Shares in the two companies, which had run up strongly ahead of the integration, initially fell sharply on details of the deal before closing down around 3 per cent in a firmer overall market.
The new firm will target annual savings of around 60 billion yen ($535 million) in 2021/22 from the integration, up by 10 billion yen from a previous projection, the two companies said.
“The latest announcement spurred feelings that the stock has run out of new material,” said a source at a Japanese brokerage.
The refiners announced in July they had finally reached a deal to merge, in April next year, after Idemitsu’s founding family dropped its long-standing opposition to the plan.
Idemitsu, Japan’s No.2 oil refiner by sales, has long been keen to merge its operations with fourth-ranked Showa Shell in response to shrinking gasoline demand in the country.