Hi y23,Hope this helps. The Geological Chance of Success (GCOS),...

Hi y23,
Hope this helps. The Geological Chance of Success (GCOS), is a systematic approach to managing prospect risking by multiplying fractions representing the likelihood of each of the following geological parameters:
Trap (Is there a subsurface container where the oil can be captured for millions of years and how good is that container in holding the oil so that it does not leak out? There is a risk/probability that the trap may not be good and oil leaks out?)
Source(Is there a source rock, like the Kudu Shale in the Orange Basin, that has the capacity to generate oil and large volumes of oil? There is a risk/probability that the source rock is not mature enough to generate oil?)
Migration (Can the oil moving around in the subsurface find its way into a trap? Is there a pathway from the oil source rock into the trap so that the oil can find its way into the trap?)
Reservoir (Does the reservoir (e.g. sandstone) in the trap have porosity i.e. small holes in the rock that can hold the oil, i.e. pore spaces between the sand grains in the case of a sandstone reservoir. When you pick up a handful of sand at the beach, the holes between the sand grains are what is called porosity). There is a risk that the reservoir is poor i.e. the holes between the sand grains / pore spaces, are filled with secondary material like carbonate or quartz which in turns reduces porosity which affects how much oil can be reservoired).
Seal (Does the trap have a surrounding/ overlying layer of rock (e.g. shale), that is impermeable and will not allow oil to leak vertically or laterally through it). There is a risk that the seal is poor and allows oil to seep through it.

Finally, you multiple the risks you have assigned to Trap, Source, Migration, Reservoir and Seal together to get an overall risk which is known as the Geological Chance of Success, or Geological Chance of finding oil.

OldGeo