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

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. 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. 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. 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. 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