Complex sentence: Shale beds that are deposited during the rift-drift stage have high porosity and low permeability enough to enclose much fluid which under pressure, the whole shale bed turns into viscous, low density and high mobility layer.
Split into simpler sentences: Shale beds that are deposited during the rift-drift stage have high porosity and low permeability. This encloses much fluid which under pressure causing the whole shale bed to turn into a viscous, low density, high mobility layer.