Natural fractures are important conduits for fluid-flow and can control the deformational behavior of rock. The accurate evaluation of natural fracture geometry in the subsurface is difficult because of the sampling problems inherent to wellbores and indirect investigation methods such as seismic. Because of these limitations in observational capability, predictive models are needed to provide more constraint on natural fracture characteristics. One approach is to look at the mechanics of fracture propagation in sedimentary rock as controlled by subcritical crack growth (also known as stress-corrosion cracking). We have measured the subcritical fracture properties of numerous sedimentary rocks, including core samples from petroleum reservoirs, in conjunction with detailed petrographic analysis. Our preliminary results suggest that carbonates and sandstones tend to have very different subcritical fracture properties, and the variation in sandstone properties can be linked to grain and cement mineralogy and volume fractions.