The Germans were never again going to make an advance comparable to Barbarossa, or even to Case Blue, if they didn't take the Caucasus oilfields. This is a battle separate but related to Stalingrad, if for entirely the wrong reasons.

Stalingrad was emphatically not considered a priority target in German planning for 1942. The main target for Case Blue was the oilfields around Maikop and Grozny in the Caucasus. The plan envisioned securing the German left flank on the Don River, clearing the Don Bend, and then turning south into the Caucasus to seize the oilfields. Perhaps the only significance of Stalingrad to this particular plan was that it was a prominent crossing point on the Volga and thus potentially an area for the Soviets to mass for counterattacks. Field Marshal von Kleist is quoted as saying "it was just a name on a map to us".

It was only midway through Case Blue that Hitler insisted on capturing Stalingrad rather than just masking it. Army Group South was split into two new army groups, imaginatively called A and B. Army Group A under Field Marshal Fedor von Bock turned south towards the Caucasus and oil in Operation Edelweiss, while Army Group B under Field Marshal Wilhelm List drove for the Volga and Stalingrad in Operation Fischreiher. The original plan was to mask the Volga crossings, then turn south. Army Group South could not do both at the same time. Indeed, Army Group B's strength was so diluted that it had to leave huge stretches of the Don's south west bank unguarded, which later became the assembly areas for the Soviets' Operation Uranus after the Sixth Army had wasted itself trying to capture Stalingrad.

That isn't to say that Case Blue in its original form could have succeeded: to advance from the Donets river to the Caucasus, Army Group South would have to travel distances greater than it had in Operation Barbarossa the previous year, over a much poorer road network, with understrength forces, against a prepared opponent, without the advantage of surprise, and in the middle of a fuel crisis.

Sources:

Antony Beevor, Stalingrad

Robert Kirchubel, Atlas of the Eastern Front, 1941-45

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