I was wondering if anyone knew how to change the spawn rate and biome for the different types of ores and minerals. I'd especially like to make oil shale appear outside the desert.
View the biomes.xml file. You can see in there how they setup the individual biomes in terms of ores that spawn in. You can also see if there are probabilities for spawn rate in that file (if not, it is probably in the source code).
Note, if you do make changes, you would more than likely need to generate a new map then (though I never tried these changes myself).