A residual basin usually means a sedimentary basin that is tectonically altered in its original feature, architecture, structure, and hydrodynamic system. Among those changed elements, the lattermost, the hydrodynamic system converted from an initial confined fluid system into a lithostatic-pressure one, is most diagnostic of the alteration of a residual basin. Preservation conditions of hydrocarbon in a residual basin are crucial to exploration evaluation. The conditions are mainly determined by the intensity of tectonic movement occurring at the basin and are closely related to the preservation degree of the basin. Pressure system and physicochemical properties of fluid in a residual basin may give indices directly to evaluate preservation conditions of hydrocarbon in it.