Chevron said a drilling rig off the coast of Nigeria partially collapsed after catching fire during drilling operations in shallow waters.
Chevron reported a fire on its K.S. Endeavor rig off the Nigerian coast this week. An investigation is under way, though Chevron said early indications point to a failure of surface equipment.
The rig has partially collapsed and the structure is on fire, the company said in a statement.
More than 150 rig workers were pulled from the platform, though two contractors are missing.
The company said a small sheen was visible near the site, which was estimated to represent about 13 barrels of liquids.
"Chevron has contracted with and is mobilizing the Transocean rig Baltic to commence drilling a relief well," a statement read. "Chevron said the time required to complete the relief well is uncertain but could extend for some period."
An "extensive search-and-rescue" effort was under way for the missing contractors.