$50 per tonne of emitted adds additional $7.50 cost per barrel of shale oil. As of November 2021, the price of tonne of exceeded $60. A 1972 publication in the journal Pétrole Informations () compared shale-based oil production unfavorably with coal liquefaction. The article portrayed coal liquefaction as less expensive, generating more oil, and creating fewer environmental impacts than extraction from oil shale. It cited a conversion ratio of of oil per one ton of coal, as against of shale oil per one ton of oil shale. A critical measure of the viability of oil shale as an energy source lies in the ratio of the energy produced by the shale to the energy used in its mining and processing, a ratio known as "energy return on investment" (EROI). A 1984 study estimated the EROI of the various known oil-shale deposits as varying between 0.7–13.3, although known oil-shale extraction development projects assert an EROI between 3 and 10. According to the World Energy Outlook 2010, the EROI of ex-situ processing is typically 4 to 5 while of in-situ processing it may be even as low as 2. However, according to the IEA most of used energy can be provided by burning the spent shale or oil-shale gas. To increase efficiency when retorting oil shale, researchers have proposed and tested several co-pyrolysis processes. Environmental considerations Mining oil shale involves numerous environmental impacts, more pronounced in surface mining than in underground mining. These include acid drainage induced by the sudden rapid exposure and subsequent oxidation of formerly buried materials; the introduction of metals including mercury into surface-water and groundwater; increased erosion, sulfur-gas emissions; and air pollution caused by the production of particulates during processing, transport, and support activities. Oil-shale extraction can damage the biological and recreational value of land and the ecosystem in the mining area. Combustion and thermal processing generate waste material. In addition, the atmospheric emissions from oil shale processing and combustion include carbon dioxide, a greenhouse gas. Environmentalists oppose production and usage of oil shale, as it creates even more greenhouse gases than conventional fossil fuels. Experimental in situ conversion processes and carbon capture and storage technologies may reduce some of these concerns in the future, but at the same time they may cause other problems, including groundwater pollution. Among the water contaminants commonly associated with oil shale processing are oxygen and nitrogen heterocyclic hydrocarbons. Commonly detected examples include