You have 3 main sources of wealth.

A basic 3000 purse that you get no matter what.
Town income which is essentially what your economy does and you have full control of that.
Trade income which is dependent on you have resources and the rest of the worlds demand for the goods. Not only can you not control part of the trade income equation it is not stable. Why? Because it can be easily blockaded.

So if you can grow your town income you are not dependent on foreign oil reserves err trade routes for income.