The project of Underground Water sealed Oil Tank in Huangdao district, in Shandong Province is taken for example in this paper, which is the biggest underground oil tank now in china. First, the discontinuities in the site of caverns were categorized. And the geological and shape characteristics of each type were respectively described on the base of practical surface investigation and deep seated discontinuities' descriptions in rock cores. Second, the compositions and optical properties, the shapes of the discontinuities as well as their interlace relationships in digital borehole images are analyzed. The characteristics of different types of discontinuities in borehole images are studied from the aspects of the shape, width, color and interlace relationships of the curves as well as the properties of the surrounding rocks, combined with the geological and shape characteristics of deep seated discontinuities. Third, the borehole images from the elevation of 120m to minus 60m in borehole ZK8 are interpreted and identified by the characteristics of each type of discontinuities in the borehole images. The results indicate that the borehole images characteristics of each type of discontinuities in deep seated rocks are different from other types. So each type deep seated discontinuities can be interpreted and studied based on digital borehole images.