This paper presents a comparative analysis of the thicknesses–displacement relationships of fault cores in non-/low-porous carbonate and porous sandstone reservoirs. Fault thickness data were collected from extensional faults in mainly Cretaceous continental sandstones and Late Cretaceous–Paleogene marine carbonates exposed along the eastern flank of the Suez Rift, Egypt. The dataset consists of 730 thickness measurements, of which 313 are from 68 faults in carbonate rocks, and 417 are from 120 deformation band/microfaults and faults in sandstones. These data show that the increase in fault core thickness with displacement for the two lithologies is, overall, similar in log–log space, consistent with a power-law trend with an exponent of 0.5. However, for smaller faults, cores in carbonates are generally thicker than those in sandstones. The observation of nearly equal core thickness–displacement relationships for larger faults (>10 m displacement) suggests that increased displacement reduces the geomechanical influence of lithology. In a statistical analysis of bins of fault displacement, small displacement faults seem to follow a skewed distribution towards smaller fault core thicknesses, moderate displacement faults have a normal distribution, while large displacement faults have a distribution skewed towards large core thicknesses. Each thickness distribution can be estimated with a mean and standard deviation. Fault core thickness variations are caused by interplaying factors such as fault geometric development, spatial and temporal positioning of shear and volumetric strains, and rheological changes. The variability in core thickness and intrinsic architecture of fault cores for any given location will cause significant variation in across-fault fluid flow, even for the same juxtaposition style.