Geological features

Oil basins

Suitable conditions for the formation and accumulation of oil exist in the down warped segments of the earth’s crust where layers of sediment have accumulated to great thick­ness, thickest in the middle and thinner towards the edges. Such areas are called ‘sedimentary basins’ and are con­sidered as potential ‘oil basins’ worth investigating for the presence of oil until its absence is definitely proved. Topo­graphically they are generally low and many occur along the continental margins and in the foothills and lowlands bordering mountain chains.

Oil occurs in various formations, although all the oil probably originated from only one formation, the Mesozoic, which covers the floor of the basin.

Several oil basins extend seaward as part of the contin­ental shelf bordering the continents, the limit of which is roughly defined by the 200 metre water depth contour. Exploration of continental shelves has progressed considerably. For example, in the Gulf of Mexico oil has been found 60 miles off the US coast and the search now extends to about 100 miles. Boundaries between states whose coasts are either adjacent or opposite to each other are generally agreed on to be the median line, i.e. a line formed by points equidistant from the nearest points of the coast line of either state.

Rock types

Rocks are divided into three main groups: igneous rocks, which include granite and volcanic rocks, consolidated from hot liquid material; sedimentary rocks, either fragments of other rocks deposited on land or under the sea by wind and water, or chemically deposited, for instance as evaporation products, or of organic origin; metamorphic rocks, which occur when igneous and sedimentary rocks are exposed to great heat and pressure thus melting and becoming magma.

Igneous and metamorphic rocks cover immense areas of the earth’s crust, forming the central nuclei of the continents, called shields, or they occur as smaller masses (massifs) located all over the world. Their nature normally prevents their bearing oil but if fractured they can act as reservoirs for oil that has migra­ted from overlying sedimentary rocks.

The oil geologist is mainly concerned with the sedimen­tary deposits filling all the world’s potential oil basins, which comprise about a tenth of the land area of the earth.

Sedimentation

The products of denudation or erosion of the higher parts of the earth’s surface are carried away by water, ice and wind in the form of pebbles, sand, silt, mud, etc., and are deposited as sediment in the valleys and plains and in the surrounding seas where they may attain thicknesses of many thousands of feet. Though slow, the action of running water and the like has brought about immense changes during the many millions of years of geological time. The eroded material is washed away by streams and rivers which carry it eventually into the sea where it may be distributed over enormous areas by ocean currents.

Under arid climatic conditions salt and anhydrite de­posits called evaporites may form by evaporation of sea water, for instance in lagoons. Shallow seas of warm clear water favour the growth of corals and algae which are im­portant contributors to the formation of carbonate rocks such as limestone and dolomite. Where vegetable matter accumulates, peat, lignite, and eventually coal beds may form.

The beds of sedimentary rocks deposited in this way are seldom uniform in thickness, composition or texture. The variations may be small or large depending on many factors

Geological age and oil occurrence

By the study of fossils, both large and microscopic, the geologist is able to assign relative ages to sediments and so to determine the succession of beds one above the other. The absolute age of the rocks can be determined from a study of radioactive minerals.

6.3. Answer the questions on the text:

1. Where do suitable conditions for the formation and accumulation of oil exist? 2. How are such areas called? 3. What are the main groups of rocks? 4. What do igneous rocks include? 5. How are sedimentary rocks formed? 6. When do metamorphic rocks occur? 7. What do igneous and metamorphic rocks form? 8. What deposits is the oil geologist mainly concerned with? 9. What deposits may form by evaporation of sea water under arid climatic conditions? 10. What favours the growth of corals and algae? 11. How can the geologist assign relative ages to sediments?

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

подходящие условия; стоящий того, чтобы исследовать; горные цепи, дно бассейна, простираться к морю, подвергаться действию тепла и давления, занимать огромные территории, быть заинтересованным в основном осадочными горными породами, вносить важный вклад, в зависимости от многих факторов.

6.5. Match the words in the left column (A) with their equivalents on the right (B):

A B

1. presence 2. expose 3. assign 1. develop 2. substance 3. stretch

4. immense 5. occur 6. progress 4. name 5. separate 6. behave

7. locate 8. comprise 9. call 7. border 8. be found 9. determine

10. matter 11. extend 12. divide 10. subject to 11. availability 12. huge

13. boundary 14. act 13. situate 14. consist of

6.6. Match each word on the left (A) with a word on the right (B) to make a word combination. Put them down and translate into Russian.

A B

1. mountain 2. continental 3. water 1. rocks 2. currents 3. minerals

4. igneous 5. organic 6. central 4. surface 5. time 6. ages

7. earth’s 8. ocean 9. geological 7. shelves 8. chains

9. contour

10. relative 11. radioactive 12. potential 10. origin 11. basins 12. nuclei

6.7. Match the words in the left column (A) with their opposites on the right (B):

A B

1.thick 2. presence 3. normally 1. above 2. opposite 3. cold

4. towards 5. adjacent 6. small 4. indefinite 5. exact 6. enormous

7. warm 8. include 9. under 7. backwards 8. exclude 9. thin

10. definite 11. rough 10. seldom 11. absence

6.8. Translate the following sentences into Russian in writing:

1. The nature of igneous and metamorphic rocks covering immense areas of earth’s crust normally prevents their bearing oil but if fractured they can act as reservoirs for oil that has migrated from overlying sedimentary rocks.

2. Oil may be generated in sedimentary rocks of almost any age, but the older rocks will naturally have lost more of their oil than have the younger ones through seepages, cracks, erosion or heat from igneous or volcanic rocks.

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

1. Topographically oil basins are normally …….and may occur along continental margins.

a) high b) low c) small d) enormous

2. Exploration of ……… has progressed considerably.

a) depth contours b) groups c) continental shelves d) boundaries

3. Sedimentary rocks are exposed to …….

a) volcanic rocks b) hot liquid materials c) evaporation products

d) heat and pressure

4. If fractured igneous rocks act as ……… for oil.

a) median line b) reservoirs c) shields d) areas

5. Evaporites may form by ……… of sea water.

a) accumulation b) denudation c) evaporation d) erosion

6. Shallow seas …… … the growth of corals and algae.

a) favour b) decrease c) slow down d) cover

7. The geologist is able to assign relative ages to ………

a) composition b) texture c) sediments d) currents

6.10. Summarize the contents of the text in a few sentences.

Дата добавления: 2015-08-13