US oil major ConocoPhillips is targeting mid-2018 for a go-ahead on a potential $US15 billion offshore gas development off northern Australia to feed its Darwin liquefied natural gas project once current supplies have run out, but says an expansion of the venture isn't viable without a sharp drop in costs. Frank Krieger, vice-president of exploration and development for Conoco in Australia, said that either the Caldita-Barossa gas fields in the Timor Sea or the Poseidon fields in the Browse Basin could be developed, but only as "backfill" for Darwin LNG, not an expansion.