The Santos-Oil Search merger has been pushed back.

Oil Search and Santos have extended their mutual period of due diligence for their proposed merger deal by one week.

Due diligence on the $21 billion merger deal will now continue until September 13, Oil Search has advised.

No reason was given for the extension.

Reports say some Oil Search shareholders are unhappy with the terms of the deal, which would see Santos acquire all shares in Oil Search at a ratio of 0.6275 Santos shares for each share.

Reports say some of the dissatisfaction relates to claims that Oil Search did not fully explore other options before agreeing to the terms.

Oil Search acting chief executive Peter Fredricson says other suitors can still approach the company with an offer, but Oil Search is not actively seeking an alternative deal,

The Oil Search board still intends to unanimously recommend the deal to shareholders. 

However, that recommendation hinges on the absence of a higher offer and on a report by an independent expert about whether the proposal is in the best interests of Oil Search shareholders.

The date for an expected final agreement has been pushed back by the latest delay.