Both joint sets and fault-related fractures serve as important conduits for fluid flow. In the former case, they can strongly influence both permeability and permeability anisotropy, with implications for production of water, hydrocarbons and contaminant transport. The latter can affect issues of fluid flow, such as whether a given fault seals or leaks, and fault mechanics. These fractures are commonly interpreted as Natural Hydraulic Fractures (NHFs), i.e., mode 1 fractures produced when pore fluid pressure exceeds the tensile strength of the rock. Various mathematical models have been a rich source of hypotheses to explain the formation and propagation of NHFs, but have provided only limited information and nothing about processes of fracture initiation in originally intact rock. Recent laboratory experiments of French et al. (2012) have advanced our understanding of mechanical controls on fracture initiation and spacing. Here, detailed analysis of both through-going fracture surfaces, non-through-going fractures, in experimentally deformed samples provide a deeper understanding of NHF processes and resulting geometric features in porous siliciclastic sedimentary rocks. Observations indicate that both fracture planarity and microcrack damage (which has not previously been reported for opening mode fractures) vary significantly depending on the degree of mechanical heterogeneity and anisotropy of the host rock. Variations reflect mechanical controls on fracture initiation and propagation, suggesting that fracture spacing may in part reflect the distribution of mechanical heterogeneities. These data indicate that the more homogeneous the rock, the greater the microcrack damage surrounding a given NHF, increasing expected fracture-associated permeability for a given fracture aperture.