The species studied were Clostridium acetobutylicum, which produces polymers, and an unidentified halophile that produces acids and gases. Studies of conductivity, permeability, porosity and capillary pressure showed that the the former species causes porethroat reduction due to biomass plugging, and the latter causes pore-throat enlargment by acid dissolution of carbonates. On the other hand, formation of insoluble mineral products, such as carbonates and sulfates, can reduce both porosity and permeability.