If a rock unit of low porosity is highly fractured, it can make a good aquifer (via fissure flow), if the water can get through it. Porosity is important, but, "alone", it does not make an aquifer. The Deccan Traps (a flood basaltic lava) in west central India are good examples of rock with high porosity but low permeability, so they are poor aquifers. The (Upper Cretaceous) Chalk of south east England has a reasonably high porosity, but low grain-to-grain permeability. Much of its good water-yielding characteristics being due to micro-fracturing and fissuring.