Methods are provided that facilitate the production of hydrocarbons from subterranean formations, involving the mobilization of an immobile heavy oil in situ by gravity displacement. In effect, heavy oil is mobilized by dense aqueous gravity displacement (DAGD), in a process that generally involves injecting a dense, heated aqueous injection fluid into the formation into an injection zone that is in fluid communication with immobile heavy oil. The injection well is operated so that the injection fluid mobilizes and displaces the immobile heavy oil, to produce an expanding upper zone of mobilized heavy oil amenable to production.