One of the last big North Sea projects sanctioned under high oil prices has begun pumping natural gas, highlighting an unexpected boom in U.K. energy production that analysts say is unsustainable. French oil company Total SA TOT -1.47 % said Monday that a deep-water natural-gas project west of the Shetland Islands called Laggan Tormore will produce the equivalent of 90,000 barrels of oil a day—almost 6% of the U.K.’s total output. Laggan Tormore is part of $50 billion of U.K. offshore projects that were commissioned in recent years when oil prices were $80 a barrel or more. Now those developments are coming onstream, helping companies working off the U.K.’s coast to pump an estimated 8% more oil and gas in 2015, about 1.6 million barrels of oil equivalent a day. The U.K. North Sea was once one of the world’s great oil basins, with production that peaked in 2000 […]