The Iberville Office of Emergency Preparedness said it received reports of seven injuries from the plant explosion, according to CNN affiliate WAFB in Baton Rouge, Louisiana.
A Louisiana State Police dispatcher told CNN that authorities were evacuating the area around the plant.
Video broadcast by WAFB showed flames and a plume of thick black smoke rising over one part of the plant. At least one other plume of smoke or steam could be seen rising from another part of the plant.
The station reported that residents in St. Gabriel, a nearby community, were being asked to shelter in place as a precaution.
The plant was still burning Thursday morning, Amy Johnson of the Ascension Parish Homeland Security and Emergency Preparedness Office told CNN.
WAFB identified the facility as the Williams Olefins plant, which produces approximately 1.3 billion pounds of ethylene and 90 million pounds of polymer-grade propylene each year, according to the company's website.
The station, citing state police, said two state routes have been closed near the plant, which is about 30 miles southeast of Baton Rouge.