You might not get any responses if people don't have any ideas on the subject.
Just looking at wikipedia, it looks like the oil fields there were discovered in the late 1970's and early 1980's. They probably used seismic techniques to explore the fields, which were developed during the 1920's in the USA. So, working from the technology needed to discover them, it probably can't be much before the mid-1930's (probably another 5-10 years after that, at least) before the technology is proved successful and makes it's way to the Soviet Union. Siberia is a big place, and the terrain and weather are pretty dreadful - a survey programme could take years, even if there were no interruptions and it was adequately-resourced.
However, this puts us into the timeframe of Germany's little jaunt into the Soviet Union, and exploration geophysics might take a back foot until a few years after the war. Assuming everything went smoothly then and they knew pretty much where to look, we might see the oilfields discovered by 1950 or so.
This is all just guesswork, of course. If you looked into the techniques used to prospect for oil and when they started to be used in Russia/USSR, you might get a better idea of when the fields could be found.