PETROLEUM EXTRACTION, TRANSPORT, AND REFINING

Cap rock

To extract petroleum, an oil company drills a well into a reservoir and pumps the oil to the surface. Fifty years

Source rock

Reservoir rock

ago, many reservoirs lay near the surface and oil was easily extracted from shallow wells. But these reserves

Figure 19–18 Most oil forms in shaly source rock. It must have been exploited, and modern oil wells are typically migrate to a permeable reservoir in order to be recovered

deeper. For example, in 1949, the average oil well drilled from an oil well.

in the United States was 1116 meters deep. In 1994, the average well was 1629 meters deep. The average cost of drilling a new oil well in 1960 was slightly more than $200,000; by 1993, the cost had risen to about $350,000.

the organic matter to liquid petroleum that is finely dis- After the hole has been bored, the expensive drill rig is persed in the rock (Fig. 19–18). The activity of bacteria

removed and replaced by a pumper that slowly extracts may enhance the process. Typically petroleum forms in

the petroleum. In 1989, the USGS reported that oil re- the temperature range from 50 to 100ºC. At temperatures

serves in the United States were being rapidly depleted. above about 100ºC, oil begins to convert to natural gas.

Petroleum companies found half as much oil as they did Consequently, many oil fields contain a mixture of oil

in the 1950s for every meter of exploratory drilling. and gas.

However, in 1995 the USGS revised these earlier assess- The shale or other sedimentary rock in which oil

ments. 2 According to the new report, due to improved originally forms is called the source rock. Oil dispersed

extraction techniques, oil reserves increased by 41 per- in shale cannot be pumped from an oil well because

cent between 1989 and 1995.

shale is relatively impermeable; that is, liquids do not On the average, more than half of the oil in a reser- flow through it rapidly. But under favorable conditions,

voir is too viscous to be pumped to the surface by con- petroleum migrates slowly to a nearby layer of perme-

ventional techniques. This oil is left behind after an oil able rock—usually sandstone or limestone—where it can

field has “gone dry,” but it can be extracted by secondary flow readily. Because petroleum is less dense than water

and tertiary recovery techniques. In one simple sec- or rock, it then rises through the permeable rock until it

ondary process, water is pumped into one well, called is trapped within the rock or escapes onto the Earth’s

the injection well. The water floods the reservoir, driving surface.

oil to nearby wells, where both the water and oil are ex- Many oil traps form where impermeable cap rock

tracted. At the surface, the water is separated from the oil prevents the petroleum from rising further. Oil or gas then

and reused, while the oil is sent to the refinery. One ter- accumulates in the trap as a petroleum reservoir. The cap

tiary process forces superheated steam into the injection rock is commonly impermeable shale. Folds and faults

well. The steam heats the oil and makes it more fluid so create several types of oil traps (Fig. 19–19). In some re-

that it can flow through the rock to an adjacent well. gions, large, lightbulb-shaped bodies of salt have flowed upward through solid rocks to form salt domes. The ris-

2 R. C. Burruss, Geotimes, July, 1995, p. 14.

348 CHAPTER 19 GEOLOGICRESOURCES

Oil Gas

Cap rock

Water

Reservoir rock

Cap rock Water

Water Oil

Gas

Cap rock

Oil Cap rock

Gas Salt

(c)

(d)

Figure 19–19 Four different types of oil traps. (a) Petroleum rises into permeable limestone in a structural dome. (b) A trap forms where a fault has moved impermeable shale against permeable sandstone. (c) Horizontally bedded shale traps oil in tilted limestone. (d) In a salt dome, a sedimentary salt deposit rises and deforms overlying strata to create a trap.

Because energy is needed to heat the steam, this type of islands (Fig. 19–20). Despite great care, accidents occur extraction is not always cost effective or energy efficient.

during drilling and extraction of oil. When accidents Another tertiary process pumps detergent into the reser-

occur at sea, millions of barrels of oil can spread through- voir. The detergent dissolves the remaining oil and car-

out the waters, poisoning marine life and disrupting ries it to an adjacent well, where the petroleum is then

marine ecosystems. Significant oil spills have occurred recovered and the detergent recycled.

in virtually all offshore drilling areas. In addition, tanker In the United States, more than 300 billion barrels

accidents have polluted parts of coastal oceans. of secondary and tertiary oil remain in oil fields. In 1994,

Although all oil refineries use expensive pollution- people in the United States consumed 6.4 billion barrels

control equipment, these devices are never completely of petroleum. Thus, if all the secondary petroleum could

effective. As a result, some toxic and carcinogenic com-

be recovered, it would supply the United States for nearly pounds escape into the atmosphere.

50 years. However, not all of the 300 billion barrels can

be recovered, and the energy yield from secondary and

PETROLEUM EXTRACTION, TRANSPORT, AND REFINING