price dropping ??, page-50

You would need to understand what he means by that statement. When he says:
"..realistically there is only a 5% chance for Zeus-1 to strike gas with this first hole and confirm how much is there."
there are really two parts to this statement; the first part about striking gas and the other part about confirming how much is there.

Zeus is different to Goodwyn because Goodwyn is a conventional trap resulting from a large block having been uplifted relative to the surrounding geology and sealed above preventing hc's from migrating upwards eventually to surface.

Perseus is NOT like Goodwyn because it is a stratigraphic trap which relies on lateral changes in the reservoir sand for its seal. This can happen in several ways but, commonly, the reservoir sand thins out at the edges to become non-existant or becomes less porous & permeable. It is also sealed above by an impervious formation like shale, for example. Note that this is all highly simplified!

Zeus is believed to be a similar kind of trap to Perseus.

Dealing with the second part of the statement, normally you would have to drill a number of wells into a structure to get a clear idea of how big it was; these are called appraisal wells. The first (discovery) well rarely enables an accurate assessment of reserves, although it can do in small simple structures. What's interesting with Zeus is that a very similar-looking large gas discovery exists nearby (Perseus) and this normally increases the probability of success.

Personally, I don't like putting % numbers on chance of success because every situation is different. I think Zeus has a 'better than average' chance of success though.