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In a typical sedimentary basin, oil generation starts at about 65˚C and ends at about
150˚C. If the source rock is buried deeper, where temperatures are above 150°C, thermogenic
gas is generated. It is the gas that is often trapped. The zone in the earth’s crust where the oil is
generated is called the oil window. It occurs from about 2,100 to 5,500 m deep on land.
Where does heavy oil come from? It is formed later when bacteria, along with chemical
and physical processes, degrade the good oil to form heavy oil. Below the oil window,
thermogenic gas is generated from thermal cracking of crude oil and organic matter left in the
source rock. Wet gas is formed under more shallow depths and cooler temperatures. Under
higher temperatures at deeper depths, dry gas is formed.
At temperatures higher than about 150˚C, crude oil is irreversibly transformed into
graphite (carbon) and natural gas. The process is similar to thermal cracking in a refinery. This
temperature occurs at a depth of about 5,500 m in a sedimentary rock basin and is a floor below
which only gas can occur in the reservoir. Deep wells are drilled for natural gas. In several
instances, a deep well has discovered a gas reservoir, and the sand grains in the sandstone
reservoir rock are coated with carbon. Apparently, there was oil originally in the reservoir, but it
was buried too deep and was thermally cracked.
Many sedimentary basins are unproductive. An unproductive basin might not have an
organic-rich source rock that could generate petroleum. Even if an unproductive basin has a
source rock, it might never have been buried into the oil window. Maturity is the degree to which
petroleum generation has occurred in a source rock. A mature source rock has experienced the
temperature and time to generate petroleum in contrast to an immature source rock. In
sedimentary rock basins, between 30 to 70% of the organic matter in the source rock that has
been buried deep enough generates gas and oil.

Task 3. Tcf in the text means ‘trillion cubic feet’ (триліони кубічних футів). Try and
turn this measure into trillion cubic kilometers.

Task 4. Answer the following questions, using the vocabulary from Task 1.
1. What is a source rock?
2. What do you know about source rock?
3. Where does the black colour in sedimentary rocks come primarily from?
4. What do black-coloured, organic-rich sedimentary rocks include?
5. Why are coal mines dangerous?
6. What is shale? What colour can it be?
7. What do black shales contain?
8. What is the most important factor in the generation of crude oil from organic
matter in sedimentary rocks?
9. How does the temperature depend on the depth?
10. What gas is called biogenic or microbial? What do you know about it?
11. When is usually thermogenic gas generated?
12. What is oil window?
13. Where does heavy oil come from?
14. When is crude oil irreversibly transformed into graphite (carbon) and natural gas?
15. Are wells drilled for natural gas deep?
16. What is the degree to which petroleum generation has occurred in a source rock?

Task 5. Talk about oil and gas generation.

Task 6. Find English equivalents for the following (see the text). Try to build up
your own sentences with them.
Древня органічна речовина; піски та багнюка; отримувати кисень з повітря;
зберігатися (про органічну речовину); осадові породи; вапняки; вугільні родовища; за