By Arathy Somasekhar and Georgina McCartney

HOUSTON, Feb 26 (Reuters) - Spot prices of a key medium crude grade in the U.S. Gulf Coast spiked on Wednesday as refiners sought alternatives following President Donald Trump's termination of a license allowing U.S. major Chevron CVX.N to export Venezuelan oil.

Trump said he was reversing an authorization granted by former President Joe Biden's administration in November 2022, without mentioning the company's name. Venezuela's Vice President Delcy Rodriguez said the U.S. had made a "damaging and inexplicable decision" by going against Chevron.

In January, Chevron exported almost 300,000 bpd of Venezuelan oil to the United States on average, according to LSEG vessel tracking data and reports from state company PDVSA.

While imports of Venezuelan crude accounted for only 3.5% of total U.S. crude imports in November, it was about 13% of crude oil imported by U.S. Gulf Coast refineries, according to the U.S. Energy Information Administration.

Venezuela was the fourth largest crude supplier to the U.S. in the first 11 months of 2024.

Prices for Mars, a medium sour crude that U.S. refiners favor, spiked 70 cents higher to a $1.70 premium over U.S. West Texas Intermediate crude on Wednesday, the grade's largest daily rise in about a month.

While the United States can find other supplies, the price of the alternative barrels and the cost of shipping will pressure U.S. refinery margins amid extensive U.S. sanctions on Russian oil that have already tightened heavy crude supplies.

Trump's announcement to block Venezuelan oil purchases comes just days before his month-long pause on Canadian and Mexican oil tariffs ends. Any tariffs on those would significantly tighten heavy crude supplies along the U.S. Gulf Coast.

Gulf Coast refiners will look to alternative heavy crude oil such as Colombian, said Rohit Rathod, a senior oil market analyst at Vortexa. Refiners may also look as far as the Middle East for alternatives, he added.

Higher volumes of Guyanese oil could also make their way into the United States, a trading source said, while another source said U.S. refiners would likely seek heavy fuel oil from Brazil.

"The news could certainly support other grades including Colombian temporarily although some refiners have been actively replacing crude with fuel oil as feedstocks, or taking lighter domestic crudes," a trader said.

Ecuador's President Daniel Noboa in January said his country could ramp up exports to as much as 250,000 bpd to countries currently buying Venezuelan oil to make up for any shortfall from license terminations in Venezuela.

Valero Energy Corp VLO.N was the top processor of Venezuelan crude in 2024, followed by Chevron and then PBF Energy PBF.N.

Trump said the license termination would take effect on March 1. It was not immediately clear what would happen with cargoes of Venezuelan crude currently navigating to U.S. ports or about to depart from Venezuela through the end of the month.

As has happened before, Washington might allow Chevron to discharge those cargoes shipped before the deadline, Rathod said, adding that in the worst case, they will have to resell them, likely to Europe or India.

Chevron typically ships five to six cargoes of Venezuelan oil per week to its own U.S. refineries and to others, contributing significantly to shipping activity at Venezuelan and nearby ports such as Aruba.

Chevron said it was considering the implications of the Trump administration's announced termination of its deal with Venezuela.

(Reporting by Arathy Somasekhar and Georgina McCartney in Houston; Editing by Sam Holmes)

((arathy.s@thomsonreuters.com; +1 832 610 7346; Twitter: @ArathySom;))