If you're talking about the dark blue part of the graph, that represents currently active oil fields only, so of course it's going to drop starting the day afte

If you're talking about the dark blue part of the graph, that represents currently active oil fields only, so of course it's going to drop starting the day after the graph was made because it's defined to exclude new production capacity! The light blue apparently represents the expected production from known but untapped fields, so the dark and light blue combined represent current known oil reserves, and don't show any spurious drops.