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THE MODERN SEARCH FOR OIL

 

Wildcatters discovered the earliest oil fields either by blind luck or by searching for surface seeps. But in the twentieth century, when all known seeps had been drilled and blind luck became too risky, oil companies realized that finding new oil fields would require systematic exploration. The modern-day search for oil is a complex, sometimes dangerous, and often exciting procedure with many steps.

Source rocks are always sedimentary, as are most reservoir and seal rocks, so geologists begin their exploration by looking for a region containing appropriate sedimentary rocks. Then they compile a geologic map of the area, showing the distribution of rock units. From this information, it may be possible to construct a preliminary cross section depicting the geometry of the sedimentary layers underground as they would appear on an imaginary vertical slice through the Earth.

To add detail to the cross section, an exploration company makes a seismic-reflection profile of the region. To construct a seismic profile, a special vibrating truck or a dynamite explosion sends seismic waves (shock waves that move through the Earth) into the ground (Fig. 9a). The seismic waves reflect off contacts between rock layers, just as sonar waves sent out by a submarine reflect off the bottom of the sea. Reflected seismic waves then return to the ground surface, where sensitive instruments (geophones) record their arrival. A computer measures the time between the generation of a seismic wave and its return, and from this information defines the depth to the contacts at which the wave reflected. With such information, the computer constructs an image of the configuration of underground rock layers. Technological advances now enable geologists to create 3-D seismic reflection profiles of the subsurface both under land and under water (Fig. 9b, c). Unfortunately, it may cost millions of dollars to create just one profile.

FIGURE 9. (a) The basic concept of seismic-reflection profiling. A signal sent by a source truck (e.g., a “Vibroseis” truck, which sends energy into the crust via a heavy plate placed on the ground beneath the middle of the truck) reflects off a bedding plane in the subsurface. Seismic receivers detect the reflections of different rays, and computers in a recording truck record the information. The computers process the data to determine the location of the bedding plane. (b) Seismic-reflection studies can be obtained in submarine strata by use of a special ship that tows airguns (devices that send sound pulses into the water) and strings of detectors. When a ship tows several rows of detectors at once, a 3-D image of the strata can be produced. (c) A modern 3-D seismic-reflection image of the subsurface. Each vertical face of this block is a seismic-reflection profile. The horizontal surface shows a map-like image at a specified depth beneath the surface. The colored bands represent beds of sedimentary rock.


Geologists continue to discover new tools that help in oil exploration. For example, tiny seeps of oil host communities of hydrocarbon-eating bacteria. Mapping the distribution of bacteria species in the soil may, therefore, indicate the presence of an oil reserve below.

If geological studies identify a trap, and if the geologic history of the region indicates the presence of good source rocks and reservoir rocks, geologists make a recommendation to drill. (They do not make such recommendations lightly, as drilling a deep well may cost over $10 million.) Once the decision has been made, drillers go to work. These days, drillers use rotary drills to grind a hole down through rock. A rotary drill consists of a rotating pipe tipped by a bit, a bulb of metal studded with industrial diamonds or hard metal prongs (Fig. 10a, b). As the bit rotates, it scratches and gouges the rock, turning it into powder and chips. Drillers pump “drilling mud,” a slurry of water mixed with clay, down the center of the pipe. The mud squirts out of holes in the drill bit, cooling the bit and flushing rock cuttings up and out of the hole. The weight of the mud also keeps oil down in the hole and prevents “gushers,” fountains of oil formed when underground pressure causes the oil to rise out of the hole on its own.

Drillers use derricks (towers) to hoist the heavy drill pipe. To drill in an offshore oil reserve, one that occurs in strata of the continental shelf, the derrick must be constructed on an offshore-drilling facility. Offshore-drilling facilities can be immense (Fig. 10c); some tower more than 100 stories from base (below sea level) to top. They serve as artificial towns for housing the drilling crew. In shallower water, facilities anchor on the sea floor, but in deeper water, they float on huge submerged pontoons. Drill holes can be aimed in any direction (not just vertical), so drillers can reach many traps from the same facility. On completion of a hole, workers remove the drilling rig and set up a pump. Some pumps resemble a bird pecking for grain; their heads move up and down to pull up oil that has seeped into the drill hole (Fig. 10d). You may be surprised to learn that simple pumping gets only about 30% of the oil in a reservoir rock out of the ground. Thus, oil companies use secondary recovery techniques to coax out up to 50% more oil. For example, a company may drive oil toward a drill hole by forcing steam into the ground nearby. The steam heats the oil in the ground, making it less viscous, and pushes it along. In some cases, drillers create artificial fractures in rock around the hole so that the oil has easy routes to follow from rock to well.

 

 

FIGURE 10. (a) The derrick in this drilling platform is used to hoist the drill pipe, which comes in segments. (b) A drill bit, typically consisting of three diamond-studded parts, connects to the bottom of the pipe. The diameter of the bit is greater than the diameter of the pipe. Drilling mud is pumped down through the pipe, comes out through holes in the bit, and then rises between the pipe and the walls of the hole, thereby flushing cuttings out of the hole. (c) An offshore-drilling facility in southern California. (d) A row of pumps in an Illinois oil field.

 

Presently, the vast majority of known oil reserves are distributed among only twenty-five fields, known as super-giant fields (Fig. 5). The largest occur around the Persian Gulf (in Saudi Arabia, Kuwait, Iraq, and neighboring countries; see Table).

The United States is the largest consumer of oil (at a rate of 7 million barrels per day, about 25% of world consumption), but lost its position as the largest producer in the 1970s. Oil reserves in the United States now account for only about 4% of the world total. Thus, today the United States must import more than half of the oil it uses. The industrialized countries use vastly more oil per capita than do the developing countries. For example, 1,000 people in the United States use 70 barrels per day. In China, 1,000 people use 4 barrels per day, and in Ethiopia, 0.3 barrels per day.

 

Once extracted directly from the ground, “crude oil” flows first into storage tanks and then into a pipeline or tanker, which transports it to a refinery. At a refinery, workers distill crude oil into several separate components by heating it gently in a vertical pipe called a distillation column. Lighter molecules rise to the top of the column, while heavier molecules stay at the bottom. The heat may also “crack” larger molecules to make smaller ones. Chemical factories buy the largest molecules left at the bottom and transform them into plastics.

 




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ENERGY RESOURCES | SOURCES OF ENERGY IN THE EARTH SYSTEM | What Are Oil and Gas? | Where Do Oil and Gas Form? | Reservoir Rocks and Hydrocarbon Migration | Traps and Seals | Types of Oil and Gas Traps | CAN OTHER ENERGY SOURCES MEET THE NEED? |


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