The reserve is effectively empty at 250M-300M barrels, and currently sits at 284M. The rapid deceleration to 400K could mean that there’s no quick way to keep withdrawing the remaining reserves.
Also, the bottom of the reserve is mostly sludge.
(a) as the levels drop below 300M barrels, pumping out of the reserve is much less efficient, so we can’t necessarily draw it down as quickly as when it’s full. So the lower it gets, the less useful it is
And (b) there are a lot of varying estimates for the real effective floor, anywhere from the 180ish to 300M barrels is within the range of a safe minimum level.
The problem is that as you pump oil and water in and out of it, in the intervening layer between the salt water and oil there forms a water/oil/impurities sludge which is useless (it ruins the pumping equipment). The thing is, they have no way of measuring it, nobody knows how much good oil is left and how much is sludge (or if they do, none of that information is public). And the closer you get to the sludge, the harder the good oil is to extract, slowing down the extraction pace.