West Texas Intermediate (WTI) and Brent crude oil futures posted a series of gains during the second half of last week, but those overall increases were erased Monday.

The April WTI contract price shed $1.78 Monday, settling at $55.48 per barrel. The benchmark traded within a range from $55.08 to $57.53.

Brent crude oil for April delivery ended the day at $64.76 per barrel, reflecting a $2.36 decline.

“OPEC is losing its market power right before our eyes,” Tom McNulty, Houston-based managing director with Great American Group, told Rigzone. “WTI dropped three percent today, and the market assumed it was all about the tweet from President Trump.”

As this Bloomberg article posted on Rigzone notes, Trump used an early Monday tweet to urge OPEC to take steps to moderate crude oil prices. McNulty acknowledged that the assumption linking Trump’s tweet to Monday’s price decline could be correct, but he added that oil market dynamics are clear.

“But the reality is that as OPEC cuts production, U.S. producers increase theirs,” McNulty explained. “It’s becoming harder and harder to push producers here out of the global market.”

Like crude oil, the price of reformulated gasoline (RBOB) ended Monday’s session lower. The March RBOB fell nearly seven cents to settle at $1.545 per gallon.

Henry Hub natural gas futures bucked the trend of the other benchmarks tracked by Rigzone and posted a gain Monday. March gas futures rose 12 cents, settling at nearly $2.84.