I think a lot of people around here are forgetting some crucial bits that need to be pointed out:
1. If Germany is to have any chance of reaching Astrahan and/or Baku, that means Fall Blau has to have gone far better - my suggestion would be that, in their august 29th attack, the Germans manage to trap the 64th and a large part of the 62nd Soviet armies just outside Stalingrad (instead of falling just short).
2. If Fall Blau has gone better, it means
- Stalingrad has been secured
- no grinding battle of attrition, preserving a lot of German strength
- Axis forces can now focus on consolidating their hold of the Don and Volga lines by eliminating all the leftover Soviet salients and bridgeheads
- lack of existing bridgeheads will seriously hamper any future Soviet counteroffensive come winter
3. With less intense fighting in the north, the Italian alpini divisions can be sent to where they excel, i.e. the mountains, instead of having to cover the steppes - this release German formations (which were inadequate for mountain warfare) for use up north
4. No street fighting in Stalingrad will also mean that the Germans will have a considerable strategic reserve for use during the winter. If used correctly, this force has a chance to stop the Soviets in a battle of maneuver once they finally managed to cross the fortified river lines of the Don and/or Volga
So, while it is possible that winter brings a huge German defeat, it's also entirely possible that the Germans manage to hold on and restore the front.