MEXICO CITY, Nov 22 (Reuters) - Mexico's state-run oil company Pemex will announce in three weeks the auction of six blocks in its massive Chicontepec basin which contain roughly 400 million barrels of crude reserves each, the company said on Thursday. The blocks make up 15 percent of the basin's total reserves, and Pemex says it expects to award the contracts in July 2013. The Chicontepec basin, discovered more than 80 years ago, is located on the Gulf coast of Mexico and is home to about 40 percent of Mexico's crude reserves. Mexico is the world's No. 7 oil producer but production has stagnated in recent years, dropping by roughly a quarter since hitting a peak of 3.4 million barrels per day in 2004. Mexico's 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.