The light crude settled down $1.69 at $55.31 per barrel in the week and Iran Heavy, one of the country's main export grades, fell $1.26, or 2.3% to $53.27 per barrel in the week, IRNA reported on Saturday, citing a report by the Oil Ministry.
In its latest monthly report published last week, calculations by OPEC showed the price of Iran's heavy crude rose 7.3% to $52.27 a barrel in September from the previous month.
The price of OPEC basket of 14 crudes stood at $54.44 a barrel on Thursday, compared with $54.54 the previous day, according to the latest OPEC Secretariat calculations.
Oil prices were buoyed in recent weeks by shutdowns in US refineries as well as Turkey's threat to cut off oil flow from Iraq's Kurdistan region. However, gains were limited as most US refineries continue to come back on stream following a string of hurricanes that swept the US oil heartland in recent weeks.
Additional supplies by Africa's top producer Nigeria and Libya, the country with the largest oil reserves in the continent, weighed on global prices. The OPECmembers' combined output increased by 104,000 barrels per day to 2.77 million bpd in September.
International benchmark Brent futures gained 92 cents, or 1.6%, to settle at $57.17 a barrel on Friday, while US West Texas Intermediate crude rose 85 cents, or 1.7%, to settle at $51.45 per barrel.
An initiative that involves more than two dozen oil producing nations to cut supplies through the first quarter of 2018 has helped prices remain in the range of $45-55 for the best part of the year. However, that is a long shot from three years ago when oil traded above $100 a barrel.