Oil shale can be a confusing term and the reason is that it can contain crude oil, bitumen, and even natural gas. The easiest way to understand oil shale is to look at its geological origins.
Oil shale refers to a type of crude oil that can be refined from sedimentary rock. Sedimentary rock, because it is made by compression, can contain a variety of different things including organic matter. The organic matter in sedimentary rock is referred to as kerogen. Kerogen itself is usually formed from algae and other marine plant life.
Kerogen is a mixture of organic chemicals and hydrocarbons in solid form. The solid part is called bitumen. The problem with kerogen is that it is dispersed throughout sedimentary rock layers, making it a solid rather than a liquid. In some natural cases, heat from the Earth’s core or from various other processes warms oil shale and liquefies its hydrocarbons. In some cases, these can then seep through the rock and collect as traditional deposits of liquid petroleum. However, in many cases, this never happens and the kerogen remains trapped in the rock.
For sedimentary rock that contains high levels of kerogen, the potential return on investment for removing the hydrocarbon component will become financially feasible when prices of crude oil rise high enough. Removal of kerogen can occur in one of two basic ways. In the first method, open pit and strip mining are used to removed solid material that is then processed above ground to separate the hydrocarbons. In some cases, oil shale is simply burned at electrical generating plants without going through any processing and the rubble is removed when it builds up.
In the second method, called in-situ processing, the oil shale is heated underground, before extraction. Underground processing is thought to be able to extract more petroleum from given reserves and may even be less damaging to the environment. All in-situ processing technologies are currently termed experimental.
Even when fully extracted and processed, oil shale will not yield petroleum of the same quality as light crude. Across the board, sulfur content averages 0.76%, but can be as high as high 10%. The API gravity also tends to be lower for oil shale, especially for when processing takes place above ground. Oil shale also tends to have disproportionately higher quantities of oxygen and nitrogen than traditional crude. Oil shale, in general, does not lend itself to the production of gasoline and is preferentially used for “middle distillates” such as diesel and jet fuel.
Because of the high levels of energy put into extraction and processing, combined with the high levels of energy needed to remove contaminants during refining, the EROEI ratio for oil shale is much lower than for traditional petroleum. This means oil shale produces more carbon dioxide and more pollution for the same amount of end product, making it a much less environmentally friendly alternative.Because world oil reserves, particularly of light crude, are being depleted and cannot produce petroleum at a rate fast enough to keep up with demand, mining of oil shale is becoming increasingly profitable and common. Large deposits of oil shale exist in Canada, the United States, and Venezuela. Canada is currently the world’s leading producer of petroleum products derived from oil shale.