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.
The Role of a Good Cap Rock
Whereas a good reservoir rock is expected to have good porosity and permeability, a cap rock should have very low porosity and permeability. Without a cap rock, hydrocarbon fluids will continue migrating upwards from the source rock, to the reservoir rock and upwards until they get to the surface. This means that cap rocks perform the role of trapping any fluid below and within it.
Examples of cap rocks include shale, anhydrite or salt. These rocks form common cap rocks in petroleum systems. To free the trapped hydrocarbon fluid, a well has to be drilled through the cap rock to create a pathway for fluid flow as well as generate a pressure differential between reservoir pressure and surface pressure that will release the trapped fluids.
Properties of a Good Cap Rock
The presence of faults on a cap rock ruins its chances of creating a perfect seal for hydrocarbon fluids. When the cap rock forms a poor seal then hydrocarbon fluids will seep to the surface making the cap rock ineffective.
Similarly, formations selected for CO2 injection are those whose cap rocks are unfaulted and ductile. A faulted cap rock cannot be trusted to hold fluids. Cap rock ductility ensures that the cap rock will not fracture under building pressure from injected gas. This is very important in CO2 injection and storage.
Another important factor for cap rocks generally, is that the rock should stretch as long as the horizontal length of the reservoir. A type of hydrocarbon trap called structural traps have cap rocks that have spill points; when the trap created by the cap rock is full, hydrocarbon fluids will escape freely from the spill point and migrate upwards.