Are you sure about this? Japan's forces took part in the Allied intervention in the Civil War in the Soviet Union, and was part of a force including US troops which invaded the Far East, occupying Vladivostok, for the purpose of securing the supply lines of Czechoslovak forces heading west into Russia. I've no idea what Czech forces were doing in this region - how did they get so far from home?
Further reference is made to Japan sending troops into Blagoveshchensk, on the Amur River on the border with Manchuria which was still part of China. But they did not stay long. There is some suggestion that some Japanese troops traveled as far as the Trans-Baikal province, but not in large numbers and not for long. Most of their forces were in Vladivostok and the surrounding Maritime province. They evacuated Vladivostok in 1922 and the recapture of the city by the Far Eastern Republic marked the official end of the civil war.
But during the civil war Japan also found an excuse to occupy Northern Sakahlin, where they stayed until 1925. Unlike the Maritime province where there were civilian Russian authorities, there appears to have been none in Sakahlin. No doubt the Japanese had the time and manpower from the south of the island to survey it, and were well aware of the resources there. But there is nothing I can see that they did the same in the Maritime or Amur provinces. No doubt they picked up valuable and detailed intelligence about these areas, but whatever they knew about the mineral resources of the region, it still wasn't enough to make them consider it worthwhile to invade in 1941, never mind earlier. They came to (parts of) Siberia, they saw, but they never thought it worthwhile to conquer.
