This demonstrates how BP reports reserves, failing to backdate the revisions. It has misled many analysts. The large increases in the late 1980s were simply due to the OPEC quota wars. Nothing particular was actually added.
Kuwait added 50% in 1985 to increase its OPEC quota, which was based partly on reserves. No corresponding new discoveries had been made. Nothing particular changed in the reservoir.
Venezuela doubled its reserves in 1987 by the inclusion of large deposits of heavy oil that had been known for years, forcing the other OPEC countries to retaliate with huge increases. Note too how the numbers have changed little since despite production.