TOKYO — China’s opposition to a U.S.-proposed oil embargo on North Korea has generally been explained in political terms, but Beijing may also have a hidden motive involving its own petroleum infrastructure.

Crude extracted from the Daqing Oil Fields in northeastern China contains a large amount of paraffin used for candles. “If production is halted, the pipes will clog up,” wrote energy information company Rim Intelligence, referring to infrastructure there in a report released last year.

“If the wax ingredient hardens, reopening the pipes will require heating them” to unclog, explained Takayuki Nogami, chief economist at Japan Oil, Gas and Metals National Corp. China may fear that it will not be able to easily recover these pipelines if it stops sending oil through them.

“China supplies most of North Korea’s crude oil imports, although China’s customs records have not reported any amounts in the past few years,” according to the U.S. Energy Information Administration. Daqing oil is transported from Dandong to North Korea via underground pipes, making it hard to verify whether export restrictions are being kept.

North Korea’s Pongwha chemical factory receives more than 500,000 tons of crude from China annually, according to South Korea’s Chosun Ilbo newspaper. “If China decreases oil exports to North Korea, Russia will just increase them,” said Naoya Abe of Rim Intelligence. “China wants to keep its influence over North Korea.”

There are reports that Russia is also illegally sending oil to North Korea by land.