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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, 04:34
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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, 19:09

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Re: After being formed deep within the earth, hydrocarbons migrate upwards &nbs [#permalink] 02 Oct 2018, 19:09
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After being formed deep within the earth, hydrocarbons migrate upwards

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