Chinese oil giant Sinopec on Wednesday became the first Asian firm to get a stake in Qatar's expansion of North Field East, the Gulf country's energy company announced.
North Field contains the world's biggest natural gas reserves and extends under the Gulf into Iranian territory.
China is already the biggest customer for Qatar's liquefied natural gas and one of the world's top LNG importers.
China Petroleum and Chemical Corporation (Sinopec) signed a 27-year supply deal last year that Qatar said is the longest ever made in the natural gas industry.
With the North Field East expansion, Qatar has embarked on a $28.75 billion project that will see annual output grow from 77 million tonnes a year to 110 million by 2027, according to state-owned giant QatarEnergy.
QatarEnergy said Sinopec will get a five percent stake in the equivalent of an eight-million-tonne production complex.
QatarEnergy chief Saad Sherida Al-Kaabi, Qatar's energy minister, called it a "milestone agreement" and a new "landmark" for relations between the two countries.
France's TotalEnergies, Britain's Shell, Italy's ENI and the United States' ConocoPhillips all have bigger shares in North Field East.
QatarEnergy did not give a value for the Sinopec deal but said it "will not affect the participating interests of any of the other shareholders".
Al-Kaabi and Sinopec chairman Ma Yongsheng signed the accord on Wednesday.
"Today's event underscores QatarEnergy's commitment to deepening its relationships with key LNG consumers, while prioritising long-term strategic partnerships and alignment with world class partners from China, represented by Sinopec here today," Al-Kaabi said in a QatarEnergy statement.
Ma said the stake purchase would "enhance the security, stability, and reliability of clean energy supply. I hope that the two companies will continue to explore new LNG cooperation opportunities based on the solid foundation we have laid together."