TOKYO, Nov 17 (Reuters) - Tokyo Gas ( TKGSF ), Japan's

largest city gas supplier, said on Monday it sold its U.S.-based

subsidiary, TVL LLC, involved in gas development and production

to Grayrock Energy for $255 million.

The unit, operating in Louisiana's Haynesville and Cotton

Valley formations, was sold as part of Tokyo Gas's ( TKGSF ) strategy to

optimise its asset portfolio, it said in a statement.