Oil prices dropped on concerns over the global oil oversupply. The oil driller Baker Hughes reported on Friday that the number of active U.S. rigs increased by 2 rigs to 574 last week. It was the first rise in 11 weeks.
Market participants were concerned about that terror attacks in Paris could have a negative impact on the economic growth.
The European Central Bank (ECB) Vice President Vitor Constancio said in Frankfurt on Monday that terror attacks in Paris could have a negative impact on markets.
"It can compound all the problems that we were already facing," he said.
"Markets, so far, are taking it calmly," Constancio added.
WTI crude oil for December delivery dropped to $40.19 a barrel on the New York Mercantile Exchange.
Brent crude oil for December fell to $43.31 a barrel on ICE Futures Europe.