An oil tanker sits beside transfer pipes at a terminal as it prepares to unload its cargo of fuel on July 4, 2018 in Zhoushan, China. VCG | Getty Images

Oil may not have been on China's official tariff list, but the country appears to have not imported any U.S. crude in August, according to Bimco's chief shipping analyst, Peter Sand, who cited U.S. Census data. Bimco is the world's largest shipowners association with members in more than 120 countries that control around 65 percent of international tonnage. According to Sand's research, Chinese imports accounted for 23 percent of total U.S. crude exports in 2017 and were averaging 22 percent for this year, up until August. "In the first seven months of 2018 China imported an averaged 10.6 million barrels," Sand said.

August was "a massive change to the export pattern seen since early 2017. Chinese buyers, led by the world's top tanker charter Unipec, were rumored to have stayed away and this new data proves it." Sand told CNBC: "China is keeping its oil import data closer than ever before. Up until March we had a good idea of where they were getting crude and that has all stopped. Now China is only releasing their import volumes. Not the sources of their imports."

Replaced with crude from Africa