UNIT 7 OIL TRAPS

7.1. Remember the meaning of the following words:

density - плотность

survey – изыскания, исследования

bear witness - свидетельствовать

offshore areas – прибрежные районы

assume - предполагать

displace – вытеснять

interrupt - прерывать

folding - складчатость

faulting - образование сбросов

fault trap – ловушка, обусловленная наличием сбросов

salt dome – соляной купол

ductile crustal deformation – упругая деформация земной коры

flank - склон

closure – замкнутая структура

underlay – залегать ниже

brittle crustal deformation – хрупкая деформация земной коры

updip – вверх по восстанию пласта

intrusion - внедрение

deep-seated – глубоко залегающий

punch - пробивать

truncate - усекать

plug - пробка

remnant - остаток

lateral – боковой, поперечный

laterally – горизонтально, вбок

lenticular sand – линзовидный песок

fissure - трещина

cavern – пещера, каверна

unconformity – несогласное напластовывание

tilt – наклон

exploration drilling - разведочное бурение

development well - эксплуатационная скважина

7.2. Read and translate the text using a dictionary:

Oil traps

Hydrocarbons are of lower density than formation water. Thus, if no mechanism is in place to stop their upward migration they will eventually seep to the surface. On seabed surveys in some offshore areas we can detect craters which also bear witness to the escape of oil and gas to the surface. It is assumed that throughout the geologic past vast quantities of hydrocarbons have been lost in this manner from sedimentary basins.

The direction of movement of oil from the place it was formed to where it accumulates because its further movement is blocked by a trap, is thought to be upward. Oil or gas (or both) rises as it displaces the sea water which originally filled the pore spaces of the sedimentary rock. Its progress is interrupted when it reaches a barrier of impervious rock that traps or seals the reservoir.

Oil traps are of many kinds divided broadly into 'structural' and 'stratigraphic' traps.

Structural traps

Structural traps result from some local deformation such as folding, faulting or both, of the reservoir rock and a cap rock. Typical examples are anticlinal and fault traps and traps connected with salt domes.

In an anticlinal trap which is the result of ductile crustal deformations a reservoir sand and a reservoir limestone are capped by impervious beds which also cover the flanks of both reservoirs, providing closure and preventing the horizontal escape of oil and gas The upper part of each reservoir contains gas underlain by oil-saturated rock; the pore space of the lower part is filled with salt water.

A fault trap which is the result of brittle crustal deformations provides closure for the sand reservoir by bringing an impervious layer alongside it on the updip side but not for the limestone reservoir m which oil and gas could not accumulate be­cause they would escape updip through the sand.

Traps are sometimes formed by the local intrusion of deep-seated rocks into overlying sediment. Rock salt is a frequent intruder forming ’salt domes’ – cylindrical, steeply conical or mushroom-shaped masses of rock salt, formed when salt was forced to flow plastically under very high pressure, punching its way up from deep-seated beds through the overlying layers. Porous formations, if present, have been truncated and effectively sealed by the salt plug. Oil may accumulate against the plug or above it in reservoir formations that have been folded by the rising plug, or in the porous remnant of older strata pushed up on top of the salt.

Stratigraphic traps

Sedimentary layers may change laterally in lithologic com­position or may die out and reappear elsewhere as a different type of rock. Such changes often cause a lateral decrease in porosity and permeability, and the more porous section of the layer may form a ‘stratigraphic’ trap. Oil accumulations also occur in traps formed by lenticular sand masses completely enclosed in tight sediments.

Limestone in itself is often impervious but may contain fissures and caverns that can form stratigraphic traps. The remains of an ancient coral reef buried by impervious sedi­ments can also form a stratigraphic trap.

A different kind of a stratigraphic trap may be formed by un­conformities when a succession of layers, including a potential oil reservoir, have been uplifted, tilted, cut by erosion, and finally overlaid by impervious sediments that act as cap rock

Oil accumulations may result not only from any one of the above-mentioned types of trap, but from a combination of two or more types. Some traps are not easy to recognize and it is not surprising, therefore, that even the most mod­ern geophysical methods may fail to give an indication of their presence, in which case only exploration drilling can provide sufficient information, at a cost considerably high than that of other exploration methods.

Even after the initial discovery of an exploitable oil accumulation, geological conditions may be so complicated that it may take years to drill many development wells before the detailed pattern of oil occurrence in the area is fully understood.

7.3. Answer the questions on the text:

1. When is movement of oil interrupted? 2. What do structural traps result from? 3. What are typical examples of structural traps? 4. How are traps formed sometimes? 5. What can form stratigraphic traps? 6. What may oil accumulation result from? 7. Why do modern geophysical methods fail to give an indication of traps presence? 8. How can sufficient information be provided?

7.4. Give English equivalents to the following word combinations from the text:

таким образом, в конце концов, огромное количество, возникать в результате, заставлять насильно, исчезать и появляться вновь, заглубляться под действием, вышеупомянутые типы, предоставить достаточно информации, разрабатываемая залежь нефти.

7.5. Match the words in column A with their equivalents in column B:

A B

1. thus 2. quantity 3. lose 1. beside 2. bed 3. in this manner

4. broadly 5. remnant 6. form 4. famous 5. enough 6. waste

7. finally 8. recognize 9. sufficient 7. widely 8. supply 9. remainder

10. provide 11. connect 12. layer 10. put together 11. amount

13. alongside 14. well-known 12. identify 13 eventually 14. join

7.6. Choose the only correct variant to fill in the gaps:

1. Structural traps result from some ………

a) combination b) deformation c) accumulation d) sedimentation

2. A reservoir sand and a reservoir limestone are capped by …

a) permeable layers b) overlying sediment c) impervious beds d) pores

3. A fault trap provides …. for the sand reservoir.

a) closure b) salt dome c) sediment d) salt

4. Rock salt is a frequent …. forming salt domes.

a) strata b) intruder c) mass d) kind

5. Such changes often cause a lateral ….. in porosity and permeability.

a) increase b) failure c) cavern d) decrease

6. Limestone is often …. but may contain fissures and caverns.

a) permeable b) cut c) impervious d) buried

7. Some traps are not easy to ……..

a) change b) bury c) form d) recognize

7.7. State whether the sentences are true (T) or false (F):

1. Hydrocarbons are of much higher density than formation water.

2. The progress of oil and gas is interrupted when they reach a barrier of

impervious rock.

3. Anticlinal and fault traps are connected with associated water.

4. Rock salt seldom intrudes in formation of salt domes.

5. A stratigraphic trap can be formed by the remains of an ancient coral reef.

6. Oil accumulations may result only from one type of traps.

7.8. Retell the text using the following word combinations as a plan:

The direction of movement, to block by a trap, to be upward, impervious rock, types of traps, to result from, oil accumulations, a combination of, to fail to give, exploration drilling development wells.

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