BUENOS AIRES, Dec 9 (Reuters) - Argentina"s biggest oil workers union said on Thursday it had called off a planned 13-hour strike after reaching an agreement with state oil company YPF (YPFD.BA) to allow workers to attend a union event. The Private Petroleum and Gas Union in the provinces of Rio Negro, Neuquen and La Pampa, representing 24,000 workers, earlier said it would hold a work stoppage on Friday from 7 a.m. local time (10:00 GMT) to 8 p.m. The union, which accused YPF of banning workers from attending a ceremony for appointing a new union leadership team, said the standoff had been defused after talks between its leader Guillermo Pereyra and YPF president Pablo González. YPF is Argentina"s main energy firm and leads development and production in the huge Vaca Muerta shale formation, the fourth largest unconventional oil reserve in the world and the second largest for shale gas.