By Catherine Bremer

MEXICO CITY, Dec 11 (Reuters) - Mexico's flagging crude oil output could slide by a third in the next decade without a spurt in exploration especially in deep water, Energy Minister Georgina Kessel said on Tuesday.

Kessel told an energy event that Mexico risked seeing its oil production decline to 2.1 million barrels per day (bpd) in the worst case scenario, from 3.1 million bpd this year.

She said that if state-run monopoly Pemex ramped up its exploration and began immediately drilling at newly-discovered oil deposits, crude output could average around 3.25 million bpd and rise to 3.4 million bpd by 2016.

Pemex oil production peaked at 3.4 million bpd in 2004 but it has seen output slip since then as yields decline at its huge, but aging, Cantarell oil field.

Mexican lawmakers sat down this week to discuss long-awaited changes to Mexico's closed energy sector. The ruling party wants to enable strategic alliances particularly in deep water and cross-border oil deposits.

But opposition parties ruled out drastic changes to allow full foreign investment in the industry.

Mexico is the world's No. 9 exporter of crude and a top three supplier to the United States.

Mexico is dealing with the double headache of stagnating oil output and declining reserves.