The biggest challenge will come between 2010 and 2015, the IEA says in its 2008 World Energy Outlook. For the next couple of years, the oil pipeline is well supplied. But that trails off after 2010. By 2015, the world needs to find an additional 7 million barrels per day of oil above and beyond all the exploration projects currently in the pipeline. And to get that oil to market by the middle of the decade, those exploration projects need to get started now.

But now’s not a good time. Cheaper oil prices have already pushed back a host of new projects, from Saudi Arabia to Canada. Oil below $90 is making Iran think twice about new and much-needed oil investment. At the same time, the short-term swan dive in oil demand in the developed world is pushing OPEC in the opposite direction, with fresh talks of deeper production cuts at or even before the oil cartel’s December meeting.

The challenge, says the IEA, is made harder because of the way the world’s oil markets are evolving. Most new oil field discoveries are in small fields, or offshore, or both—in stark contrast to the onshore super-giant fields of yesteryear. Why does that matter? Because smaller fields and offshore fields decline faster than bigger, onshore fields.

That means that the oil industry has to run even harder to stay in place, because production at oil fields around the world is winding down at an ever-quickening pace. The IEA surveyed more than 800 oil fields around the globe, and estimates production at oil fields is declining at about 6.7% a year today, and will decline by 8.6% in 2030.

That’s with constant investment to keep the fields in reasonable shape; without that upkeep, the decline rate jumps to over 10%. Which means that the world needs to add the equivalent of an Algeria every year through 2030 just to offset that decline, the IEA says.