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Unformatted text preview: xygen are needed for petroleum formation to begin.
○Petroleum is produced when organic material in sediments combines with decaying marine organisms.
○Petroleum formation appears to begin in marine sediments where organic matter is present.
Paragraph 1—2 在 Petroleum, consisting of crude oil and natural gas, seems to originate from organic matter in marine sediment. Microscopic organisms settle to the seafloor and accumulate in marine mud. The organic matter may partially decompose, using up the dissolved oxygen in the sediment. As soon as the oxygen is gone, decay stops and the remaining organic matter is preserved.
Continued sedimentation—the process of deposits' settling on the sea bottom—buries the organic matter and subjects it to higher temperatures and pressures, which convert the organic matter to oil and gas As muddy .
sediments are pressed together, the gas and small droplets of oil may be squeezed out of the mud and may move into sandy layers nearby. Over long periods of time (millions of years), accumulations of gas and oil can collect in the sandy layers. Both oil and gas are less dense than water, so they generally tend to rise upward through water
saturated rock and sediment. 3在In paragraphs 1 and 2, the author’s primary purpose is to ○Describe how petroleum is formed
○Explain why petroleum formation is a slow process
○Provide evidence that a marine environment is necessary for petroleum formation
○Show that oil commonly occurs in association with gas
4 在 Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. ○Higher temperatures and pressures promote sedimentation, which is responsible for petroleum formation.
○Deposits of sediments on top of organic matter increase the temperature of and pressure on the matter.
○Increase pressure and heat from the weight of the sediment turn the organic remains into petroleum.
○The remains of microscopic organisms transform into petroleum once they are buried under mud.
Paragraph 3在Oil pools are valuable underground accumulations of oil, and oil fields are regions underlain by one 168
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or more oil pools. When an oil pool or field has been discovered, wells are drilled into the ground. Permanent towers, called derricks, used to be built to handle the long sections of drilling pipe. Now portable drilling machines are set up and are then dismantled and removed. When the well reaches a pool, oil usually rises up the well because of its density difference with water beneath it or because of the pressure of expanding gas trapped above it. Although this rise of oil is almost always carefully controlled today, spouts of oil, or gushers, were common in the past. Gas pressure gradually dies out, and oil is pumped from the well. Water or steam may be pumped down adjacent wells to help push the oil out. At a refinery, the crude oil from underground is separated into natural gas, gasoline,...
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- Fall '13