Cap rock is a rock that forms a laterally continuous impermeable seal above and around a hydrocarbon reservoir such that hydrocarbon fluids become trapped below the rock. Cap rock detection is significant when searching for a hydrocarbon reservoir and locating the perfect spot for CO2 injection.
But then, a rock that forms a good cap rock for one fluid may fail woefully when used for another fluid. For instance, a particular cap rock may form good traps for oil and/ or gas but will form poor barrier for CO2 because of several reasons ranging from capillary pressure to cap rock ductility. A good cap rock for any fluid should be able to withstand pressure exerted by the trapped fluids.