Four years into the shale revolution, the United States is on track to pass Russia and Saudi Arabia to become the world's largest producer of crude oil,Reuters reported.


Efforts to determine when that happens and by how much, though, have produced disparate estimates that depend on uncertain factors ranging from progress in drilling technology to the availability of financing and the price of oil itself.


Forecasts for U.S. shale oil production vary from an increase of 7.5 million barrels per day by 2020 - almost doubling current domestic output of 8.5 bpd - to a gain of 1.5 million bpd, or less than half of what Iraq now produces.


The disparities are a function of the novelty of the shale boom, which has consistently confounded forecasters. In 2012, the U.S. Energy Information Administration estimated that production from eight selected shale oil fields would range from 700,000 bpd of so-called tight oil to 2.8 million bpd by 2035. A year later, those predictions had been surpassed.


"The key issue is not whether production grows; it's by how much," said Ed Morse, global head of commodities research at Citigroup Inc. in New York. "We're only at the beginning of the first inning, and this is a nine-inning game."


One forecast by BP PLC and the International Energy Agency predicts that the United States could become energy independent by 2035. Coupled with growing output from oil-rich Canada and Mexico, the North American continent has a growing shield from supply shocks.