Abstract: Study of thin sections and scanning electron microscope(SEM) data confirms that the volcanic reservoir in Nanpu-5 structure has been altered with different degrees. Based on the alteration mechanism of volcanic rocks, the effects of volcanic alteration on physical property of reservoir are analyzed. The methods of empirical statistics, porosity-permeability intersection, mercury injection parameters, stress sensitivity and minimum oil throat radius are applied to determine the lower limits of physical property, which are defined by porosity of 2.5% and permeability of 0.04×10^-3 μm². The determination of lower limits is conducive to the division of reservoir and non-reservoir, statistics of effective thickness and calculation of reserves. The study provides reference for the determination of physical property lower limits of altered volcanic rocks in other areas.