Basement rock can be very old, in fact West of Shetland it is some of the oldest rock on the planet created around 2.5 billion years ago. It is largely hard rock such as granite. The natural position in geological time for the oil producing layer would be above the basement, but things don't always stay the same, illustrated below.

Here we have illustrated schematically layers of rock that build up over many millions of years. Somewhere above the basement, oil producing source rock was formed. West of Shetland it is the Kimmeridge Clay, famous for the quality and volume of oil it has produced in the UK's North Sea.
Over millions of years, movement caused by tectonic forces can cause disruption in the layers of rock. Here, the basement has been forced up by as much as a kilometre. A couple of important things happen: the movement and heavy faulting has created an extensive fracture network. It has also resulted in the oil producing layer being at a lower level than the basement.
At the top of the structure a trap is formed in which oil can accummulate with a thick seal of muds and clays above. As the oil producing rock forces out hydrocarbons they move up the flank and into the basement through the fracture network.
Gradually the hydrocarbons become trapped at the top of the basement reservoir structure by the thick layer of shale and clay that defines the seal. Once structural closure is at capacity, oil at the edges of the closure will tend to 'spill' making its way to the surface or into other shallower traps.
However, one of the great attractions of fractured basement reservoirs is that oil can be found outside of structural closure. Oil backfills down through the highly permeable fracture network. In the basement there is no permeability in the rock, so the oil cannot escape but is trapped for explorers to find. We call this the 'jellyfish' model.
Of particular interest are flank accumulations, that is, oil that accumulates on the margins of the basement structure proximal to the source rock. These accumulations have the potential to be very significant. An example of this phenomenon can be seen on our Typhoon asset with a large potential recoverable oil volume, estimated in the CPR under P10 Prospective Resources, at over a billion stock tank barrels.