London The world's top oil and gas firms are struggling to deliver the output growth they need to outpace the burgeoning cost of exploration and development.

Third-quarter results from Exxon Mobil, Royal Dutch Shell and other top international players released over the past few days mostly beat expectations thanks to a shortage of the fuels and other crude-oil based products they make. That widened the gap between fuel prices and crude oil, lifting margins in the downstream part of the business. Companies with refineries in North America are also processing less costly domestic crude oil from shale formations.