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After being formed deep within the earth, hydrocarbons migrate upwards

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After being formed deep within the earth, hydrocarbons migrate upwards  [#permalink]

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New post 22 Feb 2018, 03:34
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After being formed deep within the earth, hydrocarbons migrate upwards, following a complex path of minute cracks and pore spaces, and will eventually reach the surface and be lost unless they encounter impermeable rocks (such as dense shale) through which they cannot travel. If the rock within which they are trapped is highly permeable (such as sandstone) the hydrocarbons can be extracted by drilling through the impermeable seal, and tapping into this permeable reservoir.

The need to expand oil and gas reserves brings with it a need to find hydrocarbon reservoirs that are difficult to locate using current geological and geophysical means. To do so, geologists look for rock formations that constitute the seals and reservoirs within which hydrocarbons could be trapped.

There are a number of different types of traps, but they can be divided into two broad categories. Structural traps are formed by deformation after the rocks have been formed, for example by folding or faulting. Stratigraphic traps are formed when the loose sediments that will eventually be turned into rocks were laid down.

Structural traps tend to be easier to locate and are the source of most of the known hydrocarbon reserves. Expanding our reserves therefore means locating more stratigraphically trapped hydrocarbons. The primary means of exploring for oil where there is no surface expression of the underlying geology is by seismology. When a seismic pulse transmitted into the earth encounters an interface where the density changes, typically the surface between two beds or an unconformity with velocitydensity contrasts, some of the energy is reflected back upwards. A string of seismophones record these reflections and after extensive computation seismologists can build up a visual record of the intensity of each reflection and the time taken for it to reach the surface.

The primary limitation of the seismic method for locating stratigraphic traps is resolution: It is not possible to resolve features that are thinner than a seismic wavelet. The most common stratigraphic traps (with the possible exception of carbonate reservoirs) are in sandstone layers that are much thinner than a seismic wavelet. Seismic wavelets can be narrowed by increasing the frequency of the seismic pulse. However, high frequencies are selectively attenuated as the pulse travels through the earth, so there are limits to how much resolution can be improved by simply generating higher frequency pulses, or by filtering out the lower frequency components of the seismic source. Moreover, the density contrasts between oil-bearing sandstones and the shales that provide stratigraphic seals for the oil are often very small, so that the reflectivities, and hence the strength of the reflection, will be so low that the events may not be observable above background noise.

Recent developments such as zero phase wavelet processing and multivariate analysis of reflection waveforms have decreased noise and increased resolution. In the future it is hoped that these techniques, and greater understanding of stratigraphy itself, will prove fruitful in expanding hydrocarbon reserves.

1. As opposed to other essays written on the same topic, it is likely that the primary purpose of this passage is to:

A. explain how hydrocarbons are formed and trapped within the earth.
B. detail how seismologists can locate hidden deposits of hydrocarbons.
C. contrast the relative difficulty of locating structural traps and stratigraphic traps.
D. discuss the formation of hydrocarbon reserves and how they can be located.
E. argue for increased private investment in the location of hydrocarbons


2. According to the passage it is often difficult to distinguish reflections from the interface between oil bearing sandstones and the shales that provide stratigraphic seals from background noise because:

A. high frequencies are attenuated as they travel through the earth.
B. there is little density contrast between the oil bearing sandstone and the shales which provide stratigraphic seals.
C. the frequency of the seismic pulse is not high enough.
D. they are thinner than the seismic wavelet.
E. they are thicker than the seismic wavelet


3. According to the passage, all of the following are needed if oil is to be extracted from a reservoir EXCEPT:

A. an impermeable seal above the reservoir.
B. an original source of hydrocarbons below the reservoir.
C. high density contrast between the reservoir rocks and the stratigraphic seal.
D. high permeability within the reservoir.
E. presence of cracks and pores in the earth‘s crust


4. Based on the points made throughout the passage, which of the following best describes how the author views seismology as a tool in locating hydrocarbons?

A. Of limited effectiveness but showing promise
B. Intrinsically flawed
C. Effective and profitable
D. Theoretically useful but ineffectual in practice
E. Out-dated and archaic


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Re: After being formed deep within the earth, hydrocarbons migrate upwards  [#permalink]

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New post 02 Oct 2018, 18:09

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Re: After being formed deep within the earth, hydrocarbons migrate upwards  [#permalink]

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New post 25 Oct 2018, 11:15

Topic and Scope

- The formation and location of hydrocarbon reserves

Mapping the Passage


¶1 explains how hydrocarbons form in pockets underground.
¶2 gives some background for our global dependence on fossil fuels.
¶3 describes the two types of hydrocarbon traps: structural traps and stratigraphic
traps.
¶4 notes that new sources of hydrocarbons will come from reserves that are difficult
to locate, and describes generally how reserves are located and extracted.
¶5 notes that most new oil will be found in stratigraphic traps and outlines the method
for finding oil when surface geology doesn‘t help: seismic exploration.
¶6 describes the limitations to seismic exploration of stratigraphic traps.
¶7 notes recent developments in refining seismic exploration, and raises hope that
discovery of stratigraphic traps will be easier in the future.
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After being formed deep within the earth, hydrocarbons migrate upwards  [#permalink]

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New post 25 Oct 2018, 11:17

Answers and Explanation OE



1)

A global question: predict with topic, scope, and purpose. The author discusses how hydrocarbon reserves are formed (especially in ¶s 1 and 3) and how they can be located (throughout the passage, but especially in the second half of the passage). (D) repeats this nearly word-for-word.
(A): Faulty Use of Detail. While the passage does this, this choice says nothing about the location of reserves, which the passage spends significant time on.
(B): Faulty Use of Detail. The flip side of the above answer choice. The passage discusses seismic exploration, but it also discusses the formation of hydrocarbons before this.
(C): Faulty Use of Detail. The author argues in ¶5 that stratigraphic traps are harder to locate than structural traps, but this isn‘t itself the main idea of the passage; the author mentions this in order to explain the method for discovering stratigraphic traps.
(D): The correct answer
(E): No such argument is made in the passage

2)

A detail question; ―According to the passage…‖ tips you off. Where are difficulties mentioned? Go back to ¶6. The last sentence of ¶6 states what the question does, that it‘s difficult to distinguish reflections between the two materials. The beginning of the sentence gives the reason: ―the density contrasts between oil-bearing sandstones and the shales that provide stratigraphic seals for the oil are often very small.‖ (B) says the same.(A): Faulty Use of Detail. While the author mentions this in the same paragraph, it‘s used in the context of how resolution can be improved, not why it‘s
difficult to distinguish between the sandstone and shale.
(B): The correct answer
(C): Faulty Use of Detail. This is part of the ―primary limitation with the seismic method‖ that the author discusses towards the beginning of the paragraph, not the direct cause of the particular problem in the question.
(D): Out of Scope. As above, thinness has to do with the primary limitation of the method, not the specific problem mentioned in the question.
(E): The passage never states this.

3)

A scattered detail question. Either eliminate wrong answer choices or look for a choice that sticks out as correct. (C) should jump out immediately; since not all traps are stratigraphic, it wouldn‘t make sense for the author to have said that oil couldn‘t be extracted without a density contrast between reservoir rocks and a stratigraphic seal.
(A): Opposite. The author states in ¶1 that ―hydrocarbons…will eventually reach the surface and be lost unless they encounter impermeable rocks.‖
(B): Opposite. The author ties oil reserves to hydrocarbons in ¶s1 and 4, so it‘s reasonable to believe that it‘s not possible to get oil if an original source of hydrocarbons aren‘t present.
(C): The correct answer
(D): Opposite. The author says in ¶1 that ―if the rock within which they are trapped is highly permeable…the hydrocarbons can be extracted by drilling.‖ In other words, drilling can‘t happen unless hydrocarbons are trapped within permeable rocks.
(E): Opposite. This is mentioned in ¶1.

4)

What is the author‘s opinion of seismology? The author discusses why seismology isn‘t a great way to find stratigraphic traps in ¶6, and raises the hope that seismology will become more effective in the future in ¶7. Paraphrase: Seismology has its problems, but will hopefully improve in the future. (A) says the same.
(A): The correct answer
(B): Distortion. Though seismology has limitations, there‘s no indication that it‘s intrinsically flawed. If it were, the author wouldn‘t argue for its improvement.
(C): Distortion. The author believes that seismology has promise, but spends a significant part of the passage explaining why seismology isn’t extremely effective. Nothing at all is said about profitability, so this choice is out of scope also.
(D): Distortion. The author doesn‘t discuss the theory of seismology, instead focusing exclusively on the practical method and its limitations. Further, theauthor only suggests that seismology is ineffective for stratigraphic exploration, not completely ineffectual.
(E): The passage does not say anything to this effect.
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After being formed deep within the earth, hydrocarbons migrate upwards &nbs [#permalink] 25 Oct 2018, 11:17
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