I'm simply not sure how much oil my well is going to transmit back to my base so I'd like to be able to expand my tanks as required, is there a way I can maybe connect additional tanks to my first one if I find I need more liquid storage space?
Daisy-chaining tanks is quite easy, but your available options depend on which mod's tanks you're using.
The method that will work with any mod's tank is to setup a system (waterproof pipe, liquiducts, liquid tesseracts, etc) to pull liquid from the first tank and pump it into the second tank. You then output from the second tank to wherever the liquid needs to go.
In addition specific mods' tanks have other ways of expanding them:
Buildcraft tanks can be made taller at any time by simply putting more tanks on top of the existing stack, they will automatically link up.
Railcraft tanks can be expanded but only if the center-middle block of the new tank is the kept the same as the center-middle of the old tank (the new tank has to be centered on the old tank) otherwise all liquid that was in the tank will be lost. If you want to move the center of the new tank, relative to the old tank, by even a single block you need to pump the liquid into a separate holding tank in order to keep it.
Xycraft tanks can be expanded in the same manner as railcraft tanks except that they split the liquid data evenly between all valve blocks in the tank, removing one of n valves will remove 1/n of the tank's contents.
Two Xycraft tanks can also be automatically daisy chained together by having them share a common wall with a valve in it, the liquid will automatically balance between the tanks up to the height of the valve (put one at the very top if you want to be able to fill both tanks by pumping liquid into only one of them.