Once the hydrocarbons begin to migrate through a reservoir, continuing to rise in the water, a barrier is needed to stop them.
If such a barrier is absent the reservoir will be nothing but a transit zone and the hydrocarbons will not be able to accumulate. To stop the hydrocarbons, an impermeable rock, called a seal or a cap rock, is necessary above the reservoir. Cap rocks are often clays and sometimes layers of crystallised salt.
But any rock sufficiently impermeable will do the job. Some very compact carbonates work very well, for example.
|