Over the next 20 years or so, oil and natural gas will lose top ranking as the world's most affordable energy sources, according to a survey of energy executives. Deeper wells in more inhospitable places, both political and geological, have altered presumptions of doing business in the oil patch. Nearly three out of four executives and managers surveyed last month by Deloitte LLP said oil and gas are the cheapest available energy sources for now, though only 23 percent believe that will be the case in 25 years. The sampling revealed a growing concern about the sustainability of oil and natural gas in the coming years. Future sources of fossil fuels, the cost of producing them and the price consumers will pay are some of the biggest uncertainties facing the industry. Clearly, the oil and gas professionals involved in our survey are starting to think about the nation's transition to renewable energy and other alternative fuels," said Gary Adams, vice chairman of Deloitte's oil and gas practice. Of the executives interviewed by Deloitte, 53 percent said they think the U.S. could run out of reasonably priced oil within the next quarter century, and 56 percent said the world is likely to face the same scenario in the next 50 years. Three out of four said shifting away from the nation's reliance on fossil fuels for transportation needs is an appropriate goal for the country, yet most think the best alternative right now is natural gas. About 30 percent said electric plug-in vehicles are the most promising alternative.