Oil shale is a sedimentary rock containing solid and combustible organic matter (kerogen and bitumen) in a mineral matrix. Oil shales are much more common than is generally recognized, occurring on every continent and in every geological system. Most "oil shales" are actually bituminous, non-marine limestones or marine sandstones containing kerogen. Oil shales commonly contain abundant fish remains that were preserved in the oxygen-free environment during deposition. Most lacustrine deposits may represent meromict lakes that have a strong brine in the lower level overlain (above the chemicline) by circulating oxygenated waters enriched in calcium bicarbonate.
The term "oil shale" describes the rock in lithologic terms but also refers to the ability of the rock to yield oil. The organic and mineral matter in oil shales are difficult to separate from each other physically or chemically. However, oil is released from the shale upon heating which causes the kerogen to decompose. Oil shale has also been called black shale, bituminous shale, carbonaceous shale, coaly shale, kerosene shale, coorongite, maharahu, kukersite, kerogen shale, and algal shale. On outcrop, some oil shales weather to form stacks of thin organic-rich layers called paper shale. The color of the oil shale is usually brownish-black to yellowish-brown but ranges from black to light tan; color depends on weathering and the type and quality of organic matter in the rock.
The organic matter in oil shale has been studied extensively and the composition of kerogen in the oil shale is found to vary significantly from deposit to deposit. Oil shale consists of sapropelic (hydrogen-rich lacustrine and marine) and humic (hydrogen-poor coaly material) types of organic matter. However, most true oil shales contain, or may contain, 10 to 50% algal organic matter (sapropelic) and fall within the Type I category of organic matter. The fraction of kerogen converted into oil with increasing temperature depends upon the hydrogen content or organic matter; oil yield increases with increasing hydrogen content. Because the density of the organic matter is significantly less than minerals in the shale, oil shale density can be used to estimate oil yield by including a conversion factor for the transformation of kerogen to oil. This relationship is used to calculate oil shale reserves.
Oil shales represent a significant source of fossil energy with total resources approaching 2 x 1015 barrels (bbl) of oil. The potential size of oil shale resources is staggering with oil shale reserves estimated to be 300 x 10^11 bbl of oil. Recoverable resources are estimated to be on the order of 5 x 1011 bbl of which 20% (or 1 x 1011 bbl) falls into the highest commercial category available for exploitation in the future. In 1987, ultimate recoverable oil shale resources were estimated to be 6 x 1010 bbl which is twice the estimated ultimate recoverable conventional oil resources (2.7 x 1010 bbl).