The order of discovery while drilling is based upon density of the liquids.
1) natural gas at the top
2) oil further down
3) water underneath
You may find these in other arrangements but this is the "natural" order according to density. (this is slightly oversimplified perhaps).
So hitting natural gas early on is a good sign but no proof that oil will be underneath.
So if natural gas is hit at 4100 m then the reason for continuing to drill is because there is anticipation of finding the oil below this say at 4900 meters but once oil is hit there are other factors to consider of importance such as:
1) Pressure and
2) Porosity of the soil which controls the rate of flow of oil into the resevoir.
These are very important to the economics of a well so there is a lot of factors yet to be determined before we get too excited.
I am not in the industry so others can correct the above should they be more knowledgeable.
There are plenty who should be