MEXICO CITY - Mexico’s national oil company Pemex said on Tuesday that an alkylation plant at its smallest refinery has been shut following detection of a gas leak, adding the refinery was operating normally. The company said in a statement that the plant at the Ciudad Madero refinery located in northern Tamaulipas state has been isolated to attend to the leak, and that nearby storage tanks have also been closed down as a preventative measure. It was not clear of the leak has affected output at the refinery, Pemex’s smallest with a capacity to process 190,000 barrels per day.