Exactly.
Also, oil fields are always located in porous rock (from what I read), allowing seepage up from below (since oil is lighter). Has any oil ever been discovered isolated in solid, non-porous rock?
I can’t say that I know for sure, but generally speaking, due to the cost of exploration, you want to search in the highest probability areas.
Very little unless it is fractured. Mostly in the Texas Panhandle and Australia but they have conventional reservoirs overlying the tombstone.