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Alexander Marshall, age 13, of E1 Paso, Tex., for his question:


Oil, which is used to help make the wheels of transportation and industry turn smoothly, is made from petroleum. Petroleum is a dark liquid that comes from the earth.

Today, petroleum is usually called oil. The term "crude oil" refers to petroleum as it comes from the earth.

Drilling for oil isn't an easy job. Skilled geologists and geophysicists must study rock formations to determine which location is likely to contain oil.

Then a drilling crew puts up a derrick that may be a 40 foot wooden structure or a 200 foot steel giant. The size of the derrick depends on the expected depth of the well and it serves chiefly to help lift the drill pipe and casing in and out of the hole.

At the base of the derrick goes the drilling rig, which is usually powered by diesel engines. Cable tool drilling is done with a cutting tool called a bit, which is attached to a long steel cable by means of an iron rod called a stem. The cable raises the bit, then drops it again and again. The dropping force drives the bit deeper and deeper into the earth.

Jagged teeth on the bit crush soil and rock. From time to time, the bit is raised out of the hole and water is forced in to flush out the rock and soil. A long pipe called a bailer is used to remove water, mud and rock from the hole.

The cable tool drilling method is used chiefly to drill shallow holes through soft beds of rock.

The most important method of drilling for oil is the rotary system. It requires very complicated equipment. It bores through the earth much as a carpenter's drill bores through wood.

With rotary drilling, a drill stem or hollow steel tube about 40 feet long is used with a number of different drilling bits. An engine turns a rotary table on the floor of the derrick, which turns the bit. A draw works is used to lower and raise the drill stem in and out of the well.

The main part of a rotary drill's draw works are a hoisting drum, a hoisting cable and two large blocks or sets of pulleys. The cable runs from the drum over the crown block at the top of the derrick.

Workers change the bit when it becomes dull or when the rock formation changes. Each time they change the bit, they must also pull all the pipe out of the hole. Sometimes 10,000 feet or more of drill pipe must be lifted out before the drilling work can continue.

To prevent the well from caving in, it is lined with steel pipe called casing. The casing also prevents substances in the well from contaminating fresh water in the ground.



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