* Brazil crude output slips to lowest in 3-1/2 years
* Maintenance in giant Brazil fields cuts production (Adds production detail, share price)
RIO DE JANEIRO, Sept 28 Petrobras' oil and natural gas output fell to its lowest level in 22 months in August, slipping 0.3 percent from July to an average 2.54 million barrels a day from fields in Brazil and abroad, the company said in a statement on Friday.
Crude oil output in Brazil, the company's principal source of production, fell to an average of 2.07 million barrels a day in the month, its lowest in three and a half years.
Scheduled maintenance at the giant Roncador and Marlim Leste fields in the Campos Basin accounted in the main for falling output.
Lower output was one of the factors that led to the company posting a loss in the second quarter, its first loss in 13 years.
Output at Petrobras has been falling despite enormous spending to increase reserves and production. Earlier this year the company announced a $237 billion, five-year investment plan. The company expects to more than double output to about 5.7 million barrels of oil and gas a day by sometime in 2020.
Petrobras preferred shares, the company's most-traded class of stock, fell 1.5 percent to 22.37 reais in Sao Paulo on Friday. (Reporting by Jeb Blount; Editing by Daniel Magnowski)