Permeability, porosity, formation factor, mercury porosimetry, and stress-strain measurements were made on 10 shale samples taken at depths between 4500 m and 5600 m in three wells on the Scotian shelf. The purpose was to obtain shale permeability values for quantitative sedimentary basin modeling and to investigate the reasons for the very low permeabilities, less than 10 (super -20) m 2 (10 nD), exhibited by many tight shales. Permeabilities of 10 (super -22) - 7 X 10 (super -22) m 2 (0.1 - 0.7 nD) and porosities of 0.9-9.2 percent were measured. The results suggest that the extremely low permeabilities occur because the flow path consists of a network of very tortuous pores (true tortuosity = 3.3) with small diameters, of the order of 8-16 nm. Presence of calcite and dolomite apparently is associated with reduced porosity, possibly a result of blocking of the pores, while kaolinite shows the reverse trend.