Nigerian President Goodluck Jonathan has replaced the top four executives of Nigeria National Petroleum Corporation, according to his office said Saturday, speaking on one of the most powerful institutions in the country and source of 80 percent of state revenues. Joseph Thlama Dawha replaces Andrew Yakubu as CEO of NNPC Group. Reuben Abati, spokesman cabinet Goodluck Jonathan, declined to give further details. A senior source in the presidency, who requested anonymity, said it…