US private equity firm Harbour Energy has made a binding offer for Santos that confirms an original price tag of $US10.4 billion ($A13.8 billion).

Santos says Harbour Energy has not altered the value of its $US4.98 a share offer made on April 3, but there are a number of changes to the transaction structure.

The offer is conditional on Harbour completing final due diligence and the recommendation of independent directors of the Santos board, plus shareholder and Foreign Investment Review Board approval.

The independent directors will consider the revised proposal and update shareholders accordingly, Santos said.

‘There is no certainty that the revised Harbour proposal will result in an offer for Santos that is capable of being considered by shareholders,’ it said in a statement.

Shareholders have been advised to not take any action at this time.

Santos said the proposal has no fixed Australian dollar component and Chinese natural-gas distributor ENN Group and private-equity firm Hony Capital, which have a combined 15 per cent stake in Santos, would be able to roll their shares into a Harbour investment vehicle and subscribe for new shares.

This option is not available to other shareholders.

Shares in Santos were down 11 cents, or 1.7 per cent, at $6.21 at 1430 AEST.