General Intelligence · Reading Comprehension

Read the following passage and answer the question based on it. PETROLEUM Petroleum is a fossil fuel because it was formed from the remains of tiny sea plants and animals that died millions of years ago. When the plants and animals died, they sank to the bottom of the oceans. Here, they were buried under thousands of feet of sand and silt. Over time, this organic mixture was subjected to enormous pressure and heat as the layers increased. The mixture changed, breaking down into compounds made of hydrogen and carbon atoms - hydrocarbons. Finally, an oil-saturated rock — much l ike a wet household sponge — was formed. All organic material does not turn into oil. Certain geological conditions must exist within the oil-rich rocks. There must be a trap of non-porous rock that prevents the oil from seeping out and a seal (such as salt or clay) tha t keeps the oil from rising to the surface. Under these conditions, only two per cent of the organic material is transformed into oil. A typical petroleum reservoir is mostly sandstone or limestone in which oil is trapped. The oil in it may be as thin as gasoline or as thick as tar. Petroleum is called a non-renewable energy source because it takes millions of years to form. We cannot make new petroleum reserves for our use. Which of the following is NOT essential for the formation of oil?