Page 33 - 4567

SOURCE ROCKS, GENERATION OF PETROLEUM

Task 1. Read and memorize the following words:
decay - розпад oxidation - окислення
bury – поховати, хоронити stagnant – застійний, стоячий
shale – глинистий сланець dangerous - небезпечний
explode - вибухати coal seam – вугільний пласт
coal bed gas – вугільний газ single-celled plant – одноклітинна рослина
algae - водорості pollen - пилок
marsh gas – болотний газ biogenic or microbial gas – біогенний або
мікробний газ
permafrost – вічна мерзлота tcf (trillion cubic feet) – триліони кубічних
футів
oil window – головна зона нафтоутворення gas that is trapped – газ, який є затисненим
basin – нафтогазова провінція thermal cracking – термічний крекінг
irreversibly - незворотньо

Task 2. Read the following text, translate it into Ukrainian.
Source Rocks
A source rock is a rock that can generate natural gas and/or crude oil. Gas and oil form
from ancient organic matter preserved in sedimentary rocks. As sediments are deposited, both
inorganic mineral grains, such as sands and mud, and organic matter (dead plants and animals)
are mixed. Most organic matter is lost on the surface by decay, a process of oxidation. The
decaying organic matter on land gets oxygen from the air, and the decaying organic matter on the
ocean bottom gets the oxygen from out of the water. Some organic matter, however, is
preserved. It was either rapidly buried by other sediments before it decayed or was deposited on
the bottom of a sea with stagnant, oxygen-free waters. The black color in sedimentary rocks
comes primarily from its organic content. Black-colored, organic-rich sedimentary rocks include
coal, shale, and some limestones. When woody plant material is buried, it is transformed into
coal and methane gas (CH 4) by temperature and time. This is why coal mines are dangerous;
they contain methane gas and sometimes explode. Coal deposits are drilled to produce coal seam
or coal bed gas, which is pure methane gas.
Shale is the most common sedimentary rock, and many are black. A black shale
commonly has 1 to 3% organic matter by weight and can have up to 20%. Green or gray shale
has only about 0.5% organic matter. Black shales contain a large variety of organic matter that
includes single-celled plants and animals that live floating in the ocean, algae, spores, pollen and
bacteria. They have the right chemical composition to generate both natural gas and crude oil. In
some areas, such as North Africa and the Middle East, organic-rich dark limestones are also
source rocks.
Generation
The most important factor in the generation of crude oil from organic matter in
sedimentary rocks is temperature. A minimum temperature of about 65˚C is necessary for oil
generation under typical sedimentary basin conditions. This temperature is obtained by
burying the organic-rich source rocks. The deeper the depth the higher the temperature.
At relatively shallow depths, the temperature is not sufficient to generate oil. There, just a
few kilometres below the surface, bacterial action on the organic matter forms large volumes of
biogenic or microbial gas. It is generated very fast and is almost pure methane gas. This gas is
commonly known as swamp or marsh gas. Biogenic gas is not commonly trapped and usually
leaks into the atmosphere in enormous volumes. However, one of the largest gas fields in the
world, Urengoy in Siberia, is believed to be filled with biogenic gas. The gas is trapped below
the permanently frozen ground (permafrost). The field contains 285 tcf of gas. Generation of
biogenic gas decreases with depth as bacterial action decreases with increasing temperature.