As far as adding a 4th resource it's not a great idea as you will have to use a multiboard to display the resource count. A better idea is:
-make a ship worker (oil tanker) based off the acolyte
-give it a structure that is based off the undead gold mine
-change the model to some sort of oil rig model, there are several on this site
-place gold mines in the sea, give them a model of a oil patch
-specify that the custom mine building can only be built on blight, raise the land above the water line and place blight around the gold mine, then lower it down
-change the game interface option for "must place upon blighted ground" to read "must place on an oil patch" and change the error sound for undead to the generic error noise
A note:
-any worker you own will be able to target any mine as far as the harvest command goes, so if you have AI it will get confused