Persistent oversupply concerns knocked crude oil prices and industry shares on Monday.
The price of a barrel of Brent crude tumbled by 1.8% to US$44.86, back below the US$45 mark. US light crude fell 2% to US$43.31 a barrel.
FXTM Research analyst Lukman Otunuga said: "Oil has been fundamentally bearish for an extended period and could be poised to decline lower when the awful mix of both supply and demand fears encourages bears to install a heavy round of selling.
"Sentiment is clearly turning bearish towards West Texas Intermediate and the appreciating dollar could ensure prices trade lower towards US$40.
"A decisive breakdown and daily close below US$44 could encourage sellers to drag prices lower towards US$40."
Mike van Dulken at Accendo Markets highlighted the Baker Hughes rig count, which posted a sharp increase of 14 operational rigs last week.
He said: "Markets remain concerned by a continued glut of gasoline and other refined products."