The pilots are missing and Coast Guard and ONGC have pressed their vessels into service to locate them.
The Medivac chopper was on its last sortie of the day from a platform on the Mumbai High south oil and gas field about 160 KM off the coast.
"We don't know the reason for that," ONGC Director Offshore T K Sengupta told PTI.
The chopper was not on any crew duty and so there were no passengers on board.
"The pilots on the board are still missing. We don't know their whereabouts," Sengupta said.
No oil and gas installation was damaged in the accident.