MEXICO CITY, Oct 15 (Reuters) - Mexico's bank regulator said on Wednesday it fined Citigroup's Mexican unit Banamex nearly 30 million pesos (US$2.2 million) over lapses in oversight of loans to oil services company Oceanografia that cost the lender more than $500 million.

A February investigation showed the bank had failed in its prudential responsibilities and lacked adequate internal controls, Mexico's National Banking and Securities Commission said in a statement.

Banamex said in a statement it had paid the fine and would follow recommendations made by regulators. Banamex said its own investigations showed that the fraud was an "exceptional" case.

Citigroup said in February it had uncovered at least $400 million in fraudulent loans that Banamex made to Mexican oil services company Oceanografia. That figure has since increased to more than $500 million.

The fine, the latest in a string of problems at the bank, comes a day after Citigroup said it had uncovered another $15 million fraud at Banamex related to a private security services company the bank operated.

Banamex Chief Executive Javier Arrigunaga resigned this month, to be replaced by Banamex executive Ernesto Torres.

The Wall Street Journal reported on Tuesday that Manuel Medina Mora, head of Citigroup Inc's consumer banking and chairman of the company's troubled Mexico unit, is preparing to leave in the coming months. (1 US dollar = 13.5405 Mexican peso) (Reporting by Tomas Sarmiento and Alexandra Alper; Editing by Cynthia Osterman)