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The Origin of Petroleum
Petroleum is defined as a gaseous, liquid, and semisolid naturally occurring substance that consists chiefly of hydrocarbons (chemical compounds of carbon and hydrogen). Petroleum is therefore a term that includes both oil and natural gas. Petroleum is nearly always found in marine sedimentary rocks. In the ocean, microscopic phytoplankton (tiny floating plants) and bacteria (simple, single-celled organisms) are the principal sources of organic matter that is trapped and buried in sediment Most of the organic matter is buried in clay that is slowly converted to a fine-grained sedimentary rock known as shale. During this conversion, organic compounds are transformed to oil and natural gas.
Sampling on the continental shelves and along the base of the continental slopes has shown that fine muds beneath the seafloor contain up to 8 percent organic matter. Two additional kinds of evidence support the hypothesis that petroleum is a product of the decomposition of organic matter: oil possesses optical properties known only in hydrocarbons derived from organic matter, and oil contains nitrogen and certain compounds believed to originate only in living matter. A complex sequence of chemical reactions is involved in converting the original solid organic matter to oil and gas, and additional chemical changes may occur in the oil and gas even after they have formed.
It is now well established that petroleum migrates through aquifers and can become trapped in reservoirs. Petroleum migration is analogous to groundwater migration. When oil and gas are squeezed out of the shale in which they originated and enter a body of sandstone or limestone somewhere above, they migrate readily because sandstones (consisting of quartz grains) and limestones (consisting of carbonate minerals) are much more permeable than any shale. The force of molecular attraction between oil and quartz or carbonate minerals is weaker than that between water and quartz or carbonate minerals. Hence, because oil and water do not mix, water remains fastened to the quartz or carbonate grains, while oil occupies the central parts of the larger openings in the porous sandstone or limestone. Because oil is lighter than water, it tends to glide upward past the carbonate- and quartz-held water. In this way, oil becomes segregated from the water; when it encounters a trap, it can form a pool.
Most of the petroleum that forms in sediments does not find a suitable trap and eventually makes its way, along with groundwater, to the surface of the sea. It is estimated that no more than 0.1 percent of all the organic matter originally buried in a sediment is eventually trapped in an oil pool. It is not surprising, therefore, that the highest ratio of oil and gas pools to volume of sediment is found in rock no older than 2.5 million years—young enough so that little of the petroleum has leaked away—and that nearly 60 percent of all oil and gas discovered so far has been found in strata that formed in the last 65 million years This does not mean that older rocks produced less petroleum; it simply means that oil in older rocks has had a longer time in which to leak away.
How much oil is there in the world? This is an extremely controversial question. Many billions of barrels of oil have already been pumped out of the ground. A lot of additional oil has been located by drilling but is still waiting to be pumped out Possibly a great deal more oil remains to be found by drilling. Unlike coal, the volume of which can be accurately estimated, the volume of undiscovered oil can only be guessed at. Guesses involve the use of accumulated experience from a century of drilling. Knowing how much oil has been found in an intensively drilled area, such as eastern Texas, experts make estimates of probable volumes in other regions where rock types and structures are similar to those in eastern Texas. Using this approach and considering all the sedimentary basins of the world, experts estimate that somewhere between 1,500 and 3,000 billion barrels of oil will eventually be discovered.
本文是托福tpo43阅读答案+原文+解析【The Origin of Petroleum】。
1.According to paragraph 1, petroleum is formed in which of the following ways? 【事实信息题】
A Bacteria and tiny plants undergo a change while they are buried in clay.
B Carbon and hydrogen combine to form shale.
C Ocean rocks are converted into organic compounds.
D Oil and gas rise to the surface of sediment and are trapped in rocks.
答案： A
解析：定位句In the ocean, microscopic phytoplankton (tiny floating plants) and bacteria (simple, single-celled organisms) are the principal sources of organic matter that is trapped and buried in sediment. 在海洋中，捕获和掩埋在沉淀物中的微型浮游植物(微型浮游植物)和细菌(简单的单细胞生物)是有机物的主要来源。因此正确答案为A。
2.The word "trapped" in the passage is closest in meaning to【词汇题】
A hidden
B destroyed
C caught
D found
答案： C
解析：trap诱捕;使…受限制，使…陷入困境，对应选项C，捕捉。
3. All of the following are cited in paragraph 2 as evidence that petroleum is a product of the decomposition of organic matter EXCEPT【否定事实信息题】
A the amount of organic matter found in layers of mud below the seafloor
B the chemical changes that occur in oil and natural gas after they have formed
C the optical properties of oil
D the fact that oil contains nitrogen and other compounds believed to be of organic origin
答案： A
解析：B选项的after they have formed对应第二段最后一句;C的optical properties of oil对应oil possesses optical properties;D的oil contains nitrogen and other compounds对应contains nitrogen and certain compounds。A的amount不是证据，因此正确答案为A。
4. According to paragraph 2, which of the following is true of the change of solid organic material into oil and gas? 【事实信息题】
A It is more likely to occur along the base of continental slopes than on the continental shelves.
B It only takes place in areas where the seafloor contains at least 8 percent organic matter.
C It is a process that can be reversed through chemical changes that occur after the oil and gas have formed.
D It involves a complicated series of chemical reactions.
答案： D
解析：定位句A complex sequence of chemical reactions is involved in converting the original solid organic matter to oil and gas, and additional chemical changes may occur in the oil and gas even after they have formed.原始的固体有机物质转化为石油和天然气，有一系列复杂的化学反应参与其中, 即使石油和天然气已经形成，其他化学变化也可能会发生。complex sequence 对应选项D的complicated series，因此正确答案为D。
5. 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. 【句子简化题】
A When oil and gas are squeezed out of the rock in which they originated, it is probably because the layer of rock above them is much more permeable than shale.
B Sandstones, which are made of quartz grains, and limestones, which are made of carbonate minerals, can hold much more oil and gas than any shale can.
C When they are squeezed from the shale in which they were formed, oil and gas move easily into the much more permeable layers of sandstone or limestone above.
D Oil and gas are squeezed out of sandstones, consisting of quartz grains, and migrate readily into limestones, which consist of carbonate minerals and are much more permeable.
答案： C
解析：When引导从句，原句核心为：当它们被挤压时，油和气体能更轻易的移动至更易渗透的砂岩或石灰岩中去，对应选项C。A缺少migrate readily此信息;B 的Sandstones主语错误，不是原句核心;D的much more permeable变为次要成分。因此正确答案为C。
6. Why does the author include the information that "The force of molecular attraction between oil and quartz or carbonate minerals is weaker than that between water and quartz or carbonate minerals."? 【修辞目的题】
A To help explain why petroleum behaves differently from water in bodies of sandstone and limestone
B To illustrate why petroleum migrates more rapidly through sandstone than it does through limestone
C To help explain how water and petroleum can mix in certain aquifers
D To account for the different molecular structures of oil and water
答案： A
解析：定位句The force of molecular attraction between oil and quartz or carbonate minerals is weaker than that between water and quartz or carbonate minerals. 分子之间的吸引力在石油和石英、碳酸盐矿物之间是弱于水和石英、碳酸盐矿物的。B选项的more rapidly …than未提及;C的how…can mix未提及;D的account for…molecular structures未提及。因此正确答案为A。
7. The word "encounters" in the passage is closest in meaning to【词汇题】
A meets
B forms
C escapes
D avoids
答案： A
解析：encounter遭遇;邂逅，对应选项A，遇到。
8. The word "suitable" in the passage is closest in meaning to【词汇题】
A noticeable
B permanent
C protected
D appropriate
答案： D
解析：suitable合适的;适当的;相配的，对应选项D，合适的。
9. According to paragraph 4, what happens to most of the petroleum that forms in sediments? 【事实信息题】
A It remains in underground pools.
B It is buried under organic matter.
C It rises to the surface of the ocean.
D It combines with the minerals found in groundwater.
答案： C
解析：定位句Most of the petroleum that forms in sediments does not find a suitable trap and eventually makes its way, along with groundwater, to the surface of the sea. 大多数在沉积物中形成的石油找不到合适的存储地,最终它们自己找方式,随着地下水流到了海洋的表面。对应选项C 的rises to the surface of the ocean。因此正确答案为C。
10. Paragraph 4 supports which of the following statements about future petroleum discoveries? 【事实信息题】
A Less petroleum will be found than in the past because the ratio of petroleum pools to volume of sediment will decrease.
B Most of the petroleum will come from rocks that are less than 65 million years old.
C Petroleum that has leaked away from older rocks will be the source of most new discoveries.
D More petroleum will become available because the amount of trapped organic matter will increase.
答案： C
解析：A选项的Less petroleum… than in the past，sediment will decrease是未提及内容;B选项的less than 65 million years old错误;D选项的organic matter will increase未提及。C的对应了原文的young enough so that little of the petroleum has leaked away，即更老的岩石中可能有新发现，因此正确答案为C。
11. The phrase "is an extremely controversial question" in the passage is closest in meaning to 【词汇题】
A is a question of great importance
B is a question causing strong disagreement
C is a question that has existed for a long time
D is a question composed of many related parts
答案： B
解析：is an extremely controversial question是一个非常有争议的问题，对应选项B，是一个造成强烈的分歧的问题。
12. According to paragraph 5, eastern Texas is an example of a geologic region where 【事实信息题】
A oil is located but has not yet been pumped out
B experts accurately predicted the rock types and structures found there
C the volume of oil can only be guessed at
D intensive oil exploration has occurred over a long time
答案： B
解析：定位句…experts make estimates of probable volumes in other regions where rock types and structures are similar to those in eastern Texas. 专家做出的估计可能在其他地区会有大量产出，在那些岩石类型和结构与那些在德克萨斯州东部类似的地区。对应选项B的accurately predicted the rock types and structures，因此正确答案为B。
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage. 【句子插入题】
Because such muds are a major source of petroleum, scientists believe that petroleum originated as living organisms.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
答案： B
解析：Because因果关系， living organisms对应原文的organic matter，且B处提出Two additional kinds of evidence support the hypothesis对应句子的 scientists believe…，科学家先提出看法再用证据支持这种假设，因此B为正确答案。
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage This question is worth 2 points. 【概要小结题】
Drag your answer choices to the spaces where they belong To remove an answer choice, click on it.
To review the passage, click VIEW TEXT
Petroleum, which includes both oil and natural gas, can be a gaseous, liquid, or semisolid substance.
Answer Choices
Petroleum comes from organic matter that has undergone a complex series of chemical changes under the seafloor.
Although most of the petroleum formed leaks away into the ocean, some migrates from shale to sandstone or limestone, and is caught in pools.
Porous rocks made of quartz or carbonate minerals are particularly likely to house oil pools because of their strong molecular attraction with oil.
Petroleum forms best when organic matter is evenly distributed over a large area and does not exceed 8 percent of the material in the clay.
More than 60 percent of the petroleum discovered so far has been found in rocks that are less than two-and-a-half million years old.
It is difficult to estimate the total amount of petroleum in the world, but experts believe that 1,500-3,000 billion barrels will eventually be discovered.
答案： ABF
解析：A是原文第一二段的总结，B选项对应第四段的第一二句，F对应第五段的最后一句。
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