What are the Oil Sands?

Alberta’s oil sands are a naturally occurring mixture of sand, silt, clay, minerals, water and a very viscous type of oil. Each grain of sand is coated with a film of water that has tiny mineral particles—known as fines—suspended in it. These water-coated sand grains are in turn covered with a film of oil. Any spaces in the oil-covered sand are filled with water, minute silt and clay particles, and sometimes natural gas. There can also be a variety of other materials mixed in, including rocks, fossils and petrified wood.

The sand in oil sand is mostly quartz, but about 5% of the grains may be other minerals, such as mica, rutile, ilmenite, tourmaline, zircon, spinel, garnet and pyrite. Silt and clay are particles that are much smaller than sand. In general, as the content of silt and clay in the oil sand increases, the amount of water also increases, and the amount of oil decreases. Commercial grade mineable oil sand has a silt and clay content of less than 20% but more than 3%. A certain minimum amount of fine material is necessary in the oil sand, or it becomes more difficult to separate.

The amount of oil in the oil sands can be up to 18% by weight, although lower content is common. Anything below 6% is considered too poor to warrant extraction, although oil sand of this grade is sometimes blended with better sand for processing. On average, it takes 2 tonnes (2.2 tons) of raw oil sand to produce one barrel of oil. When it is separated from the sand, the oil is very thick. A common description is that it has the consistency of molasses. Oil from the oil sands is one of the most viscous hydrocarbons.

Even though it seems very different, oil from the oil sands was actually formed in the same way as conventional oil. In the very distant past, organic matter—mostly algae—settled to the bottom of an ancient sea and was covered up with sediment. As it became more deeply buried, heat and pressure caused chemical changes that transformed the layer of organic matter into hydrocarbons such as oil.

This is where the oil sands story begins. Geologic conditions forced the oil from its original location in Carboniferous and Jurassic rocks. It migrated across the layer of Devonian limestone and into porous sandstone in the lower Cretaceous McMurray formation. Here it remained, trapped beneath a layer of impermeable shale. Once settled in the sand, a process of thickening began.

Over millions of years the oil was reduced by as much as half to two-thirds of its original volume, as the smaller molecules in the oil evaporated or were digested by bacteria. What was left were the larger, more stable hydrocarbons that were not affected by these processes.

Extracted and upgraded, oil from the oil sands can be used just like conventional oil.