By Adriana Barrera and Ana Isabel Martinez MEXICO CITY, June 15 (Reuters) - Mexico's state oil monopoly Pemex will revise its key Chicontepec oilfields project, seen as crucial to boosting output in coming years, and may also adjust its overall investment plans, a Pemex adviser told Reuters on Friday. Chicontepec, which is located on the country's gulf coast and holds 41 percent of Mexico's total crude reserves, has been unable to boost production despite heavy investment. Pemex,