Wasn't this known for a while? The problem is that simply agitating the liquid already causes immense wear because these are natural caves selected for their potential as long-term storage, not active sources.

Not to mention that draining them reduces the quality of the remaining oil (much like how you never want to feed an engine from the bottom of a diesel tank).

It's also my understanding that the problem is not just the reserves being drained, let alone drained so much, but also how rapidly they were drained because that again makes things worse.