The Soviets absolutely were one of the major oil producing nations in the 1930s and 1940s. But the oil was almost entirely located on the wrong side of the nation from Japan.
Pulling from a 1950 CIA report (because it’s what I have at hand) the Soviet Far East produced 1,000 million metric tons of crude oil. While Eastern Siberia added another 56 million metric tons. The other 37,276 million metric tons came from the western half of the country. About half of that came from the Baku area on the western side of the Caspian Sea. The bulk of the refineries were also located in the West. So to get to Soviet oil of any significance, Japan would have to conquer the intervening 10,000km. Conquering the USSR to the Caspian before domestic oil supplies ran out would require a campaign vastly more successful than any ever accomplished by any army in history. And Japan would have to do it against an army that had manhandled it a couple times in recent history.
Then Japan would also have to figure out transport. The trans siberian railway did not have the capacity to transport enough oil to fully supply japan’s needs. Plus there’s little to no chance that it’d be captured intact. So Japan would be looking at having to build and greatly expand tens of thousands of kilometers of track. So on top of a miraculous military campaign, Japan would have to follow up with one of the greatest civil engineering challenges of the 20th century.
In comparison the Dutch East Indies were about half as far from then Japanese than Baku was. It was also comparatively undefended, and had the advantage of already being quickly connectable by the Japanese merchant fleet. As history proved it was far more than japan could conquer and hold. An invasion across Siberia would have been vastly more difficult to pull off.
Hi, I made a relatively similar question a while back, maybe the answers will help while you wait for a more specific answer:
The Empire of Japan had a pressing need for oil to fuel their industrialisation and political interests in territorial expansion. Whilst Russia might possess ample oil reserves, there was tension between the two powers following the Sino-Japanese war. Japan were the victors of that conflict, but they didn’t wish to reignite a war when richer spoils lay elsewhere.
The Japanese were allied with Germany, and had been declared “honoury aryans”, but none of this resulted in any military cohesion. Geographic distance, cultural differences and separate wartime objectives resulted in a lack of cooperation between the two powers.
The dominant force in the region were the British troops stationed in Singapore, some 60,000 - 80,000 men. The Japanese recognised this, but were also aware of the American naval base at Pearl Harbour, which was the undisputed naval power in the region. Whilst it remained, it was a knife hovering over Japan’s supply lines. They couldn’t hope to hold any territory they captured, nor defeat the British, with such a force nearby. America joining the war to defend British Singapore was considered inevitable. To realise the Empire’s dreams of expansion, it was decided that Pearl Harbour would be attacked.
It’s important to note, the ambition of the Japanese High Command hadn’t been to conquer America, or their territories. The USA was judged to be opposed to the war, preoccupied with the European theatre and ultimately unable to seize the entrenched Japanese islands.
The attack at Pearl Harbour had several objectives. To knock the Pacific fleet out of the fight for 6 months, which it did. To sink an American battleship and damage morale, which it did. And to remove Britain’s biggest ally from the region, which it did. The mission was a success, and a mistake which would bring the Empire of the Rising Sun crashing down.
Following the attack, the Japanese 25th Army were able to out manoeuvre and force the surrender of the entire British garrison at Singapore. It was a pivotal moment in the war, a major victory for Japan, and the worst defeat in British history. The attack would likely never have been attempted without the assault on Pearl Harbour.
TL;DR - Japan needed oil & resources, the only thing stopping expansion was Britain. British Singapore was Japan’s aim, the force there was a bigger threat than Russia ever was. But first, any Japanese expansion was at risk as long as the US Pacific fleet remained.
Everything everyone has said here is correct but as a corollary to the Southern Resource Area being easier to invade it required the removal of the American presence in the Philippines. The Philippines sit pretty much smack dab in between Japan and where Japan wanted to invade.
Had they invaded the Dutch East Indies and SE Asia and then America joined the war against them at a alater date, America would have been able to use the Philippines as a base to interdict critical war supplies and bomb Japan. Source: Professor Ronald Spectre and his books about the War in the Pacific
Related question, was Japan in any way motivated by "revenge" for America's annexation of Hawaii in the late 1800s?