The world's premier energy monitor is preparing a sharp downward revision of its oil-supply forecast, a shift that reflects deepening pessimism over whether oil companies can keep abreast of booming demand.

The Paris-based International Energy Agency is in the middle of its first attempt to comprehensively assess the condition of the world's top 400 oil fields. Its findings won't be released until November, but the bottom line is already clear: Future crude supplies could be far tighter than previously thought.

A...