Is the granite of igneous or metamorphic origin?
Regardless, it must be fractured, or there would be no reservoir.
Petroleum migrates--even to the surface.
It can be found in fractured volcanics, too, but they overlie the sedimentary rocks which are believed to be the source of the oil.
"Refilling" reservoirs are commonly in severely faulted areas, in relatively young rock, geologically speaking, and may be filling with oil and gas migrating along those faults and fractures from reservoirs that lie in deeper strata.
Until someone drills deeper there no one will know for certain, but why drill deeper if the oil is migrating to existing wellbores now? Fracture sets and faults which allow fluid migration really complicate drilling a well, and the production is already happening.
Many, many people have been taken in by this inorganic mantle source hype, and all they ever got was snake oil for their money.