Energy giant Royal Dutch Shell on Thursday said its net profits plunged by 57 percent in the second quarter compared with the equivalent period a year earlier. The Anglo-Dutch group said earnings after tax slumped to $1.737 billion (1.308 billion euros) in the three months to June 30 compared with $4.1 billion in the second quarter of 2012. \"These results were undermined by a number of factors -- but they were clearly disappointing for Shell\", chief executive Peter Voser said in an earnings statement. Voser said the group\'s bottom line was hit by higher costs, exploration charges, adverse currency exchange rates and unrest in major oil producer Nigeria.