Abstract Although the degree to which fractures are open and interconnected in the subsurface governs their ability to transmit fluid, information on in situ fracture apertures and connectivity is usually incomplete. Fracture apertures remaining open and effective for fluid flow (hydraulic apertures) depend on a number of factors that reflect fracture growth, diagenesis, and the modern state of stress. Of these, diagenesis—mineral precipitation and dissolution within fractures—has received little systematic study. Yet observations in petroleum reservoirs show that diagenetic minerals prop open some fractures but close others. Study of coupled diagnesis and fracture processes yield insight into fracture permeability and sealing. For example, using microstructural evidence for the timing of fracture formation relative to cementation episodes, fracture openness can be estimated from cement volumes in rock matrix, even where large fractures are not sampled. Interaction of diagenesis and fracture processes might also play an active role in governing fracture growth. This could moderate the partitioning of permeability and porosity among fractures of different size.