OIL
TRAPS
Remember the meaning of the following
words:
1. density -
плотность
2.survey – изыскания,
исследования
3. bear witness - свидетельствовать
4. offshore areas –
прибрежные районы
5. assume -
предполагать
6. displace – вытеснять
7. interrupt -
прерывать
8. folding -
складчатость
9. faulting - образование сбросов
10. fault trap –
ловушка, обусловленная наличием сбросов
11. salt dome – соляной
купол
12. ductile crustal
deformation – упругая деформация земной коры
13. flank - склон
14. closure – замкнутая
структура
15. underlay – залегать
ниже
16. brittle crustal
deformation – хрупкая деформация земной коры
17. updip – вверх по
восстанию пласта
18. intrusion - внедрение
19. deep-seated –
глубоко залегающий
20. punch -
пробивать
21. truncate - усекать
22. plug - пробка
23. remnant - остаток
24. lateral – боковой,
поперечный
25. laterally –
горизонтально, вбок
26. lenticular sand –
линзовидный песок
27. fissure - трещина
28. cavern – пещера,
каверна
29. unconformity –
несогласное напластовывание
30. tilt – наклон
31. exploration drilling
- разведочное бурение
32. development well
- эксплуатационная скважина
Read and translate the text using a
dictionary:
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.