Currently, you can get offshore oil before regular oil.
This is a balance problem since a single good patch of offshore can last until you beat the game, and if it's close enough to land you don't even need to worry about setting up tankers.
I would suggest locking the tanker behind purple or blue science so that the player can feel the progression from pumpjack to offshore.
Plus, it makes logical sense because offshore oil is pretty hard in real life.

maybe as an optional setting. I have thought about that before, but i wanted to keep the possibility for people to play with normal land based oilfields completely disabled.
In the meantime, you can change the richness of the offshore oilfields in the mod-settings.