Petroleum geology
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration).
Petroleum geology is principally concerned with the evaluation of seven key elements in sedimentary basins: source, reservoir, seal, trap, timing, maturation and migration. In general, all these elements must be assessed via a limited 'window' into the subsurface world, provided by one (or possibly more) exploration wells.
These wells present only a 1-dimensional segment through the Earth and the skill of inferring 3-dimensional characteristics from them is one of the most fundamental in petroleum geology. Recently, the availability of cheap and high quality 3D seismic data (from reflection seismology) has greatly aided the accuracy of such interpretation.
Several major subdisciplines exist in petroleum geology specifically to study the seven key elements discussed above.
Analysis of source rocks
In terms of source rock analysis, several facts need to be established. Firstly, the question of whether there actually is any source rock in the area must be answered. Delineation and identification of potential source rocks depends on studies of the local stratigraphy, palaeogeography and sedimentology to determine the likelihood of organic-rich sediments that were deposited in the past.
If the likelihood of there being a source rock is thought to be high, the next matter to address is the state of thermal maturity of the source, and the timing of maturation. Maturation of source rocks depends strongly on temperature, such that the majority of oil generation occurs in the 60° to 120°C range. Gas generation starts at similar temperatures, but may continue up beyond this range, perhaps as high as 200°C. In order to determine the likelihood of oil/gas generation, therefore, the thermal history of the source rock must be calculated. This is performed with a combination of geochemical analysis of the source rock (to determine the type of kerogens present and their maturation characteristics) and basin modelling methods.
Analysis of reservoir
The existence of a reservoir rock (typically, sandstones and fractured limestones) is determined through a combination of regional studies (i.e. analysis of other wells in the area), stratigraphy and sedimentology (to quantify the pattern and extent of sedimentation) and seismic interpretation. Once a possible hydrocarbon reservoir is identified, the key physical characteristics of a reservoir that are of interest to a hydrocarbon explorationist are its porosity and permeability. Traditionally, these were determined through the study of hand specimens, parts of the reservoir that outcrop at the surface and by the technique of formation evaluation using wireline tools passed down the well itself. Modern advances in seismic data acquisition and processing have meant that seismic attributes of subsurface rocks are readily available and can be used to infer physical/sedimentary properties of the rocks themselves.
Ex. 7. Decide whether the following statements are True or False, based on the text. Correct the false statements.
Geophysical data has been known to people for a long time.
Geologists try to understand what happened in the Earth’s crust millions of years ago.
Geological processes repeat themselves.
During erosion, rocks are pushed upwards and become exposed.
The Earth’s crust consists mainly of sedimentary rocks.
The process of weathering usually means water or air impact on the rock.
The faster the accumulation of clastic fragments, the finer the debris.
Grains of the rock deform during compaction.
Claystones have biogenic origin.
Petroleum geology concentrates on sedimentary rocks.
Important geological data can be obtained through a well.
Geologists can use the data from the wells directly in their work.
Source rock is rich in organic matter.
Geochemical analysis allows to determine the presence of a source rock.
To determine the presence of reservoir rock, it is usually not enough to drill one well.
Porosity and permeability of the rock are easier to determine today than before.
Ex. 8. Choose the correct variant.
The rock cycle begins when rocks are pushed up by (tectonic / gravitational) forces.
The exposed rocks are (consolidated / eroded) under the impact of surface processes.
The eroded particles, or (sediment / deposit) travel until they are deposited into (layers / formations).
When compacted by the overlying layers, these layers form (sedimentary / clastic) rocks.
Weathering involves (compaction / breakdown) of the rock surface.
Sandstones are generated by (rapid / slow) accumulation of (clastic / carbonate) debris.
Laminated muds are produced by slow buildup of (fine-grained / coarse-grained) particles.
Cementation of the sediment particles occurs thanks to (organic matter / carbonate minerals) squeezed from pores of a rock.
Carbonate rocks have (biogenic / abiogenic) origin.
Geologists have to reconstruct (3-dimensional / 2-dimensional) characteristics from exploration wells.
Identification of potential source rocks is aimed at determining the likelihood of (mineral-rich / organic-rich) sediments that were deposited in the past.
If the source rock has been identified, the next task is to assess the state of (thermal / geological) maturity of the source.
Maturation of source rocks depends strongly on (pressure / temperature).
Once a possible hydrocarbon reservoir is identified, the key physical characteristics of a reservoir are its (density / porosity) and permeability.
Ex. 9. VOCABULARY. In the text above, find the equivalents to the following words and phrases. (The sequence corresponds to their occurrence in the text.)
буквально
применимый
давать, приносить
недра
земная кора
открытый, наружный
горный хребет
свидетельство
ведущий принцип
закрытый цикл
цикл горной породы
тектонические силы
подвергаться эрозии
поверхность
частицы
отложение
откладываться
уплотнять
осадочные породы
масштабный
магматические породы
метаморфические породы
происходить из
выветривание
разрушение
взаимодействие
способ
обстановка осадконакопления
быстрый
накопление
обломки
грубый, крупнозернистый
мелкозернистый
тонкослоистый
уплотнение
поровое пространство
деформация
зерно
выдавливать
цементировать
песчаник
алевролит
аргиллит
биогенный
кальцит
известняк
относиться к
коллектор
покрышка
ловушка
вызревание, степень зрелости
миграция
разведочная скважина
одномерный
выводить, делать вывод
основополагающий
сейсморазведка методом отраженных волн
способствовать
точность
нефтематеринская порода
уточнение границ (залежи и т.п.)
стратиграфия
палеогеография
седиментология
вероятность
термическая зрелость
кероген
бассейн
определить количественно
структура, схема
пористость
проницаемость
штуф, образец породы
выходить на поверхность
оценка пластовых свойств
тросовый (кабельный) инструмент
приобретение
свойство, качество
Ex. 10. The Vocabulary List contains a number of important terms apart from those which are given at the beginning of the Unit. Match the terms from the box with their definitions.
a) crust |
i) stratigraphy |
b) sediment |
j) migration |
c) sedimentary rock |
k) palaeogeography |
d) igneous rock |
l) sedimentology |
e) metamorphic rock |
m) basin |
f) reservoir |
n) porosity |
g) seal |
o) permeability |
h) trap |
p) kerogen |
The study of the geography of ancient times or ancient epochs.
A depression in the crust of the Earth, caused by plate tectonic activity and subsidence, in which sediments accumulate.
A relatively impermeable rock, commonly shale or salt, that forms a barrier or cap above and around reservoir rock such that fluids cannot migrate beyond the reservoir.
The naturally occurring, solid, insoluble organic matter that occurs in source rocks and can yield oil upon heating.
The percentage of pore volume or void space, or that volume within rock that can contain fluids.
The unconsolidated grains of minerals, organic matter or preexisting rocks, that can be transported by water, ice or wind, and deposited.
One of the three main classes of rock that crystallizes from molten rock, or magma.
The science of the production, composition, transport, and deposition of sediment.
One of the three main classes of rock that form from the alteration of preexisting rocks by changes in ambient temperature, pressure, volatile content, or all of these.
The ability, or measurement of a rock's ability, to transmit fluids, typically measured in darcies or millidarcies.
The movement of hydrocarbons from their source into reservoir rocks.
A configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate.
A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids.
The thin, outermost shell of the Earth that is typically 5 km to 75 km thick.
The study of the history, composition, relative ages and distribution of strata, and the interpretation of strata to reveal Earth history.
One of the three main classes of rock that is formed at the Earth's surface through deposition of sediments derived from weathered rocks, biogenic activity or precipitation from solution.
Ex. 11. From the Vocabulary List above, find synonyms to the following items.
help
conclude, deduce
widespread
design, structure
way, manner
destruction
fast
give, produce
probability
feature
originate from
basic, essential
Ex. 12. Match words from Column A to the words from Column B in order to form phrases. Translate the resulting phrases into Russian. Make up five sentences with these phrases.
Column A
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Column B |
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Ex. 13. Insert appropriate items from the box into the following sentences. Sometimes you will have to change the form of the word.
a) maturation |
f) infer |
k) seal |
p) erode |
b) biogenic |
g) quantify |
l) accuracy |
q) kerogen |
c) yield |
h) weathering |
m) sandstone |
r) pattern |
d) fine-grained |
i) interior |
n) deposit |
s) applicable |
e) permeability |
j) attribute |
o) trap |
t) debris |
Sediments carried by the stream are __________ where the slowing water can no longer move them.
Their objective was to develop a model which would __________ the permeability of rock as drilling proceeded.
__________ mudstone looks like hardened clay and, depending upon circumstances under which it was formed, it may show cracks
Occasionally the impermeable layers are dense igneous rocks that prevent fluid flow and act as a __________.
When heated to the right temperatures in the Earth's crust, some types of __________ release hydrocarbons.
The author of the article reveals the timing between tectonic burial and thermal __________ of hydrocarbons.
Chemical __________ is a gradual and slow process as the mineralogy of the rock adjusts to the near surface environment.
The device sends back measurements of global sea surface temperatures with very high __________.
Fragments of building __________ can form a large part of the total number of finds from a site.
For a rock to be considered as an exploitable hydrocarbon reservoir without stimulation, its __________ must be greater than approximately 100 mD
Sandstone is made from sand grains __________ from older rocks, cemented together and then hardened into new rock.
He had all the __________ of a great leader: charisma, energy, discipline, and resourcefulness.
The techniques are generally __________ to all areas of seismic survey.
The theory of __________ petroleum origin has long been accepted within broad scientific circles.
From archaeological evidence we can__________ that these people used stone cutting tools.
Each of these oilfields could__________ billions of barrels of oil.
Rock formations that are primarily composed of __________ are porous enough to store large quantities, making them valuable petroleum reservoirs.
Sedimentary rocks usually have a layered __________, resembling a cake.
Structural __________ are formed by a deformation in the rock layer that contains the hydrocarbons.
Heat from the core is trapped in the Earth's __________.
Ex. 14. Translate the following sentences into English, using vocabulary from the text. (The items are underlined.)
Одним из ведущих принципов геологии считается цикл горной породы.
Геологические структуры или ловушки, где накапливается нефть, называются коллекторами.
Осадочные горные породы образуются в результате выветривания и эрозии материнских пород, переноса и осаждения вещества.
В песчаниках часто развиваются значительные деформации и уплотнения.
Осадочные породы составляют лишь небольшой процент земной коры, однако они часто обнажены на поверхности в виде горных хребтов.
Породы могут подвергаться эрозии, привнося свои частицы в осадочные отложения новых поколений.
В результате активности тектонических сил породы могут оказаться погребенными глубоко в недрах Земли.
Магматические горные породы — это породы, образовавшиеся непосредственно из магмы.
Осадочные породы можно классифицировать по размеру зерен: самым крупнозернистым, как правило, является песчаник, а самым мелкозернистым – аргиллит.
Эта порода представлена хорошо сцементированными тонкослоистыми глинистыми отложениями.
Метаморфические горные породы происходят из других пород вследствие изменения физико-химических условий.
Геологи установили, что обстановка осадконакопления этих отложений была морской, и накопление обломков породы не было быстрым.
О масштабах деятельности живых организмов свидетельствует присутствие мощных биогенных пород, в частности известняка.
Миграция нефти может происходить как по заключающим её пористым породам, так и по трещинам.
Большинство залежей в мире связано с ловушками структурного типа.
Для образования коллектора необходимо наличие непроницаемой горной породы – покрышки, ограничивающей миграцию углеводородов по вертикали.
Цвет и прозрачность могут служить показателями термической зрелости керогена.
Сейсморазведка методом отраженных волн применяется для обнаружения подземных образований.
В докладе представлена модель, позволяющая количественно определить основные параметры коллекторов для подсчета запасов углеводородного сырья.
Пористость и проницаемость породы можно довольно точно определить по штуфам.
Speaking and Communication Skills
Ex. 15. GEOLOGICAL TIME SCALE. Read the names of epochs, periods and eras. Mind the pronunciation!
Eras:
Palaeozoic – [pæliə'zəuik] – Палеозойская (эра)
Mesozoic – [mezə(u)'zəik] – Мезозойская (эра)
Cenozoic – [si:nə(u)'zəik] – Кайнозойская (эра)
Epochs:
Palaeocene – ['pæliəusi:n] – Палеоцен (эпоха)
Eocene – ['i: əusi:n] – Эоценовая (эпоха)
Oligocene – [:'ligəusi:n] – Олигоцен (эпоха)
Miocene – ['maiəusin] – Миоцен (эпоха)
Pliocene – ['plaiəsi:n] – Плиоценовая (эпоха)
Pleistocene – ['pli:stəusi:n] – Плейстоценовая (эпоха)
Periods and systems:
Precambrian –[pri: 'kæmbriən] – Докембрийский / докембрий (период)
Cambrian – [kæmbriən] – Кембрийский (период)
Ordovician – [:də'viiən] – Ордовикская (система)
Silurian – [səi'lju(ə)riən] – Силурийский (период)
Devonian – [de'vəunjən] – Девонский (период)
Carboniferous – [ka:bə'nif(ə)rəs] – Каменноугольный (период)
Permian – ['рз:miən] – Пермская (система)
Triassic – [trai'æsik] – Триасовая (система)
Jurassic – [dju'ræsik] – Юрский (период)
Cretaceous – [kri'teiəs] – Меловой (период)
Tertiary – ['tз:(ə)ri] – Третичная (система)
Quaternary – [kwə'tз:nəri] – Четвертичный (период)
Ex. 16. Work in pairs. As you look at the scheme in the next page, think of one epoch, period or era. Make dialogues. Your partner should ask questions and guess what you think about. Be careful with the order of epochs and periods! The latest ones are placed higher in the table.
Model:
Student A: Is it the Pliocene epoch?
Student B: No, it is earlier than the Pliocene epoch.
Student A: How many million years ago was this period or epoch?
Student B: It was between 208 and 144 million years ago.
Student A: Is it Jurassic period?
Student B: Correct!
Eras |
Periods / Epochs |
Events |
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Cenozoic - Quaternary |
Holocene Epoch 10kya till present |
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Pleistocene Epoch 1.6-0.01 Mya |
Climate fluctuating cold to mild. The era of ice ages. Sahara formed. |
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Cenozoic - Tertiary 66.4-1.6 Mya |
Pliocene Epoch 5-2 Mya |
Cooler climate; continued uplift and mountain building, with widespread glaciation in Northern Hemisphere. |
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Miocene Epoch 25-5 Mya |
Moderate climate; extensive glaciation begins again in Southern Hemisphere. Moderate uplift of Rocky Mountains. |
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Oligocene Epoch 38-25 Mya |
Rise of Alps and Himalayas. Lands generally low. Volcanoes in Rocky Mountains. South America separates from Antarctica. |
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Eocene Epoch 55-38 Mya |
Mild to very tropical climate. Many lakes in western North America. Australia separates from Antarctica; India collides with Asia. |
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Paleocene Epoch 65-55 Mya |
Mild to cool climate. Wide, shallow continental seas largely disappear. |
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Mesozoic 245-66.4 Mya |
Cretaceous Period 144-66.4 Mya |
Tropical to subtropical climate. Elevation of Rocky Mountains at end of period. Africa and South America separate. |
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Jurassic Period 208-144 Mya |
Mild climate. Continents low, with large areas covered by seas. Mountains rise from Alaska to Mexico. |
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Triassic Period 245-208 Mya |
At first, deserts stretched out over most of the land, slowly giving way to a mild, moist climate with great areas of forested plains. Continents mountainous and joined in one mass. |
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Paleozoic 540-245 Mya |
Permian Period 286-245 Mya |
Extremely violent climate changes: deserts, swamps, ice. Extensive glaciation in Southern Hemisphere. Seas drain from land; worldwide aridity. Urals formed. |
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Carboniferous Period 360-286 Mya |
Slower earth movements. Seabeds began to rise. Climate warm; conditions like those in subtropical zones; little seasonal variation, water plentiful. |
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Devonian Period 408-360 Mya |
Violent change in the Earth's landscape by volcanic activity and crustal movements, folding and mountain forming. The Earth started to look green. |
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Silurian Period 438-408 Mya |
Mild climate. Continents generally flat; again flooded. Modern groups of algae and fungi. |
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Ordovician Period 505-438 Mya |
Mild climate. Shallow seas. All plants and animals still restricted to the water. First fungi. Possible invasions of land by plants. |
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Cambrian Period 540-505 Mya |
Mild climate; extensive seas, spilling over continents. |
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Precambrian 3960-590 Mya |
Proterozoic Period 2500-540 Mya |
Dry and cold climate to warm and moist. Earliest known fossils. |
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Archean Period 3960-2500 Mya |
Extensive mountain building. Shallow seas. Accumulation of free oxygen. Origin of life. |
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Azoic 4500-3960 Mya |
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Planet forms from accumulation of cosmic dust and from meteorite impacts. |
Ex. 17. On the basis of information in the table above, tell about the following, beginning from the earliest times of the planet’s existence. Remember to mention periods and epochs, when the particular changes occurred.
Changes in climate
Changes in the Earth geology
Ex. 18. Translate the following passage into English.
Месторождение открыто в 2005 году. Бурением вскрыт разрез мезозойско-кайнозойских карбонатно-терригенных пород максимальной толщиной 1730 м, который представлен образованиями триасовой, юрской, меловой, палеогеновой, неогеновой и четвертичной систем. В вертикальном разрезе выделяются три главных тектонических уровня. Нижний структурный уровень (фундамент) представлен палеозойскими отложениями. Верхний тектонический уровень – платформенный чехол (platform mantle), сложен осадками юрско-неогенового возраста. Между нижним и верхним тектоническим этажом залегает промежуточная метаморфизованная пермо-триасcовая толща.
Ex. 19. Read the text below and fill in the gaps with the words from the box.
a) pressure |
g) strata |
m) carbonate |
b) land area |
h) breakdown |
n) parent rock |
c) cementation |
i) igneous |
o) calcite |
d) living organisms |
j) water |
p) superposition |
e) erosion |
k) fossils |
q) lithification |
f) unconformities |
l) suspension (суспензия, взвесь) |
r) reaction |
Sedimentary rock is one of the three main rock types (the others being (1) __________ and metamorphic rock). Rock formed from sediments covers 75-80% of the Earth's (2) __________, and includes common types such as chalk, limestone, dolomite, sandstone, conglomerate and shale.
Sedimentary rocks are classified by the source of their sediments, and are produced by one or more of:
clastic rock formed from fragments broken off from (3) __________, by
weathering in situ (на месте) or
(4) __________ by water, ice or wind, followed by transportation of sediments, often in suspension, to the place of deposition;
biogenic activity; or precipitation from solution.
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The sediments are then compacted and converted to rock by the process of (5) __________. Sedimentary rocks are formed because of the overburden (6) __________ as particles of sediment are deposited out of air, ice, wind, gravity, or water flows carrying the particles in (7) __________. As sediment deposition builds up, the overburden (or 'lithostatic') pressure squeezes the sediment into layered solids and the original connate fluids are expelled. |
The term diagenesis is used to describe all the chemical, physical, and biological changes, including (8) __________, undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.
Sedimentary rocks are laid down in layers called beds or (9) __________. That new rock layers are above older rock layers is stated in the principle of (10) __________.There are usually some gaps in the sequence called (11) __________. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.
Sedimentary rocks contain important information about the history of Earth. They contain (12) __________, the preserved remains of ancient plants and animals. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time.
All rocks disintegrate when exposed to mechanical and chemical weathering at the Earth's surface.
Mechanical weathering is the (13) __________ of rock into particles without producing changes in the chemical composition of the minerals in the rock. Ice is the most important agent of mechanical weathering. (14) __________ percolates into cracks and fissures within the rock, freezes, and expands. The force exerted by the expansion is sufficient to widen cracks and break off pieces of rock.
Chemical weathering is the breakdown of rock by chemical (15) __________. In this process the minerals within the rock are changed into particles that can be easily carried away. Air and water are both involved in many complex chemical reactions.
Organic sedimentary rocks contain materials generated by (16) __________, and include (17) __________ minerals created by organisms, such as corals, mollusks, and foraminifera, which cover the ocean floor with layers of (18) _________ which can later form limestone.
Ex. 20. Read the following text about types of traps and note down the highlighted words describing the traps.
There are three basic forms of a structural trap in petroleum geology: Anticline Trap, Fault Trap, and Salt Dome Trap. The common link between these three is simple: some part of the earth has moved in the past, creating an obstacle to oil flow.
Anticline Trap
An anticline is an example of rocks which were previously flat, but have been bent into an arch. Oil that finds its way into a reservoir rock that has been bent into an arch will flow to the crest of the arch, and get stuck (provided, of course, that there is a trap rock above the arch to seal the oil in place).
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A cross section of the Earth showing typical Anticline Traps. Reservoir rock that isn't completely filled with oil also contains large amounts of salt water. |
Fault Trap
Fault traps are formed by movement of rock along a fault line. In some cases, the reservoir rock has moved opposite a layer of impermeable rock. The impermeable rock thus prevents the oil from escaping. In other cases, the fault itself can be a very effective trap. Clays within the fault zone are smeared (размываются) as the layers of rock slip past one another. This is known as fault gouge (сбросовая глина).
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A cross section of rock showing a fault trap – in this case, an example of gouge (оторочка). This is because the reservoir rock on both sides of the fault would be connected, if not for the fault separating the two. In this example, it is the fault itself that is trapping the oil. |
Salt Dome Trap
Salt is a peculiar substance. If you put enough heat and pressure on it, the salt will slowly flow, much like a glacier that slowly but continually moves downhill. Unlike glaciers, salt which is buried kilometers below the surface of the Earth can move upward until it breaks through to the Earth's surface, where it is then dissolved by ground- and rain-water. To get all the way to the Earth's surface, salt has to push aside and break through many layers of rock in its path. This is what ultimately will create the oil trap.
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Here we see salt that has moved up through the Earth, punching through and bending rock along the way. Oil can come to rest right up against the salt, which makes salt an effective trap rock. However, many times, the salt chemically changes the rocks next to it in such a way that oil will no longer seep into them. In a sense, it destroys the porosity of a reservoir rock.
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Ex. 21. Retell the information above, using the highlighted words and phrases.
Ex. 22. DESCRIBING PICTURES. Look at the following pictures representing different types of traps. Describe one of them to your partner in as many details as possible. Your partner should guess what trap you are describing.
Useful vocabulary:
strata / layer – слой
parallel / crossing – параллельный / пересекающий
overlying – вышележащий
underlying – нижележащий
sloped / inclined – наклонный
vertical – вертикальный
horizontal – горизонтальный
dome-shaped – куполообразный, сводовый
wave-like – волнообразный
fault – разлом (геол. структура)
fold – складка (геол. структура)
fracture – трещина (геол. структура)
2-D image / 3-D image – двухмерное изображение / трехмерное изображение
arch – куполовидное поднятие
crest of the arch – хребет, перегиб складки
to pinch – выклинивать
to bend – гнуться
A |
B
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С |
D |
Grammar
Ex. 23. Study the following information.