Nigeria, Africa's largest oil producer, has signed a memorandum of understanding, or MOU, with U.S. energy firm Vulcan Petroleum Resources to build six oil refineries in a deal worth $4.5 billion.

The refineries are aimed at replacing Nigeria's decaying refining infrastructure, which has turned the country into a net importer.

The MOU with Vulcan signified the beginning of changing our old paradigm of exporting just raw materials and exporting jobs to Western countries, said Nigeria's Trade and Investment Minister Olusegun Aganga.

The development follows a decision last week by Nigerian President Goodluck Jonathan to dismiss the boss and several other executives of NNPC, the state-owned oil company, for corruption and fraud.