“We’re accessing what were once considered just source rock.”. Many oil reservoirs are pooled from seeps out of source rock where the oil formed. Pooled reservoirs are not the whole of the reserve, but only the easily accessed and freest flowing of the petroleum.
We’re still learning about subsurface oil and gas. The amounts of oil and gas to be recovered are still incalculable. The extent of petroleum to be discovered isn’t known. How much oil is in source rock from the reservoirs already found is still a big question without hard answers.