Four workers were taken by ground transport to hospitals and three were flown by helicopter to be treated for burns, the sheriff said.
Jeff Shields, a Sunoco spokesman, says part of the investigation into Friday night's fire will review whether the contractors followed safety practices.
Authorities say seven people have been injured in a flash fire at a Southeast Texas oil refinery.
The workers were injured while doing some welding at the terminal according to the Marcus McLellan of the sheriff's office. Officials from the company have not yet released any comments about the flash fire and the conditions of the employees.
"There is now no danger to the area around the plant", the sheriff's office said.
According to the company that owned the marine terminal, Sunoco Logistics, it appeared to involve a crude-oil pipeline connection.
The Nederland plant is used to store crude oil and other natural gases, with a storage capacity of approximately 24 million crude oil barrels, according to Sunoco's website. There were no fatalities.