Origins, compositions, and technological and environmental problems of utilization of oil shales

Oil shale (OS) is a large global resource of low-quality fossil fuel. According to the World Energy Council (WEC), the total world resources of shale oil (SO) are conservatively estimated at 3.3 trillion barrels, or around 500 billion tonnes. During 2005 and 2006, over 4.1 billion tonnes of natural oil was produced yearly world-wide. Analyses show that world oil production will start to fall sometime during this decade and will never rise again, decreasing down to about 15% in 2050. The same will happen with the natural gas resources somewhat later, and with coal – much later. In the near and farther future the other main competitors of SO recovery are tar sands that are under development and hypothetic gas hydrates of ocean floor as a challenge. The present yearly SO production is around 0.5 million tonnes. The expansion of SO recovery, e.g. onto the 10% level of the present oil production (400 million tonnes per year) would require construction of around 133 oil retorting plants producing 3 million tonnes per year (50 000 barrels per day) each and using 5.7 billion tonnes OS altogether. For this purpose, recoverable reserves of OS with Fisher oil yield of 9%, corresponding to the expected industrial retort oil yield of 7%, in the amount of almost 150 billion tonnes (as a minimum for 25 years of operation) are required. At the usual overburden to OS volume ratio between 1 : 1 and 3 : 1, the yearly amount of rock excavation would reach some 12–25 billion tonnes or around 5–10 km 3 . When mining approximately 25 m thick OS seams (around 40 tonnes shale/m 2) in open pits, the area exhausted each year would reach some 100 km 2 . Yearly around 5 billion tonnes or 3 km 3 of hot treatment mineral wastes should be allocated. The WEC estimate (2005) of commercial-grade OS suggests the organic matter (OM) content of 13–23 wt%, which in the best cases corresponds to 8.5–15% Fisher oil yield, or to 7–12% oil yield by industrial retorting, or to 6.3–10.5 MJ/kg. No commercial technologies exist for complete utilization of OS of this grade – neither for oil retorting nor for burning in electric power plants. In Estonia, SO is produced from OS with oil yield over 13–16% and electric power is obtained by direct burning of OS, yielding 8.5 MJ/kg on average. Oil shales are either shallow marine or fresh-water and salt lake sedimentary deposits. The mineral matter (MM) content of the WEC commercial-grade OSs varies between 77 and 87 wt%. Some national lists of OS deposits include even resources with Fisher oil yield as low as 3–5 wt% or OM content around 4–7% and MM up to 93–96%. National data on WEC commercial-grade resources are randomly available. Depending on the element composition (C, H, N, O, S) of OM, its oil yield may range from 20 to 66%. The MM composition depends on the origin of OS. According to the MM composition, the three main types of wide-spread marine OSs are: a) calcareous (chalk, limestone), b) siliciclastic (claystone–siltstone–sandstone mixtures), and c) mixed calcareous–siliciclastic (marls) types. The MM of the largest salt lake Green River OM has a specific composition of alternating sodium plus Ca + Mg + Fe carbonate, plus silicate. Not depending on the composition of OM and bulk composition of MM, different marine OS-bearing basins and deposits may have or may not have high contents of sulphur (S) or a large spectrum of trace (including toxic and radioactive) elements. Uniform and specific features of OS composition cause similar and different, presently mainly unsolved problems of OS utilization under various geological, environmental, and economic conditions.