Reuters By Marianna Parraga and David Alire GarciaACAPULCO, Sept 28 (Reuters) - Mexico will need to double to about $4 billion its annual oil exploration investment to reverse a 14-year decline in output, a move that will require more funding by Pemex and private producers, a top official with the state-run firm said Friday.The nation's oil industry needs Petroleos Mexicanos to invest more than $2.5 billion per year and another $1 billion to $1.5 billion from private companies to fully replace its reserves, Jose Antonio Escalera, the firm's chief of exploration . . .