The survey of 43 participants forecast Brent would average $68.02 a barrel in 2021 versus a forecast in July for $68.76. It is the first downward revision to the 2021 price view since November 2020.
Brent has averaged about $67 this year.
The International Energy Agency said this month oil demand was set to increase more slowly for the rest of 2021 on the spread of the Delta variant.
But the Organization of the Petroleum Exporting Countries has so far stuck to its prediction of a strong demand recovery.
OPEC and its allies, or OPEC+, agreed in July to boost output by 400,000 barrels per day a month starting from August until its current reductions of 5.8 million bpd are phased out.
Some analysts said the group, which meets on Wednesday, could reconsider hiking output due to the impact of rising COVID-19 cases. Others downplayed the hit to consumption.
U.S. crude was forecast to average $65.63 in 2021 versus $66.13 in July, amid a slow recovery in production.
Commerzbank analyst Carsten Fritsch predicted a "gradual, unspectacular increase (in U.S. output), not strong enough to cause any headaches for OPEC+."
The U.S. Energy Information Administration had forecast output to fall 160,000 barrels per day (bpd) in 2021 to 11.12 million bpd.