Denmark's Maersk Drilling extended the contract with BP Plc (BP) to operate the NA Heydar Aliyev drilling rig in the Shah Deniz gas field until May 2021, a press release said July 20. The contract extension costs about US$523 million.

"The company will continue to develop the Shah Deniz field in the Caspian Sea. The new contract is a continuation of the existing one, which will expire in May 2016," the company's website says.

"We are very pleased to extend [the operation contract] on drilling rig NA Heydar Aliyev, and we hope to continue our cooperation with BP," said Claus Hemmingsen, the general director and a member of the executive board, in the company's message.

Shah Deniz' gas reserves are estimated to be about 1.2Tcm.

The contract to develop Shah Deniz was signed on June 4, 1996.