OSLO (Reuters) - Norway's $1.3 trillion sovereign wealth fund is unlikely to offer the same high returns in coming decades as it has over the past 25 years, its chief executive Nicolai Tangen said. Monday marked the 25th anniversary of the Norwegian government's first cash injection to the central bank to help establish what has since become the world's largest such fund. Set up to pool state's revenues from Norway's oil and gas production and prevent the economy from overheating, the fund was then turned into a sovereign wealth fund in 1998.