In 1942, the Germans captured a relatively small oil complex at Maikop in the North Caucasus, and got within 25 miles of a larger one at Grozny. But the biggest prize was at Baku, on the "far" (southwest) side of the Trans-Caucasus region, on the Caspian Sea. I'm assuming that the Germans could not have captured it in 1942, and only with great difficulty thereafter.
Historians like William L. Shirer in "The Rise and Fall of the Third Reich" believed that the Germans could have interdicted Soviet oil shipments from the south by getting a firm grip on Stalingrad, and choking off transportation in the Volga River, and for some miles beyond.
With regard to the first issue, how close would the Germans have had to approach Baku to be able to interdict the oil from the source by bombing, and how far were they from this hypothetical point in the Caucasus?
With regard to the second, were there viable alternate north-south routes to the Volga, or could one have been built? If the Germans controlled Stalingrad, could this have been bypassed by "night runs" to avoid bombing?