Hydrocarbons

The Earth is not the fixed, solid mass that we usually envision. It is actually a sphere of solids and molten rock fluids that are gradually but continuously moving and changing. For example, South America is drifting away from Africa at about the speed your fingernails grow. Earthquakes and volcanoes are reminders of the Earth's instability and changing face.

The planet's crust is divided into numerous tectonic plates. These push against each other, rise and fall, tilt and slide, buckle and crumple, break apart and merge together. As a result, sediments from the bottom of ancient seas can today be found in rocks on the tops of mountains. In fact, the summit of Mount Everest is marine limestone, formed just this way.

For more than half a billion years, photosynthesis has made life possible on Earth. Plants absorb solar energy and use it to convert carbon dioxide and water into oxygen and carbohydrates, such as sugar, starch and cellulose. These carbohydrates and other organic materials eventually settle on the ground and in stream, lake and sea beds.

As these organic materials become more deeply buried, heat and pressure transform them into solid, liquid or gaseous hydrocarbons known as fossil fuels—coal, crude oil or natural gas. Coal is generally formed from the remains of terrestrial (land-based) plants. Oil is typically derived from marine (water-based) plants and animals, mainly algae, that have been gently "cooked" for at least one million years at a temperature between 50° and 150° Celsius. Natural gas can be formed from almost any marine or terrestrial organic materials, under a wide variety of temperatures and pressures.

Due to the force of gravity and the pressure created by the overlying rock layers, oil and natural gas seldom stay in the source rock in which they are formed. Instead, they move through the underground layers of sedimentary rocks until they either escape at the surface or are trapped by a barrier of less permeable rock.

Most of the world's petroleum has been found trapped in porous rocks under relatively impermeable formations. These reservoirs are often long distances away from the original source.

A seep occurs when hydrocarbons migrate to the Earth's surface. Over time, huge amounts of these hydrocarbons have escaped into the atmosphere. Flowing water can also wash away hydrocarbons. Sometimes only the lighter, more volatile compounds are removed, leaving behind reservoirs of heavier types of crude oil.

The Athabasca oil sands in northeastern Alberta are one example of a petroleum resource that has lost its lighter components or fractions. The tar-like bitumen in the oil sands was formed largely by the effects of bacterial processes, water flows and oxidation on the petroleum in the reservoir.

Petroleum Communication Foundation. Our Petroleum Challenge: Exploring Canada's Oil and Gas Industry, Sixth Edition. Calgary: Petroleum Communication Foundation, 1999. With permission from the Centre for Energy.