WASHINGTON — Global and U.S. oil prices tumbled sharply to four-year lows Friday on news that the oil-producing cartel OPEC has opted not to cut production, raising the prospects of a world oversupplied with crude for the foreseeable future.

The price for a barrel of West Texas Intermediate crude, the U.S. oil reference price, fell by $7.70 to $65.99 in post-Thanksgiving trading on the New York Mercantile Exchange, a dizzying drop considering it was above $100 a barrel over the summer.

The price for Brent crude, an international reference price that U.S. gasoline producers use to set their own prices, tumbled $1.01 to $71.57 in European trading Friday after falling 6.7 percent the previous day.

Shares of companies across the energy industry fell. Chevron Corp. slid 5 percent while Exxon Mobil fell 4 percent. Newfield Exploration fell the most among companies in the Standard & Poor’s 500 index, dropping 16 percent.

The drops follow a decision by the oil cartel Thursday to leave its production target of 30 million barrels per day in place.

The price plunge signals even lower gasoline prices for U.S. consumers, who are enjoying the cheapest Thanksgiving travel period in five years. A gallon of regular unleaded gasoline averaged $2.792, the AAA motor club said Friday, compared with $3.034 a month earlier and $3.283 a year ago.

Riding high are the airlines, package delivery services and cruise lines, which are spending less on fuel. Southwest Airlines rose 6.5 percent Friday, the most in the S&P 500 index, while Delta Air Lines Inc. was the second-biggest gainer, climbing 5.5 percent.