How can oil can escape from a shale source rock into a reservoir rock if the shale itself is impermeable?
Shale typically has a low permeability but shale is not technically impermeable. Natural occurring faults, fractures and joints in shale can greatly increase the permeability of the rock. This is why hydraulic fracturing (or fracking) is used to increase fluid flow (for both water, natural gas and petroleum wells) in host rock with low permeability.
Fractured shale can allow a significant amount of fluid because some shale can have porosity values greater than 10%. Given the grain size of shale, this can be a significant amount of space available to hold fluids.