Since peak oil has been one of the main concerns on PP for a good while, an occasional status report might be useful. Gail Tverberg's recent article and the following comments provide a good look at the long term prospects for new oil supplies from shales and tight sands. A good look at the current situation in the U.S. is provided here by Frank Holmes.
Although there are other tight formations, such as the Mississippi lime in Kansas and Oklahoma that will soon be added to the mix, it is likely that collective tight formation production will peak below about 3 million barrels per day. Sustaining that level of production would require that drilling continue at the peak pace indefinitely. The reason for this is that about 90% of the ultimate recovery of a well occurs within two years. At best, the U.S. will be left to import at least half of the oil that it uses.
One of the side effects of the oil production from tight sands and shales has been the production of huge amounts of associated natural gas. This has depressed natural gas prices below the levels required for shale gas wells to be economical. Shale gas production has already peaked, however a huge resource remains and will eventually be recovered when price levels permit.
Tight sands and shales are ubiquitous throughout the world and they will eventually be developed by many other countries besides the U.S. As supplements to the production from older conventional fields, they might sustain world oil production at current levels for decades. But, as Gail Tverberg pointed out, there are both physical and economic reasons why they cannot replace the production from the older fields. With $100 per barrel oil and some additional conventional oil production from Iran and Iraq, we can probably sustain the current level of production for another decade, but don't expect economies to grow without additional new supply.