Opec believes it is pumping enough oil to satisfy demand
Brent crude has fallen below $100 a barrel for the first time since April, as traders predict Opec will stick to current output levels.
With Opec now meeting in Vienna to discuss forthcoming production, Saudi Arabia has already hinted that it sees no need for a cut.
Brent crude was down $4.14 to $99.30 a barrel, while US crude lost $3.08 to $103.26.
Prices have sunk from a record of more than $147 a barrel seen in July.
Before the meeting, the Venezuelan energy minister said he would support keeping production levels unchanged.
"We need to keep things as they are," said Rafael Ramirez.
Earlier, the Saudi oil minister Ai al-Nuaimi said: "The market is fairly well balanced."
"Inventories are in a healthy position, everything is in balance."
OPEC members including Kuwait and the United Arab Emirates, have called for no change in output levels though Algeria, Iran, Venezuela and Libya have suggested a cut is needed - claiming the market is oversupplied.
A formal announcement is due early on Wednesday.
Oil analyst Stephen Schork said part of the reason why Opec would keep prices on hold, was so not to become an election issue in the US.
"I don't think the Saudis or Opec in general want to project themselves into the US presidential election, which is what would happen if you saw a production cutback."
Opec is currently thought to be producing about a million barrels per day (bpd) more than its official ceiling of 29.67 million bpd.
In May and June Saudi Arabia agreed to increase production by 500,000 bpd to help calm markets.
Opec produces about 40% of world crude.
In July, the exporters' group said world demand for oil would grow by 50% between now and 2030 as people in developing countries drive more cars.