have you actually read the full report? the only reason they say that there is that much gas there is because of drilling activities by BP and shell in the Nile delta where they found a lot of gas, considering they share the same basin called Herodotus, they have presumed that there will be that much gas on their half of the basin, no drilling or surreys or anything has occurred on the Greek side, so how on earth can they estimate $427 billion worth of gas?
Not only do they believe what was mentioned above, but also that they will become the main supplier of Gas to Europe, if there is that much Gas.