Four years into the shale revolution, the U.S. is on track to pass Russia and Saudi Arabia as the world's largest producer of crude oil, most analysts agree. When that happens and by how much, though, has 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 a gain of 1.5 million barrels per day, or less than half of what Iraq produces, up to an increase of 7.5 million bpd by 2020 — almost doubling current domestic output of 8.5 million bpd.

The disparities are a function of the novelty of the shale boom, which has consistently confounded forecasts. In 2012, the U.S. Energy Information Administration (EIA) 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. Just a year later, those predictions had already been surpassed.