Germany doesn't have much oil but they do have coal. There is a chapter on this in yergin's book which is certainly worth a read. Essentially the basics is they invested heavily in coal hydrogenation, by 1940 this was about 46% of their total oil supply. Most of the rest came from the ploste oil fields in Romania. By 1944 about 125,000 bpd were from synthetic fuel plants, this was 92% of aviation fuel and 57% of total supply.

Alied bombing was significant and this was a priority target, by September of '44 synthetic production was down to 5000 bpd. With the Russians taking Romania, Germany was basically screwed. They could produce planes and tanks but without fuel they entered a death spiral where they didn't even have the fuel to fly and protect the fuel plants.

This was a major consideration with why they invaded Russia, most of the oil at the time came from either the Us, Baku in Russia or the Dutch east indies. Romania was largest producer in Europe, but not on the same scale, and the Middle East while a goal if they could push through the suez was more in it's infancy(ghawar wouldn't be discovered until after the war). Although the allies did invade Iran to secure their oil supplies, but I think that is more complicated than I understand.