3.1.Remember the meaning of the following words:
warped - , earths crust
edge margin , foothill
lowland - border ;
boundary - igneous rock - ()
consolidate - evaporation products -
nucleus (. . nuclei) shield -
pebble , limestone -
valley - plain -
arid climatic conditions
favour - algae
peat - fossil ,
assign- , (.) succession , equidistant -
3.1. Read and translate the text using a dictionary:
Geological features
Oil basins
Suitable conditions for the formation and accumulation of oil exist in the down warped segments of the earths 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 earths 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 worlds 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 earths 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.
3.3. 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?
3.4. Give the English equivalents to the following word combinations from the text:
; , ; , , , , , , , .
UNIT 4 OIL TRAPS
4.1.Remember the meaning of the following words:
density - survey ,
bear witness - offshore areas
assume - displace
interrupt - folding -
faulting -
fault trap ,
salt dome
ductile crustal deformation
flank - closure
underlay
brittle crustal deformation
updip intrusion -
|
|
|
deep-seated punch -
truncate - plug -
remnant - lateral ,
laterally , lenticular sand
fissure - cavern ,
unconformity
tilt exploration drilling -
development well -
4.2. 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.
4.3. 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?
4.4. Give English equivalents to the following word combinations from the text:
, , , , , , , , , .
