President Enrique Pena Nieto proposed historic changes to Mexico's state-run energy sector Monday, cracking open the door for global oil giants such as Exxon Mobil and Shell to invest in Mexico's lethargic 75-year-old state oil monopoly, Pemex, a symbol of deep nationalist pride.

In a highly anticipated address, Pena Nieto stopped short of offering foreign oil firms what they really want: a right to own and sell the oil they drill in Mexico.

Instead, he proposed constitutional changes that will allow for risk and profit-sharing partnerships between foreign firms and Pemex, the eighth-largest oil business in the world, and a move aimed at luring the money and technology necessary to exploit Mexico's huge but hard-to-reach deep-water and shale oil fields.