Locating hydrocarbons is difficult, and by now all obvious reserves of

Question

Locating hydrocarbons is difficult, and by now all obvious reserves of oil and gas have been discovered. The need to expand oil and gas reserves, therefore, brings with it a need to find hydrocarbon reservoirs that are difficult to locate using current means.

When geologists seek hydrocarbons, they look for rock formations that form the seals and reservoirs within which hydrocarbons can be trapped. After forming deep within the earth, hydrocarbons migrate upward, following a complex path of minute cracks. They reach the surface and are lost unless they encounter impermeable rock through which they cannot travel. If the rock within which they are trapped is highly permeable, drilling through the impermeable seal and tapping into this reservoir can extract the hydrocarbons. There are a number of different types of traps, but they can be divided into two broad categories. Structural traps are formed through the deformation of rocks, for example, by folding or faulting. Stratigraphic traps are formed when the loose sediments that will eventually be turned into rocks are laid down.  For example, if the sands of a beach are covered with estuarine mud, the buried sediments will become sandstone capped by impermeable siltstones, thus forming an ideal reservoir and trap.  Structural traps tend to be easier to locate, and most known hydrocarbon reserves are held within such traps. Expanding our reserves therefore requires locating more stratigraphically trapped hydrocarbons.

Finding oil when surface geology does not indicate its presence is done through seismology. A seismic pulse is transmitted into the earth; when the pulse encounters an interface where density changes, some of the energy is reflected upward. A string of seismophones records these reflections, and, with computer processing, seismologists can build up a record of their intensity and the time it takes them to reach the surface. The primary limitation of the seismic method is resolution; it is not possible to resolve features that are thinner than a seismic wavelet. The most common stratigraphic traps are found in sandstone layers thinner than a seismic wavelet. Increasing the frequency of the seismic pulse can narrow seismic wavelets, but high frequencies are selectively attenuated as the pulse travels through the earth. Moreover, the density contrasts between oil-bearing sandstones and the shales that provide stratigraphic seals for the oil are often small, so 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. It is hoped that these techniques will prove fruitful in expanding known hydrocarbon reserves.    1 
 1. 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 that provide stratigraphic seals
C. the sandstones have much greater permeability than the shales
D. the frequency of the seismic pulse is not high enough
E. they are thinner than the seismic wavelet

2 
 2. The example referred to in the  highlighted text  is used to

A. explain how such a trap might typically be found
B. contrast a typical stratigraphic trap with a typical structural trap
C. explain why sandstones covered by siltstones make an ideal reservoir and trap
D. illustrate the point that stratigraphic traps are formed when sediments are laid down
E. show why stratigraphic traps can be difficult to locate seismically

3 
 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. a means of penetrating the seal
E. high permeability within the reservoir

4 
 4. Which of the following best describes how the author views seismology as a tool in locating hydrocarbons?

A. Presently ineffective but showing promise
B. Intrinsically flawed
C. Effective and profitable
D. Beneficial but problematic
E. Theoretically useful but ineffectual in practice

RC Butler 2023  -  Practice Two RC Passages Everyday.   
Passage # 290 Date: 22 Aug-2023
 This question is part of RC Butler 2023 .  Click here for Details

Ruchi007 · Answer

Sajjad1994  Explanation for #4?

Sajjad1994 · Answer

Official Explanation

4. Which of the following best describes how the author views seismology as a tool in locating hydrocarbons?

Explanation

Analyze the question stem

This Inference question asks you to infer the author's attitude toward seismology.

Research

Refer to the passage map. The third paragraph is about seismology, and the fourth is about ways to potentially improve it.

Make a prediction

The author discusses how seismology works, but admits it has some problems currently and suggests a few ways those problems might in future be solved. The correct answer will be one that expresses this cautiously optimistic attitude.

Evaluate the answer choices

(D)  sums up well the whole of the attitude in the third and fourth paragraphs and is the correct answer.

(A)  and  (E)  say the approach is ineffective and are thus too negative; there are problems at times, but the author does not seem to feel the whole process is  "ineffective."   (B)  is far too negative.  (C)  fails to note that there are problems, which may impact both effectiveness and profit.

TAKEAWAY:  Watch out for extreme choices, especially ones that are partly correct.

Answer: D

otk · Answer

where can i find the suggested solution of all RC? Answers explanation can only be found when you post it.

Lodz697 · Answer

Sajjad1994

Could you post the Q3 explanation?

Thanks in advance!

Sajjad1994 · Answer

Official Explanation

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

Explanation

Analyze the question stem

This is a Detail EXCEPT question, which means that all of the incorrect answers will be stated in the passage. The correct answer is either not stated in the passage or is stated but is not a requirement for extracting oil.

Research

Review the passage map: the second paragraph is all about how oil is trapped, while the third covers how to extract oil.

Make a prediction

It is difficult to make a precise prediction for questions that ask what is not stated by the passage, but you can predict possible wrong answers. For oil to be extracted from a reservoir, it first has to get into the reservoir— "hydrocarbons migrate upward,"  according to  paragraph 2.  Then the reservoir itself must be made of permeable rock but have impermeable rock above it. And of course one must be able to drill through the impermeable rock to get to the oil. Choices that relate to any of these details can be eliminated.

Evaluate the answer choices

Review each answer choice until one is found that is not necessary for oil extraction according to the passage.  (C)  is not supported by the passage: while it sounds as if a high-density contrast makes it easier to find the reservoirs with seismic wave, that doesn't mean it's necessary to extract the oil; perhaps a reserve could be found with different means.

(A)  is supported; it is said that without an impermeable seal, hydrocarbons are lost.

(B)  is supported, because without a source of hydrocarbons rising up to the reservoir, there would be no reservoir at all.

(D)  is necessary to get to the oil, and  (E)  is necessary for the reservoir to form in the first place.

Answer: C

dhruva09 · Answer

Sajjad1994  I do get the term ineffective is towards extreme, but there is present mentioned along with it, also in no sentence in the passage has the author mentioned that there is any benefit obtained from it yet. Also presently ineffective seems absolutely apt with the way, the author has placed the point on the drawback of resolution, which is now being worked on. Can you pls provide a better reasoning why option 'D' is correct ?

Locating hydrocarbons is difficult, and by now all obvious reserves of

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Reading Comprehension (RC) Questions

Locating hydrocarbons is difficult, and by now all obvious reserves of

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards