Texas is the nation's largest oil-producing state, accounting for about 25% of the total. Fueling that output is a combination of legacy oil fields and new shale discoveries, especially those in the Permian Basin of west Texas, which alone contributes 14% of the total U.S. oil output.

While there are thousands of oil fields spread across the state, many don't produce that many barrels of oil each month. Instead, the bulk of the state's production comes from just two monster oil fields: the Permian Basin and the Eagle Ford shale. Here's a closer look at why these are the two largest oil-producing fields in the Lone Star state.