Definition - What does Impermeable mean?
Impermeable is a property of rocks which makes them not able to transmit fluids due to low permeability. High porosity shale is impermeable because of its small and disconnected pores. Rocks with this characteristic are believed to be desirable sealing cap rocks for reservoirs as hydrocarbons are not allowed to effortlessly pass through them. This is because the rocks contain small or no pore space which makes it difficult for fluids to pass through. In reservoirs, a layer of such rocks prevents the lateral or upward escape of petroleum.
Petropedia explains Impermeable
The Impermeable characteristic of some geologic materials limits their ability to transmit a significant quantity of fluids. A form of structural trap is created by an anticline due to local deformation of the reservoir rocks and impermeable cap rocks. When the slippage and fracture of rocks along the fault line bring a layer of permeable rock in contact with the impermeable stratum, a barrier is formed for petroleum migration known as a fault trap. Overpressure may occur in the buried channel of a river with coarse sand which is sealed by impermeable shale on all sides.