These new oil fields would make them the fifth- and sixth-biggest oil fields ever discovered in the United States.
The fields are attracting billions of dollars of investment from foreign oil giants like Royal Dutch Shell, BP and Norway's Statoil, and also from the smaller U.S. drillers who developed the new techniques like Chesapeake, EOG Resources and Occidental Petroleum.
Last month China's state-owned oil company CNOOC agreed to pay Chesapeake $570 million for a one-third stake in a drilling project in the Niobrara. This followed a $1 billion deal in October between the two companies on a project in the Eagle Ford.
Drillers have unlocked decades' worth of natural gas, an abundance of supply that may keep prices low for years.