Systems and methods for placing well bores and/or portions of completed well systems in a subterranean formation relative to tar mats or other phenomena in the formation are provided. In some embodiments, the methods comprise: identifying a reservoir characteristic associated with a chemical or physicochemical property of a fluid in or proximate to the reservoir characteristic; measuring the property of a fluid in at least a portion of a subterranean formation in two or more locations in the subterranean formation; identifying a variation of the property of the fluid at one or more of the locations in the subterranean formation; and determining a target location or direction for a portion of a well system based at least in part on the variation of the property of the fluid, the well system comprising a borehole penetrating the subterranean formation.