International benchmark Brent crude traded at $66.59 per barrel on Tuesday at GMT 0648 while American benchmark West Texas Intermediate (WTI) saw prices of $61.58 per barrel at the same time.
On Monday, Brent crude was trading at $67.64 per barrel at 0625 GMT on Monday while WTI was at $61.79 a barrel at the same time.
Oil prices slightly decreased despite the U.S. and China reaching a phase one trade deal, which encourages investors that global oil demand could be stronger than the expectations for 2020.
In addition, OPEC and its allies, dubbed as OPEC+, agreed on Dec. 6 to curb their production by an additional 0.5 million barrels per day (bpd) starting from Jan. 1.
The existing output cut is already a 1.2 million bpd.
Saudi Arabia, OPEC's de facto leader, also said this month that it would voluntarily lower its individual output level by 400,000 bpd.
As member countries of OPEC+ prepare to lower their oil production, investors expect some oversupply in the global oil market to fall at the beginning of 2020, which create an upward push on crude prices.
In the meantime, Iran's Revolutionary Guard on Monday announced it had seized an oil tanker near the island of Abu Musa in the Persian Gulf, according to Iranian television.
The oil tanker was stopped by Iran over allegedly "smuggling 1.312 million liters of fuel," TV channel Al-Alam reported without elaborating.
By Gulsen Cagatay