Brent crude was 105.10 dollars, its weakest since July 15 and was down 80 cents at 105.22dollars a barrel.
U.S. crude futures fell 85 cents to 97.32 dollars a barrel, following 6.8 per cent decline last month, the biggest monthly loss since May 2012.
The outage at Coffeyville refinery, a major crude consumer, could last up to four weeks, according to its operator.
“A stronger dollar and concerns about stalling oil demand especially from China have been the bear factors at work, eroding prices,” said David Hufton, Managing Director of brokerage PVM Oil Associates.
Oil prices remained steady as U.S. job growth slowed more than expected in July.
Non-farm payrolls increased 209,000 last month, less than expected, after surging by 298,000 in June, the Labor Department said on Friday.
The front of the Brent futures price curve is trading at a heavy discount to later barrels in a formation known as a contango.
This discount has now lasted longer than any since early 2011, Morgan Stanley analyst Adam Longson said.
Oil investors say they are less worried now than a month ago about the risk to oil supplies from conflicts and civil turmoil despite heavy fighting in many countries.
OPEC’s second largest producer, Iraq, is battling an Islamic insurgency in the north and west.
The conflict threatens to split the country, but has yet to have an impact on near-record oil exports from the south.
Baghdad is also embroiled in a dispute with Iraqi Kurdistan over oil exports via Turkey.
In Libya, oil output remains around 500,000 barrels per day (bpd), down from one million bpd in 2012, following weeks of clashes between rival militias.