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Daily RC Article 66

Origin of Oil

Paragraph 1

Since the early 1920s, most petroleum geologists have favored a biogenic theory for the formation of oil. According to this theory, organic matter became buried in sediments, and subsequent conditions of temperature and pressure over time transformed it into oil.

Paragraph 2

Since 1979 an opposing abiogenic theory about the origin of oil has been promulgated. According to this theory, what is now oil began as hydrocarbon compounds within the earth’s mantle (the region between the core and the crust) during the formation of the earth. Oil was created when gases rich in methane, the lightest of the hydrocarbons, rose from the mantle through fractures and faults in the crust, carrying a significant amount of heavier hydrocarbons with them. As the gases encountered intermittent drops in pressure, the heavier hydrocarbons condensed, forming oil, and were deposited in reservoirs throughout the crust. Rock regions deformed by motions of the crustal plates provided the conduits and fractures necessary for the gases to rise through the crust.

Paragraph 3

Opponents of the abiogenic theory charge that hydrocarbons could not exist in the mantle, because high temperatures would destroy or break them down. Advocates of the theory, however, point out that other types of carbon exist in the mantle: unoxidized carbon must exist there, because diamonds are formed within the mantle before being brought to the surface by eruptive processes. Proponents of the abiogenic theory also point to recent experimental work that suggests that the higher pressures within the mantle tend to offset the higher temperatures, allowing hydrocarbons, like unoxidized carbon, to continue to exist in the mantle.

Paragraph 4

If the abiogenic theory is correct, vast undiscovered reservoirs of oil and gas—undiscovered because the biogenic model precludes their existence—may in actuality exist. One company owned by the Swedish government has found the abiogenic theory so persuasive that it has started exploratory drilling for gas or oil in a granite formation call the Siljan Ring—not the best place to look for gas or oil if one believes they are derived from organic compounds, because granite forms from magma (molten rock) and contains no organic sediments. The ring was formed about 360 million years ago when a large meteorite hit the 600-million-year-old granite that forms the base of the continental crust. The impact fractured the granite, and the Swedes believe that if oil comes from the mantle, it could have risen with methane gas through this now permeable rock. Fueling their optimism further is the fact that prior to the start of drilling, methane gas had been detected rising through the granite.

Topic and Scope:  

The origins of oil; specifically, the abiogenic theory of the origins of oil

Purpose and Main Idea:

The author’s purpose is to describe the details and implications of the abiogenic theory of oil formation. His main idea is that this theory is an unverified alternative to the more accepted biogenic theory of oil formation.


paragraph 1 describes the biogenic theory of oil formation—the idea that subsurface organic matter is transformed into oil over time by the twin forces of pressure and temperature. This paragraph  clues you into the author’s topic—the origins of oil—but not much else. It’s not until we hit paragraph 2 that we get a real sense of what the passage is going to be about—the abiogenic theory of the origins of oil, an alternative to the dominant biogenic theory. The rest of this paragraph  goes on to describe this theory. Essentially, it holds that oil forms out of sub-surface hydrocarbons.

paragraph 3 discusses the views of opponents and advocates of this theory. Opponents argue that oil couldn’t come from hydrocarbons because hydrocarbons can’t exist in the earth’s mantle. Advocates, on the other hand, argue that hydrocarbons can indeed exist in the mantle. paragraph 4 switches gears: it examines the implications of the abiogenic theory. If this theory is correct, the author says, there’s a lot of oil out there just waiting to be discovered. He then describes one Swedish company’s attempts to validate the theory by looking for oil in a place where the theory suggests it might be.

The Big Picture:

  • Most CAT science passages contain a lot of intricate scientific details and unfamiliar scientific jargon. Resist the temptation to assimilate all of these details and all of this jargon. You won’t need to understand the passage as a scientist would in order to answer the questions. Instead, be content to acquire a broad overview of the passage as you read through it.
  • It’s useful to scan question stems before reading a passage. A brief look at the stems will clue you in to which parts of the passage are most important to answering the questions. The bottom line: by scanning the stems in advance, you’ll quickly get a sense of where to find the answers to many of the questions.

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