(Adds quote from executive)

MEXICO CITY, Nov 12 (Reuters) - Mexican state oil company Pemex will invest more than 250 billion pesos ($18.9 billion) a year in coming years, Chief Executive Juan Jose Suarez said on Thursday.

Pemex is trying to reverse a steep drop in output as its once-largest oil field, Cantarell, declines.

"We estimate that Pemex will have to invest sums larger than $250 billion pesos per year," Suarez told a conference.

The slide in production at Cantarell and a lack of major new discoveries has reduced Mexican oil exports to their lowest level outside hurricane season since 1990 and poses a major threat to the federal budget, which relies on oil revenues to fund more than a third of spending.

($1 = 13.2 pesos) (Reporting by Robert Campbell; Editing by Gary Hill)