Oil Shale and Oil Sands: Unconventional Fossil Fuel Frontiers

Beneath vast stretches of the Canadian and American west, beneath the mountains of Estonia and Scotland, and under the deserts of Brazil and China lie enormous deposits of hydrocarbon-bearing rock and sand that have transformed the global energy landscape since their large-scale development began in the twentieth century. Oil shale and oil sands — the two major categories of "unconventional" hydrocarbon resources — contain quantities of oil equivalent to, or exceeding, the entire conventional crude oil reserves that have powered industrial civilization for a century and a half.

Oil sands (also called tar sands) are deposits of a dense, thick form of petroleum known as bitumen mixed with sand, clay, and water. Unlike conventional crude oil, bitumen is too viscous to flow through pipelines at ambient temperatures and must be extracted either through surface mining (in shallow deposits) or through in-situ thermal recovery methods (in deeper deposits) that heat the bitumen to reduce its viscosity. The Athabasca Oil Sands in northern Alberta, Canada, are the world's largest known reservoir of oil sands, with reserves exceeding one hundred and seventy billion barrels of recoverable oil — the third-largest oil reserve in the world after Saudi Arabia and Venezuela.

Oil shale is a fine-grained sedimentary rock containing kerogen — a solid, waxy organic material that has not been subjected to sufficient geological heat and pressure to convert it to conventional oil. When oil shale is heated (a process called retorting), the kerogen converts to a synthetic crude oil (shale oil) and gas. Oil shale deposits exist on every inhabited continent and are estimated to contain several trillion barrels of equivalent oil — many times the world's entire conventional oil reserves. However, the energy required to extract and retort oil shale is typically a substantial fraction of the energy in the product, making oil shale one of the most energy-intensive of all oil production methods.

The development of oil sands and oil shale as energy resources represents a defining challenge of the twenty-first century energy transition: these resources are enormous in scale, economically significant at current oil prices, but accompanied by environmental impacts — land disturbance, water use, greenhouse gas emissions — that are substantially greater than conventional oil production. The debate over oil sands and oil shale development encapsulates the central tension of the energy transition: the economic and geopolitical value of domestic hydrocarbon resources versus the environmental costs of exploiting the most unconventional, energy-intensive, and environmentally disruptive end of the fossil fuel spectrum.

The Geology of Oil Sands

Oil sands — also called tar sands in many contexts, though "oil sands" is preferred by the industry as reflecting the product (oil) rather than the texture (tar) — are sedimentary deposits in which porous sandstone or carbonate rock has been impregnated with a heavy, viscous form of petroleum that has been degraded by the action of bacteria and water over millions of years.

Conventional oil forms when organic matter in marine sediments is buried to depths sufficient for temperature and pressure to convert it to liquid petroleum, which then migrates upward through permeable rock until it is trapped beneath an impermeable cap rock, forming a conventional oil reservoir. In oil sands deposits, the upward-migrating petroleum reached the surface (or near-surface) where it was exposed to groundwater, bacteria, and atmospheric oxygen, which stripped away the lighter fractions and degraded the remaining petroleum into heavy bitumen. The result is a near-surface deposit of oil-impregnated sand with a very high bitumen content (typically ten to fifteen percent by weight) but extremely low permeability (the bitumen fills the pore spaces between sand grains, preventing flow).