The tug boat sank on Sunday due to what the ship owner described as “rough seas” off the coast of Nigeria.
Chevron had confirmed in a statement that the Jascon-4 capsized early on Sunday due to "heavy ocean swells" while the vessel was "performing towing operations" at a mooring point around 30 kilometres off oil-producing Delta State.
Reuters quoted a spokeswoman for the ship owner as saying Tuesday that “unfortunately all 12 crewmembers are still missing.”
"Rescue operations involving sea divers are still ongoing and a full investigation into the incident is taking place," she said.
Chevron Nigeria was however yet to provide an update on the incident.
Chevron Nigeria Limited’s 2012 net daily production from Nigeria was 238,000 barrels of crude oil, 165 million cubic feet of natural gas and 4,000 barrels of liquefied petroleum gas.
West African Ventures Limited is an indigenous operating company of Sea Trucks Group, an international group of companies offering offshore installation services to the oil and gas industry worldwide.
The company founded in 1977, has employed more than 2,500 employees, according to a statement on its website.
The Sea Trucks Group has offices in Nigeria, Angola, Ghana, United Arab Emirates, The Netherlands, United States, Australia, Singapore and China.
The group also owns and operates a large and versatile fleet of marine construction and supply vessels, large cargo barges, as well as supply bases and fabrication facilities in Nigeria and the United Arab Emirates.