Nigeria ’s new president, Muhammadu Buhari, on Friday dissolved the board of the state oil company from which billions of dollars is reported to be missing. A statement from the head of the Civil Service, Danladi Kifasi, announced the presidential directive regarding the Nigerian National Petroleum Co., the government agency in charge of nearly all aspects of the country’s oil industry , the seventh largest in the world. Mr. Buhari took office last month promising to halt corruption, and he said this week that Western governments had promised to help recover looted state money. The president’s action came before he has named a cabinet and amid speculation that he may be planning to take charge himself of the petroleum portfolio. A former Central Bank director last year largely blamed the state petroleum company for missing federal revenues of about $20 billion — an amount that the company disputes.