Mexican officials say they expect to open bidding on the first round of private oil contracts in the first half of 2015.

A constitutional reform passed last year opened the state-owned energy sector to private investment. But enabling legislation is still before congress.

Mexico's assistant energy secretary says the first round of bidding will offer a range of shale gas, onshore and deep water fields.

Lourdes Melgar said Monday that Mexico hopes to have private companies producing a half million barrels of oil a day by 2018, while state-owned oil company Petroleos Mexicanos should maintain production at about 2.5 million barrels a day.

Mexican tax authorities say they expect government revenue, on average, to be about 50 percent of production value from private projects operated under contract, license or concession.