We don't discover gold or diamond mines when drilling for oil but that's not to suggest we don't believe they don't exist.
The number of holes made to get oil out is quite small in comparison to the surface area of the globe.
The number of holes made to get oil out is quite small in comparison to the surface area of the globe.
The point of all those holes is to log layers, horizons, sediments, etc and to map out the geology of very large areas .. much much much larger than the combined bore hole diameter areas.
Of course boreholes are the final step in "proofing" siesmic results that map out many layers across large areas and allow geologists to rule out many areas as not having the structures required to trap gases.