|Fig. 1: Example of an oil shale. (Source: G. Glatz.)|
Generally speaking, oil shales (see Fig. 1) are a mixture of fined grained sedimentary rocks containing organic material that can be refined into fuels.  A more formal definition of oil shale was provided by Gavin who described oil shale as a
"… compact laminated rock of sedimentary origin yielding over 33 percent of ash and containing organic matter that yields oil when distilled …" [2,3]
The inorganic matrix of the oil shale can consist of clays, carbonates, feldspars, quartz, pyrite and other minerals. With respect to the organic matter we can distinguish between two constituents. On the one hand we have kerogen that is the macromolecular and organic solvent-insoluble (e.g. carbon disulphide) organic matter. On the other hand we have bitumen characterized as the solvent-soluble portion of oil shale organic matter.  It is believed that when kerogen undergoes thermal transformation during catagenesis hydrocarbons and other organic compounds are released. [1,2,5]
Technically, shale oil is different from oil shale in the sense that a shale-oil resource commonly refers to producible oil from organic-rich mudstones, calcareous mudstones and so on.  Prominent examples of this type of oil-bearing formations are the Bakken, Barnett and the Eagle Ford shale. Contrary, oil shale should be considered as a kerogen resource system. In order to produce oil, the kerogen needs to be heated up first. A possible solution to avoid this confusion is to refer to shale oil as tight oil given that the oil is trapped in relatively low permeability rocks and has to be produced using special techniques like hydraulic fracturing and horizontal drilling.  Thus, in this report we will refer to shale oil as oil derived from kerogen i.e. the retorted product. The Green River formation spanning Colorado, Utah and Wyoming (see Fig. 2) is one of the most prominent oil shale resources in the United States. It is estimated to hold the equivalent of 1.38 trillion to 1.8 trillion barrels of oil in place. [1,8-10]
|Fig. 2: Colorado, Utah and Wyoming oil shale deposits. (Source: Wikimedia Commons.)|
Oil shale has a mixed history with various ups and downs. Prior to the discovery of crude oil in 1859 the oil shale industry was an integral part of the United States economy.  Scotland's shale industry also came to a halt in 1864 when cheap foreign crude oil entered the market. [2,11] Interest in oil shale re-surged during the Second World War and after oil embargos in the seventies. [2,12] The industry took another hit with the declining oil prices in the eighties. [2,12] Oil shale gained momentum in the United States again in 2005 when the Bureau of Land Management participated in an oversight hearing on oil shale development efforts before the Senate Energy and Natural Resources Committee in an effort to discuss the bureaus efforts
"… to facilitate and promote oil shale research and development on public lands." 
The 2005 Energy Policy Act recognized oil shale as a strategically important domestic resource. In early 2006 the proposals of Chevron Shale Oil Co., ExxonMobil Corp., and Shell Frontier Oil & Gas among others were accepted to develop oil shale technologies. 
The significance of oil shale stems from its enormous potential as a resource. The World Energy Council conservatively estimated total world resources at 4.8 trillion barrels in 2013. Crude oil resources are estimated to be at 1.3 trillion barrels with the United States accounting for 3.7 trillion barrels alone in 2011.  In 2013 the United States Geologic Survey increased the estimate for the aforementioned states to a total 4.2 trillion barrels of shale oil.  Already in 1920, Victor Clifton Alderson commented that if oil is the "king", then "oil shale" is the "heir apparent". 
|Fig. 3: Oil shale processing plant in Kiviõli. (Source: Wikimedia Commons|
It has to be pointed out, though, that it is not easy to assess the extent to which shale reserves are economically recoverable or, in other words, what it takes to turn a resource into a reserve. Assuming an oil price of 50$ per barrel, global oil shale reserves may yield up to 1000 billion barrels. 
Another important aspect of oil shale is the comparatively wide distribution worldwide giving countries like Israel, Jordan, or Mongolia with few to no traditional hydrocarbon resources the opportunity to develop indigenous energy sources. [14,15] A prominent example is the Estonian oil shale industry with its distinguished history. [16,17]
As mentioned above, kerogen is converted to hydrocarbons during catagenesis. At this stage geological forces have buried the organic matter deep enough to reach temperatures associated with the so-called oil window. Depending on the type of kerogen the opening of the oil window - or the initiation of catagenesis - happens at about 60 degree Celsius. The window for oil closes at about 170 degree Celsius, the window for gas at approximately 225 degree Celsius. This marks the end of catagenesis.  Depending on the geothermal gradient this corresponds to a burial depth of about 1.5 to 4 km.  The rate upon which kerogen is converted to oil is usually approximated by the Arrhenius equation
where K is the reaction rate, A the Arrhenius constant, Ea the activation energy, R the universal gas constant, and T the absolute temperature. The reaction order is considered to be one and oil and gas formation can be modeled as a series of parallel or successive reactions. [4,20] At moderate temperatures of about 100 degree Celsius it takes millions of years to convert kerogen to hydrocarbons.  Raising temperatures to 500 degree Celsius at atmospheric pressure and anaerobic conditions reduces the time required to hours. 
|Fig. 4: Estonian Kukersite. Kukersite was the major energy resource for northwestern Soviet Union being mined and retorted for fuel oil or burned directly for electricity. [23,24] (Source: Wikimedia Commons.)|
With respect to kerogen conversion, we can distinguish between in-situ and conventional or ex-situ methods for inducing the maturation process. Aforementioned conditions especially apply for ex-situ applications where shale is mined and retorted on surface.  In-situ methods aim to convert the kerogen in place i.e. subsurface. A comprehensive summary of both in-situ and ex-situ technologies is given by Burnam and McConaghy. 
From a chemical perspective, conversion of kerogen into synthetic crude can be achieved by thermal dissolution, hydrogenation, or pyrolysis, the latter one being the most common one. [10,26] Pyrolysis essentially approximates natural conversion of kerogen using higher temperatures to compensate for the geological time frame.  During pyrolysis, kerogen is heated in the absence of oxygen to yield fuel decomposing the kerogen. For Green River shale the decomposition reaction is given by Leavitt, Tyler and Kafesjian as 
As mentioned above, higher temperatures allow for a faster conversion of the kerogen. This is also reflected in the Arrhenius equation. Interestingly, it has been found that processing shale at lower temperatures for longer times improves product quality. Higher pressures also improve oil quality because vaporization is impeded fostering secondary cracking reactions. Lower temperature and higher pressure will, however, reduce the overall yield. 
Commercial projects have been limited to ex-situ projects e.g. Estonia. No in-situ method so far was able to demonstrate large-scale production on a commercial level.  Both Chevron (Chevron CRUSH process) and Shell (Shell in-situ conversion process or ICP for short) recently stopped their oil shale engagements at the Western Slope. 
Companies have conceived different approaches to produce oil from shale. An overview of companies being invested in oil shales (especially in the US) and their respective approaches is given in the U.S. DOE secure suels survey.  Oil yields of different approaches are sometimes compared with respect to the yield of the Fischer assay method.  During Fischer assay a crushed sample is heated to a final temperature of 500 degree Celsius within about 40 minutes an held at this temperature for the same amount of time. Products are collected, weighted and reported as weight percentages of oil shale.  A relation between the mass fraction of kerogen and the Fischer assay F estimated in gallons of oil recoverable per ton of shale is given by Cook with
|F||=||2.216 × wp - 0.7714 gal/ton|
where wp is the weight percent of kerogen in the shale.  The Fischer assay method, however, is only able to approximate the energy potential and does not automatically reveal the maximum amount of oil that can be produced from the shale under investigation.  The Chevron CRUSH process as well as Sheel ICP are reported to have an oil yield of 80% of Fischer assay.  The ex-situ Hytort process reportedly increases oil yields to 300% of the Fisher assay yield.  These numbers, however, do not provide an economic context. Energy ratios might be a better way to provide key figures to ascertain the competitiveness of a production process. For example, energy return on investment - or EROI for short - is defined as 
|EROI||=||Energy returned to society
Energy required to obtain that energy
Accordingly, a process with an EROI of 2 would yield 2 Joules on an investment of 1 Joule. A comprehensive analysis of the Shell in-situ conversion process with respect to energy inputs as well as greenhouse gas emission is given in Hall and Klitgaard.  For Shell's ICP, Brandt estimates that energy outputs as refined liquid fuel are 1.2 to 1.6 greater compared to the total primary energy input. Shell's own estimate yields a higher energy balance of 3.5 to 1 due to different assumptions. Depending on technology used, however, up to 5.5 units of energy can be produced per unit of energy spent. 
Oil shale has experienced a mixed history. The significance of oil shale stems from its enormous potential as a energy resource and its worldwide distribution offering countries with no traditional hydrocarbon resources the opportunity to develop native energy resources. So far, only ex-situ methods have been commercialized. Companies, however, are working hard to overcome the technological challenges associated with in-situ methods that are required to turn the vast underground resources into a reserve.
© Guenther Glatz. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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