Shanghai - China Petrochemical Group, parent of Shanghai and Hong Kong listed oil giant Sinopec Corp, plans to spend an estimated maximum $17.7 billion to buy back a 2 percent stake in the Shanghai-listed entity over the next year, in an apparent move to support the mainland's sagging stock market.

The State-owned parent started the purchase on Tuesday, buying 6.06 million shares, or 0.005 percent of the listed arm, Sinopec said in a filing to the Shanghai Stock Exchange dated November 6.

A 2 percent stake amounts to 2.33 billion shares, based on Sinopec's total capital base of 116.6 billion share.

Sinopec's Shanghai-listed, yuan-denominated A shares closed at 4.62 yuan per share on Tuesday. Based on these calculations, the parent could spend as much as 107.7 billion yuan ($17.7 billion).