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During the refining process, the refiner must remove excessive sulfur as the crude oil is being
processed. If not, the sulfur will harm some of the chemical equipment in the refinery. Crude oils
are classified as sweet and sour on the basis of their sulfur content. Sweet crudes have less than
1% sulfur by weight, whereas sour crudes have more than 1% sulfur. In general, heavy oils tend
to be sour, whereas light oils tend to be sweet. At a refinery, low-sulfur crude has 0 to 0.6%
sulfur, intermediate-sulfur crude has 0.6 to 1.7% sulfur, and high-sulfur crude has above 1.7%
sulfur.

Natural Gas Composition
Natural gas on the surface is composed of hydrocarbon molecules that range from one to
four carbon atoms in length. The gas with one carbon atom in the molecule is methane (CH 4),
two is ethane (C 2H 6 ), three is propane (C 3H 8), and four is butane (C 4H 10). All are paraffin-type
hydrocarbon molecules.
Reservoir Hydrocarbons
Chemists classify reservoir hydrocarbons into (1) black oil, (2) volatile oil, (3) retrograde
gas, (4) wet gas, and (5) dry gas. Laboratory analysis of a sample is used to determine the type.
Both black and volatile oils are liquid in the subsurface reservoir.
Black oil or low-shrinkage oil has a relatively high percentage of long, heavy, nonvolatile
molecules. It is usually black but can have a greenish or brownish color.
Volatile oil or high-shrinkage oil has relatively more intermediate size molecules that are
shorter than black oil molecules. The color is brown, orange, or green.
Retrograde gas is a gas in the reservoir under original pressure, but liquid condensate
forms in the subsurface reservoir as the pressure decreases with production.
Wet gas occurs entirely as a gas in the reservoir, even during production, but produces a
liquid condensate on the surface.
Dry gas is pure methane. It does not produce condensate either in the reservoir or on the
surface.

Task 4. Answer the following questions, using the vocabulary from Task 1.
1. Where does the word petroleum come from?
2. What does petroleum include?
3. What are the two most common elements in both crude oil and natural gas?
4. What are hydrocarbons?
5. What is the difference between crude oil and natural gas?
6. Talk about the structure of crude oil and natural gas.
7. What are two types of crude oils? What is the difference between them?
8. What is sulfur?
9. How are crude oils classified on the basis of their sulfur content?
10. Try to classify natural gas.

Task 5. Talk about reservoir hydrocarbons. What unites them? What makes them
different?

Task 6. Find English equivalents for the following (see the text). Try to build up
your own sentences with them.
Містити (включати); молекули, що утворюють неочищену нафту; різниця між
неочищеною нафтою та природнім газом; тиск; траплятися у вигляді рідини; суміш
декількох сотень молекул вуглеводнів; формувати прямі ланцюги; забруднювати повітря;
прибрати зайву сірку; відносно високий відсоток; мати (містити) відносно більше молекул
середніх розмірів; на поверхні.

Task 7. Give a short summary of the text from Task 2.