Japan's top power generator JERA said on Thursday it will acquire natural gas production assets in the United States for $1.5 billion, in its first foray into shale gas production. JERA said it has reached an agreement with pipeline operator Williams and GEP Haynesville II to acquire 100% of their interests in the South Mansfield gas field in western Louisiana's Haynesville Shale basin. GEP Haynesville II is a joint venture between GeoSouthern Energy, backed by Blackstone , and Williams.