An expandable downhole tool is disclosed that employs naturally occurring organic matter as the expandable material. The expandable material is located within an enclosure formed in part by an impermeable element, which may form a hydraulic seal, and a permeable membrane that permits fluids present in the wellbore to pass and interact with the organic matter. The heat and fluids present within the wellbore may cause the expandable matter to swell, and thus causing the enclosure to swell and possibly form a hydraulic seal against a wellbore or inner annulus. The expandable tool may be used as packer, plug, or other downhole tool, among others. Methods of manufacturing and using such tools are also disclosed.