>Wait, those big oil tanks don’t have lids? Doesn’t that mean rain would mix with the oil??

Modern tanks have floating lids. The lid is as much for keeping the volatiles in as keeping water out. Water gets into all sorts of places in oil refining anyway. It wouldn't really matter anyway since oil and water being famously good at mixing. You can just draw water off the bottom, filter it or boil it off depending on the situation and amount. Obviously they don't want more water (you're wasting money processing everything that you can't sell) but some isn't a big deal.