Im no expert but Id say the oil doesnt just fill up the well... that seems like nonsence.
it is most likely simply crude that was locked in the nearby soils which is infiltrating/leaching slowly back into the well.
because surely the well doesnt create crude oil that's ready to go? the process takes eons to complete, so logic would suggest that if it was creating new oil, it would first be filled with a substance that slowly transforms into crude?
but either way, sand-oils which are filling the well is amazing! becuase oil thats trapped in soil is very expensive to process.