* Pemex's 3rd discovery of light crude since August
* Navegante-1 began drilling in April 2011
MEXICO CITY, Nov 25 (Reuters) - Mexico on Sunday announced the discovery of light crude oil in an onshore well in the southern state of Tabasco, the country's third light crude find since August.
The oil, which lies at a depth of 6 km (3.7 miles) in the Navegante-1 well, has proven, probable, and possible reserves (3P) of up to 500 million barrels of crude oil equivalent (MMBoe), according to the state oil monopoly Pemex.
"This is one of the biggest discoveries in the last 10 years, particularly onshore," outgoing President Felipe Calderon said at an event.
Pemex, which began drilling in Navegante-1 in April 2011, said it plans to begin production on the new find next month.
Mexico is the world's No. 7 oil producer but output has stagnated in recent years, dropping by roughly a quarter since hitting a peak of 3.4 million barrels per day in 2004.
Incoming president Enrique Pena Nieto has ambitious plans to open the energy sector to more private investment, but he may struggle in a divided Congress to secure a constitutional reform needed to forge ahead with sweeping changes.
In the meantime, Calderon, who leaves office at the end of the month, has been quick to tout the country's successes.
In late August, Calderon announced the discovery of Mexico's first deepwater oil deposit - light crude in the Gulf of Mexico's Trion-1 well, in the Cinturon Plegado de Perdido area, which lies near the U.S. border.
In early October, Calderon made public a second discovery of deepwater light crude in the Gulf, this time in the Supremus-1 well, near the Tamaulipas coast, and not far from Trion-1.
Pemex said plans to exploit the new deposit are under way.
A discharge line is under construction and the company plans to drill two border wells to determine the size of the deposit, Pemex said in a press release.
It also said it is acquiring seismic information to see whether the deposit extends as far south as the Antonio Bermudez complex, which has estimated reserves of 700 to 1,000 MMBoe.
But an overhaul of Pemex, which many analysts say is needed to boost production, may not come as swiftly as the raft of recent deposit discoveries.
Senate Energy Committee President David Penchyna, from the same party as the president-elect, told Reuters this month that the new Mexican government would send Congress an energy reform initiative in the first half of 2013.
But he warned that it could be more watered down than what Pena Nieto outlined in his campaign - a possible constitutional change to allow increased private investment in the oil industry.