The global energy landscape is shifting, prompting the pursuit of hydrocarbon sources once considered too difficult or costly to recover. These deposits, known as unconventional oil resources, exist outside traditional, easily accessible reservoirs of crude oil. Extracting fuels from these non-traditional sources requires specialized, often energy-intensive processes before conventional refining. Tar sands and oil shale are two significant examples of these unconventional accumulations being developed today. Their vast potential reserve volume offers a substantial extension to the world’s supply of liquid fuels.
The Composition and Geology of Tar Sands
Tar sands, also called oil sands, are a naturally occurring mixture of sand, fine clay minerals, water, and an extremely dense form of petroleum called bitumen. Bitumen is a black, highly viscous hydrocarbon too thick to flow or be pumped out under normal reservoir conditions. The deposit consists of individual sand grains coated in a thin film of water, which is then enveloped by the sticky bitumen.
The bitumen content within a deposit varies, but anything less than about six percent is considered uneconomical for mining. While these deposits are globally distributed, the largest and most commercially developed reserves are found in the Athabasca region of Alberta, Canada. This Canadian deposit holds the majority of the world’s total natural bitumen reserves. Because bitumen is nearly solid at room temperature, unique thermal and mechanical methods are necessary for its separation and recovery.
Methods for Extracting Oil from Tar Sands
The method used to extract bitumen depends primarily on the depth of the deposit. When oil sands are relatively shallow (typically less than 75 meters deep), producers use large-scale surface mining techniques, often involving open-pit excavation. Massive shovels move the excavated oil sand to crushers, which break down the clumps before transport to an extraction facility.
At the extraction plant, the crushed oil sand is mixed with hot water to create a slurry. This mixture is agitated to separate the bitumen from the sand and clay, often using a process similar to the hot water extraction method. The hot water causes the bitumen to separate and float to the surface as a froth, which is skimmed off for further processing. The remaining mixture of sand, water, and clay is pumped into large containment areas called tailings ponds.
For reserves too deep for open-pit mining, in-situ, or “in place,” recovery methods are employed. The most widely used technique is Steam-Assisted Gravity Drainage (SAGD), which involves drilling two parallel horizontal wells into the reservoir. High-pressure steam is injected into the upper well, heating the bitumen and significantly dropping its viscosity. The now-fluid bitumen flows due to gravity into the lower production well and is pumped to the surface.
Regardless of whether the bitumen is recovered through mining or in-situ methods, it is an extra-heavy crude that must be chemically altered, or upgraded, before it can be refined. This upgrading involves either adding hydrogen or removing carbon to transform the thick bitumen into a lighter synthetic crude oil.
The Composition and Geology of Oil Shale
Oil shale is a fine-grained sedimentary rock that contains a solid organic material called kerogen. Kerogen is not crude oil but a precursor that has not been exposed to the geological heat and pressure required for conversion into liquid petroleum. The rock must be heated to high temperatures to trigger a chemical decomposition process that yields a liquid hydrocarbon.
The presence of solid kerogen distinguishes oil shale from other fine-grained sedimentary rocks. Significant global deposits exist, with the largest known resources found in the Green River Formation across Colorado, Utah, and Wyoming in the United States. Other major deposits are located in Estonia, China, and Brazil. The geology of these deposits often relates to ancient lakes where organic matter accumulated and was preserved in sediment layers.
Methods for Extracting Oil from Oil Shale
Converting the solid kerogen within the shale rock into usable liquid oil requires pyrolysis, or retorting, which involves heating the rock in an oxygen-free environment. One approach, known as ex-situ retorting, requires first mining the oil shale through surface or underground operations. The excavated rock is then transported to a surface facility where it is crushed to increase its surface area.
The crushed shale is heated in specialized vessels, or retorts, to temperatures around 450 to 500 degrees Celsius. This heating causes the kerogen to thermally decompose, producing a vapor that is cooled to condense into shale oil. The resulting shale oil is typically lighter than bitumen but still contains impurities like sulfur and nitrogen. Therefore, it requires further processing, or upgrading, before it can be accepted by standard refineries.
An alternative approach is in-situ retorting, where the oil shale is heated while still underground. This technique minimizes the need for extensive mining and the surface disposal of spent shale rock. Electrical heaters or circulating heated gases are introduced into the formation through a network of wells to pyrolyze the kerogen deep below the surface. The resulting shale oil and gas are collected and pumped out through separate production wells, similar to conventional oil extraction.
The Global Context of Unconventional Oil Resources
The pursuit of tar sands and oil shale is driven by the immense size of their reserves, which collectively exceed the world’s known conventional oil reserves. Tapping into these resources provides a vast, geologically secure supply that can substantially extend the global availability of fossil fuels. Their development introduces specific logistical and technical challenges that distinguish them from traditional petroleum operations.
Extracting and processing these unconventional hydrocarbons require a considerably higher input of energy and water compared to producing conventional crude oil. Both SAGD and retorting processes rely on large volumes of heat, often generated by burning natural gas, to mobilize the resource. The intense industrial processes and the need for water highlight the specialized infrastructure required to bring these fuels to market.