MEXICO CITY, Feb 15 (Reuters) - State oil monopoly Pemex said on Friday it expects Mexico's Ku Maloob Zaap oil fields, its most productive wells, to maintain output at 850,000 barrels per day (bpd) through 2017. Pemex said in a statement that Ku Maloob Zaap's 170 producing wells contributed a third of Mexico's overall crude output last year, and in December achieved 865,000 bpd of crude and 349 million cubic feet of gas. The aging but still highly productive Ku Maloob Zaap fields are located in the shallow waters of the Bay of Campeche. 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. President Enrique Pena Nieto has ambitious plans to lure significant private investment into Mexico's energy sector, under state control since 1938. But he may struggle in a divided Congress to secure the reforms needed to significantly boost production.