The world’s largest oil and gas company Saudi Aramco has filed an inter partes review (IPR) against a Korean petrochemical business in what is a highly unusual move by one of the energy majors.

The Saudi national oil giant, which produces 12.5 million barrels per day, has brought the IPR against SK Innovation, which started life as the Korea Oil Company before morphing into a broad-based energy and chemicals business. The patent in question, number 9,023,979, relates to a method of preparing epoxide/CO2 polycarbonates and was issued in 2015.

It’s not clear what has prompted the review - there is no ongoing patent litigation between the two companies, which might mean that it is related to licensing negotiations that have broken down and Saudi Aramco has brought the IPR in order to gain some leverage in the talks.

While the Saudi national oil company has recently been earning headlines for a much-anticipated IPO, it has also been quietly growing its patent portfolio. In 2016 it appeared for the first time in the Intellectual Property Owners Association’s (IPO) ranking of the top recipients of US patents and this year saw the number of grants it received jump by a little over 43% to 171 (placing it at 195 on the list).

Whatever the reason for the IPR, it is highly unusual for an oil major to petition for a review, which remain dominated by the high-tech sector and, to a lesser extent, brand name and generic pharmaceutical businesses. In a search of Lex Machina I couldn’t find one IPR filed by any of the 10 largest oil producing companies. Where post-issuance reviews have become increasingly common is in the services sector involving the companies that supply a lot of the equipment to the majors or carry out much of the drilling for oil and gas.

Halliburton is among the most active of these, with 36 IPRs including 32 this year, mostly against its rival Schlumberger. Baker Hughes meanwhile has been involved in 27 IPRs either as petitioner or patent owner.

It remains to be seen if Saudi Aramco’s review is a sign of things to come among the world’s energy producing giants as they look to follow tech and pharma’s lead or it remains a one off.