurations. The carbon atoms may be linked in a ring or a chain, each with a full or partial complement of hydrogen atoms. Some hydrocarbons combine easily with other materials, and some resist such bonding.

There are three essentials in the creation of a crude oil field:

First, a "source rock" whose geologic history allowed the formation of crude oil. This usually is a fine-grained shale rich in organic matter.

Second, migration of the oil from the source rock to a "reservoir rock," usually a sandstone or limestone that's thick and porous enough to hold a sizable accumulation of oil. A reservoir rock that's only a few feet thick may be commercially producible if it's at a relatively shallow depth and near other fields. However, to warrant the cost of producing in more challenging regions (the Arctic North Slope, for example) the reservoir may have to be several hundred feet thick.

Third, entrapment. The earth is constantly creating irregular geologic structures through both sudden and gradual movements - earthquakes, volcanic eruptions and erosion caused by wind and water. Uplifted rock, for example, can result in domelike structures or arched folds called anticlines. These often serve as receptacles for hydrocarbons. The probability of discovering oil is greatest when such structures are formed near a source rock. In addition, an overlying, impermeable rock must be present to seal the migrating oil in the structure.