An investigation continues after a house fire in Petrolia that sent one woman to hospital with serious injuries.

Lambton OPP and the Enniskillen Fire Department were called to the 200 block of Eureka Street around 9:45 a.m. on Saturday for a structure fire.

Most of the structure was already engulfed in flames when emergency crews arrived.

Witnesses at the scene reportedly located and assisted an adult female, who had collapsed at the front door of the residence.

The woman was taken to hospital and treated for serious, but non-life threatening injuries.

The Office of the Ontario Fire Marshal has been called in and will investigate the cause of the fire.