The blaze, late on Sunday in Rivers State, occurred when a home-made refinery ignited a nearby oil reservoir, leaving victims severely burned, a security official said.
"The fire outbreak started at a very late hour... 18 victims were burnt beyond recognition while 25 injured persons were rescued," Olufemi Ayodele, spokesman for the local Nigeria Security and Civil Defense Corps, said.
"Most of the victims were youths... a pregnant woman and a young lady getting ready for her marriage ceremony next month were all casualties," he said.
Local residents in Emohua district said the toll could be higher as dozens of people were at the site scooping up spilt crude oil.
Oil theft is a major problem in Nigeria, an OPEC member and one of the continent's largest petroleum producers.
Accidents and spills are common in the makeshift refineries hidden in swamps in the Niger Delta, where smugglers tap into pipelines to steal crude and refine it into fuel for sale.