The $8 billion black market for Venezuelan oil is suddenly closing down

President Trump’s partial oil blockade threatens to devastate this black market, which U.S. officials say lines Venezuelan leader Nicolás Maduro’s pockets and props up the impoverished country’s fragile economy.

U.S. officials said the military would be going after a network of ships already sanctioned by the Treasury Department. Such tankers account for about 70% of Venezuela’s oil exports, mostly sent to Asian buyers who pay in cryptocurrencies, Venezuelan economist Asdrúbal Oliveros said on the country’s public radio Wednesday.

Losing access to that network of ships would reduce Venezuelan revenue by some $8 billion a year, Oliveros estimated.

Some sanctioned tankers have already turned around to avoid the U.S. Navy flotilla in the Caribbean, which has already seized one vessel last week. At least one tanker has left Venezuela since that ship, the Skipper, was confiscated, but it was carrying fuel oil, not the more valuable crude.

About 75 tankers are loitering in Venezuelan waters, and half are on Treasury’s blacklist for sanctions violators, according to TankerTrackers.com. About two dozen are typically used to export crude oil.

The “blockade"—the word Trump used—is better called a quarantine, U.S. officials said, since legal tankers will still have free passage to Venezuela, and a real blockade is considered an act of war under international law. But it still marks the most extraordinary use of U.S. military might to enforce oil sanctions against Venezuela.

The hard truth for Venezuela is that it is more vulnerable to an oil embargo, because it isn’t a military power like Russia or Iran, which also face stiff U.S. oil sanctions that are loosely enforced.

Maduro and his aides have condemned the U.S. actions as part of an effort to overthrow the government and loot Venezuela’s oil reserves, some of the largest in the world. Caracas also accused Washington of theft after last week’s seizure of a tanker carrying 1.9 million barrels of Venezuelan crude to Cuba.

“For 25 weeks, Venezuela has been condemning, confronting and beating back a multidimensional aggression campaign that goes from psychological terrorism to the piracy of oil," Maduro said late Tuesday, promising not to back down. “Venezuela has shown it is strong. We are prepared to continue on our accelerated path toward a hardened revolution."

Venezuela’s national oil company said its shipping continued “with full insurance, technical backing and operational guarantees." Western diplomats in Venezuela said the company told them the Venezuelan navy would escort tankers into port, but the diplomats said the country’s military was unlikely to challenge U.S. operations.

Trump has said Maduro should leave office and that his “days are numbered." He has also stepped up military activity off Venezuela, including with more than 20 strikes on alleged drug-carrying boats, killing scores of people. However, Trump hasn’t explicitly linked his actions to regime change in Venezuela. He has said U.S. military strikes on Venezuela itself will soon come.

Cutting off the flow of sanctioned vessels would be a blow to Maduro’s personal fortune and the money he uses to enrich his inner circle. It would also devastate the national economy.

Economists warn that strict enforcement by the U.S. would curtail hard-currency inflows, stoking food and fuel shortages in a largely import-dependent country while exacerbating inflation, which the International Monetary Fund estimates will reach nearly 700% next year.

Venezuela pumps about 900,000 barrels of oil a day, a far cry from its peak of 3.5 million barrels a day in the late 1990s. Now, the only non-sanctioned oil leaving the country is shipped by Chevron, which has a narrow exemption from U.S. financial restrictions.

To ship oil itself, Venezuela taps into a so-called shadow fleet of 900 vessels that was pioneered by Iran when it came under heavy sanctions over a decade ago and then by Russia after the Ukraine war began. The network uses old tankers that sometimes falsely run under a country’s flag, conduct ship-to-ship crude transfers to conceal its origin and turn off transponders that give up their locations.

U.S. sanctions normally deter aboveboard oil traders, but a black market flourished. Now that the U.S. is threatening to strictly enforce the sanctions, some of the shadow fleet appear to be reconsidering.

The Bella 1, a vessel sailing to Venezuela from Iran, made an abrupt U-turn on Monday. The tanker, which is under U.S. sanctions for previously transporting Tehran’s oil, changed its route again toward South America on Wednesday, giving Curaçao as its destination.

Other oil disruptions took place on Nov. 29, the day Trump said Venezuelan airspace should be considered closed, a major escalation.

That day, the Star Twinkle 6, a Panama-flagged tanker, started loitering in the Caribbean, loaded with 830,000 barrels of light Venezuelan oil, according to Kpler data. The vessel, which normally delivers Venezuelan oil to China, was sanctioned by the U.S. in May for having previously transported Iranian crude.

With 11 U.S. warships in the Caribbean, the Trump administration already has the resources to enforce a quarantine, experts say. Navy destroyers, which are highly maneuverable and carry powerful weapons, would likely be used to intercept oil tankers and escort them to a designated marshaling area. Once the tankers are in a concentrated zone, military helicopters could patrol the area to make sure the tankers comply with the quarantine, the experts say.

“The Navy has a whole process for how you go up alongside a ship" and intercept it, said Bryan Clark, a naval strategist at the Hudson Institute. First, U.S. sailors would use passive measures and contact the tanker by radio on an international channel. If the tanker doesn’t comply, more active measures can be employed, such as blocking the ship from its destination.

“You don’t anticipate any resistance, because these tanker operators are just people hired to operate tankers, and there’s nothing for them to gain if they fight off one of these inspection parties," Clark said.

If a tanker does resist the U.S. quarantine demands, special teams like Marines, Navy SEALs and Coast Guard Law Enforcement Detachments could be brought in to board the ship, the experts said.

Still, fully enforcing a quarantine or blockade is difficult and could take time to have an impact, prolonging the standoff. “Maduro may only blink if the threat of military action is credible," Nicholas Watson, director at the risk consulting firm Teneo, said in a note Wednesday.

Write to Benoit Faucon at benoit.faucon@wsj.com, Costas Paris at costas.paris@wsj.com and Shelby Holliday at shelby.holliday@wsj.com