Tight oil
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Tight oil is light crude oil contained in unconventional petroleum-bearing formations of low permeability, often shale or tight sandstone. Economic production from tight oil formations requires the same hydraulic fracturing and often uses the same horizontal well technology used in the production of shale gas. While sometimes called "shale oil", tight oil should not be confused with oil shale or shale oil. Therefore, the International Energy Agency recommends using the term "light tight oil" for oil produced from shales or other very low permeability formations, while the World Energy Resources 2013 report by the World Energy Council uses the terms "tight oil" and "shale-hosted oil".
Shown are conceptual illustrations of types of oil and gas wells. A vertical well is producing from a conventional oil and gas deposit (right). Also shown are wells producing from unconventional formations: a vertical coalbed methane well (second from right); a horizontal well producing from a shale formation (center); and a well producing from a tight sand formation (left).
Unconventional (oil and gas) reservoir
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Unconventional reservoirs, or unconventional resources are accumulations where oil and gas phases are tightly bound to the rock fabric by strong capillary forces, requiring specialised measures for evaluation and extraction.
Schematic cross-section of general types of oil and gas resources featuring unconventional as well as conventional reservoirs