WAFRA, Kuwait—The Arabian Peninsula has fueled the global economy with oil for five decades. How long it can continue to do so hinges on projects like one unfolding here in the desert sands along the Saudi Arabia-Kuwait border.

Saudi Arabia became the world's top oil producer by tapping its vast reserves of easy-to-drill, high-quality light oil. But as demand for energy grows and fields of "easy oil" around the world start to dry up, the Saudis are turning to a much tougher source: the billions of barrels of heavy oil trapped beneath the desert.

That the Saudis are even considering such a project shows how difficult and costly it is becoming to slake the world's thirst for oil. It also suggests that even the Saudis may not be able to boost production quickly in the future if demand rises unexpectedly. Neither issue bodes well for the return of cheap oil over the long term.

"The easy oil is coming to an end," says Alex Munton, a Middle East analyst for the Scottish energy consulting firm Wood Mackenzie. The major oil fields in the Gulf region, he says, have pumped more than half their oil—the point at which production traditionally begins to decline.