RE:Does the seismic give them an idea of the size of the oilSeismic absolutely will be used. GLJ will make a total volumetric calculation of all the reservoir depths of pay and come up with a total calculation of the rock volume.
They'll then apply porsity, water saturation (3%) in this case and come up with a total OOIP original oil in place.
Then a recovery factor based on analogous fields in the area will be applied (like you say 20%) to come up with the recoverable oil volume.