At the end of 1997 several nations stated that their oil reserves had not changed since the end of 1996  But oil reserves gradually drop as old oil fields are drained and rise suddenly as new oil fields are discovered  Ttherefore, oil reserves are unlikely to remain unchanged from one year to the next
So most of the nations stating that their oil reserves were unchanged are probably incorrect
It is likely that in 1997, in most of the nations that stated that their oil reserves were unchanged, old oil fields were drained or new oil fields were discovered, or both.