(Adds background on budget proposal, oil output)

MEXICO CITY, Sept 13 (Reuters) - S&P Global Ratings said on Friday that the Mexican government's 2020 economic growth forecast of 1.5% to 2.5% likely is too optimistic.

The government's growth forecast is part of its 2020 budget blueprint unveiled last weekend, which also slightly loosened a primary surplus target as the government aimed to balance big welfare promises with the reality of a stagnant economy.

"It's probable that our updated growth projections will be more conservative," S&P Global Ratings said in a statement, referring to the agency's upcoming forecast for the third quarter.

S&P added that the government's estimates for financing requirements and a 2020 primary budget surplus are in line with its own expectations. But it said it sees a risk that the government's oil output forecast of 1.9 million barrels per day will not be met next year.

Current oil output, the vast majority of which is produced by state-owned Pemex, averages about 1.7 million barrels per day. (Reporting by Frank Jack Daniel; Writing by David Alire Garcia; Editing by Leslie Adler)