It depends on the rock volume counted as net pay, the porosity, the oil saturation (one minus the water saturation), and the oil formation volume factor that converts reservoir volume to surface volume. Change any one of those and the in-place number moves.
It is not a reserves number. Reserves are the part expected to be produced commercially, and the recovery factor is that part divided by this volume. Quoting an in-place volume as if it were producible overstates what the field will give up.