The petrochemicals can be synthesised from CO2 and water with known methods. While nobody bothers with this right now, SpaceX basically has to make this economical as one sub-goal of their Mars colony, so I expect it in a decade or two, well before we run short of natural petroleum.

I’m more concerned with the long-term need to recover the potassium and phosphorus that washes into the ocean, which, despite still being theoretically solvable, is not currently economic enough to bother with even for the gold and uranium that’s also in the seawater.