What is the natural replenishment rate of oil?
Almost nil on human time scales.

The total estimated reserves (this number keeps changing over time) are of the order of 300E9 m^3. For convenience lets say all of this oil formed over the last 300 million years at a constant rate. Then the rate of formation is  1000 m^3 per year or ~6000 barrels of usable oil.

This obviously is a simple back of the envelope calculation and doesn't account for oil that has been lost/degraded/converted to other hydrocarbons over time. Also the rate of formation varies a lot (depends on climate, position/structure of plates and what not).