The Canadian Athabasca tar sands (oil sands) contain the makings of an enormous reserve of oil, which the Alberta government estimates at 174 billion barrels that are economically recoverable (the second largest "oil reserves" after Saudi Arabia), or some 10% of the total of the 1,700 - 2,500 billion barrels worth there is thought to be in total. This is not in fact oil per se, but bitumen which needs to be recovered from the mineral solid ("sand"), and then refined into a material that is light enough to be used as a fuel. The actual sand is a mixture of sand or clay, water and bitumen, and the process requires an enormous amount of energy, usually provided in the form of natural gas, and water, the latter drawn from the Athabasca river.
The natural gas is used both to provide heat to form steam to extract the bitumen, and also as a hydrogen source to refine-up the product into oil suitable as a fuel. The wikipedia site on tar sands, (en.wikipedia.org/wiki/Athabasca_Tar_Sands) reckons that it takes up to 1,200 cu feet of gas to produce one barrel of oil, which is the equivalent of 6,000 cu feet of gas in terms of it's energy. So a simple division would suggest an EROEI of 5. However, a figure of 1.5 is often quoted, due presumably to the need to take account of the energy used in the mining, processing and recovery, etc. Hence if you add-in the whole lot the figure might well fall to 1.5, which many think is not enough to make the process viable. However the EROEIs quoted (usually with no break-down of a calculation given to explain them), that I have seen, vary from about 5 down to 1.5, but I think it depends on exactly what is factored-in.
A supply of Canadian gas that is expected to dwindle (as it will elsewhere in the world), against a rising requirement for it, if the amount of tar sands oil production is expected to increase by five times over the next 20 years, might be offset if an alternative source of heat could be provided with which to generate the steam. I stress that since more gas is used to supply hydrogen with which to "reform" the bitumen into "oil" for fuel than is used for steam generation, the demand on natural gas is likely to remain enormous and to grow. One alternative being considered is to gasify the bitumen into syn-gas ( a mixture of H2 + CO), but this is an energy demanding process. Hence if an alternative energy source is available, both problems are in principle addressed.
To this end, Canadian companies AECL and Energy Alberta have proposed the construction of a nuclear reactor near the sites of the huge Athabasca tar sands development, controlled by Shell, and while it has not been stated explicitly that it is Shell who are the "large company" that will take 70% of the electricity that it generates, a spokeswoman from Shell has confirmed that the company is considering a range of alternatives, including nuclear. The reactor is estimated to cost C$6 billion (£2.8 billion).
Not everyone is an enthusiast of the proposal, however, including Walt Patterson, associate fellow at think-tank Chatham House who said: "Extracting oil from tar scares the pants off me. The whole idea is fundamentally perverse in the context of the present environmental situation. To then power it with nuclear, it seems the worst of all worlds."
Shell and its partner companies in Athabasca presently produce 155,000 barrels of oil per day from the tar sands there, and the proposed increase in output by five times over the next 20 years would require an additional 1,000 MW of generating capacity - i.e. about one nuclear reactor's worth!
It is necessary to dig about 4 tonnes of raw material out of the ground to make one barrel of oil, and something like 4 barrels of water is needed too. Hence because it takes resources to extract resources, it may well be that the depletion of natural gas and a water supply that is unable to meet demand, especially if the environmental clean-up factors are included as it is a rather dirty process overall, will hit the viability of the tar sands "oil fields" before the low EROEI does.
(1)"Shell could take nuclear option to mine oil from Canadian tar sands," By Tim Webb: http://news.independent.co.uk/business/news/article2944305.ece