Germany was not going to mobilize Caucas oil, ship it to Romania for processing, and then to Germany for use on single track rail lines, of a different gauge then their own, with almost no locomotives or rolling stock to spare, without sufficient refining capacity in Romania, without the steel to upgrade that capacity, on routes harassed by partisans, with factories being bombed from the sky, during wartime with a time window of at most 2 years.
The different gauge of Soviet rail was devastating to resupply as you mentioned, they were dependent on captured trains and cars. Germany was doing a slow process of changing the gauge of the rail but that was still a difficult and expensive jobs.
Another really important aspect of logistics I want to add is how inaccurate the maps Germany had of the Soviet infrastructure. Where the map would show concrete/asphalt roads would in fact be dirt roads, and I think just about all of us have seen the famous videos of German trucks and sometimes even tanks being stuck in muddy roads in the USSR.
Also lets not act like everyone in the German intelligence and higher ranks underestimated how hard Barbarossa was going to be. Many in the various intelligence services and even high ranking Generals pleaded Hitler not to invade the USSR.
- 1