BTW I found another source that listed the top ten countries in terms of reserves. That number came to circa 900 billion barrels, but included about 175 billion barrels of Canadian tar sands in the total. There are another 45 nations with at least some reserves but those are mostly small numbers. This leaves us a long way from 3 billion and takes the OPEC numbers at face value.
There is an apples v orange discussion here - 'proven reserves' versus a calculation of total final economically recoverable resources available. Current total proven reserves, as another poster cited, is around 1200 billion barrels.
The IEA study I've been speaking of was tying to calculate the actual recoverable oil resources over time. Beyond proven reserves, it looks at how reserves and resources might be found and recovered. They speak of 5.5 trillion barrels of oil resources recoverable, 1 trillion of which has been produced ... leaving 4.5 trillion:
http://www.iea.org/textbase/nptable/Oil%20cost%20curve,%20including%20technological%20progress,%20availability%20of%20oil%20resources%20as%20a%20function%20of%20economic%20price.pdf
It shows ...
OPEC+ME = 1000 billion barrels
other conventional oil = 900 billion barrels
deepwater = 100 billion barrels
Arctic= 200 billion barrels
EOR = 300 billion barrels (above $20/barrel)
heavy oil bitumen (tar sands) = 1000 billion barrels
oil shale = 1000 billion barrels (above $30/barrel)
I've already made my points about the difference between 'proven reserves' (always conservative based only on current knowledge and technology of fields) versus
'ultimately recoverable resources'. If we want to talk about what will happen in the next 50 years, we have to talk
about the latter concept. My 3 trillion number was including the tar sands, which is starting to be included in even 'proven reserves' estimates. This is reasonable, we are producing it now.
Still, if you want to back that out, you still have about 2 trillion of conventional oil economically recoverable oil (meaning we are really 1/3 of the way through).
See their publication:
http://www.iea.org/bookshop/add.aspx?id=204