EXERCISE 10 Prepare a presentation about petroleum and its history.

CHAPTER II

ORIGIN AND ACCUMULATION OF HYDROCARBONS

EXERCISE 1 Read the text below and match the headings to the paragraphs of the text. There is one extra heading.

1.Arrangement of hydrocarbon molecules.

2.What is crude oil?

3.Production of chemical feedstock.

4.Modification of hydrocarbon molecules.

5.Oil formation.

6.Oil and gas products.

7.Different mixtures of hydrocarbons.

FORMATION OF OIL

A) Crude oil is a complex mixture of hydrocarbons with minor proportions of other chemicals such as compounds of sulphur, nitrogen and oxygen. To use the different parts of the mixture they must be separated from each other. This separation is called refining.

B) Crude oils from different parts of the world, or even from different depths in the same oilfield, contain different mixtures of hydrocarbons and other compounds. This is why they vary from light colored volatile liquids to thick, dark oils - so viscous that they are difficult to pump from the ground.

C) Hydrocarbons vary in structure depending on the number of carbon atoms and the way in which the hydrogen atoms combine with them. Hydrocarbons can be arranged as straight chains, branched chains or closed rings. There are two main chemical families of hydrocarbons - the alkanes and the alkenes.

D) As the structure of hydrocarbons varies so much, thousands of synthetic products can be manufactured with many different properties. Hydrocarbons with small molecules make good fuels. Methane (CH4) has the smallest molecules, and is a gas, used for cooking and heating and generating electricity. Gasoline, diesel, aviation fuel and fuel oil are all liquid fuels.

E) Hydrocarbon molecules can be split up into smaller ones, or built up into bigger ones, or altered in shape, or modified by adding other atoms. This is why they are a very useful starting point (called a chemical feedstock) for making other materials. Even the thick black tarry residue left after distillation is useful. It is called bitumen, and is used in tarmac for road surfacing, and for roofing.

F) Oil is formed from the remains of tiny plants and animals (plankton) that died in ancient seas between 10 and 600 million years ago. After the organism died, they sank into the sand and mud at the bottom of the sea. Over the years, the organisms decayed in the sedimentary layers. In these layers there was little or no oxygen present. So microorganisms broke the remains into carbon-rich compounds that formed organic layers. The organic material mixed with the sediments, forming fine-grained shale, or source rock. As new sedimentary layers were deposited, they exerted intense pressure and heat on the source rock. The heat and pressure distilled the organic material into crude oil and natural gas. The oil flowed from the source rock and accumulated in thicker, more porous limestone or sandstone, called reservoir rock. Movements in the Earth trapped the oil and natural gas in the reservoir rocks between layers of impermeable rock, or cap rock, such as granite or marble.