Mexico's Democratic Revolution party (PRD) lawmakers occupy the chamber of deputies on Dec.11, 2013 in Mexico City. Leftist lawmakers locked themselves inside Mexico's chamber of deputies Wednesday to prevent the lower house from debating a controversial oil reform bill that passed the Senate hours earlier. LIBRADO BAEZ/AFP/Getty Images

Mexico's Senate on Wednesday overwhelmingly approved an energy reform to permit the biggest oil industry opening in 75 years, sending it to the lower house where leftists padlocked doors to the chamber to stop lawmakers from debating the bill.

The reform in the world's 10th largest oil producer, backed by the ruling Institutional Revolutionary Party (PRI) and the opposition conservative National Action Party (PAN), was sent to the lower house for debate and final approval.

The government aims to pass the bill this week, but after it arrived in the lower house, leftists occupied the speaker's podium and piled up chairs to block access routes to the chamber and sealed other doors with chains and padlocks.

The stalling tactics by leftists are unlikely to do more than cause a slight delay in approving the bill, which passed the Senate with more than two thirds of the lawmakers' votes.

If the bill is passed by the lower house, it would then need to be approved by the legislatures of 17 of Mexico's 31 states.

The overhaul would be the crowning piece of President Enrique Pena Nieto's first year of reforms, which have also targeted education, the tax system and telecommunications. But the energy overhaul is considered most crucial to the overall economy and the remaining five years of Pena Nieto's presidency.

Under the legislation, contracts could be made directly with the state rather than with the state-run oil company, which would lose its monopoly on Mexican oil. The bill would allow contracts for profit- and production-sharing as well as licenses under which companies would pay royalties and taxes to the Mexican government for the right to explore and drill.

Private companies would have to specify in contracts that all oil and gas found belongs to Mexico. The constitution would continue to prohibit oil concessions, considered the most liberal kind of access for private oil companies.