Sakalin: It represents about 20% of Soviet oil production. This increased during the war to about 25%.

Forces:
Japanese
The Japanese Kwantung Army (the overall command in Manchuria) had about 10 divisions in 1939, 9 infantry and one cavalry. The biggest armored unit was a single tank brigade with about 90 light tanks and tankettes. Additionally, there were 8 brigade sized border guard units occupying various fortified regions of the border. The 2nd Air army with about 100 aircraft total supported the ground troops.
By 1945 the size of the Japanese army in Manchuria had about doubled, at least on paper. However, many of the units were of poor quality and their armament was often inadequite.
In 1941 the Kwantung Army is still, more or less, the size of it was in 1939. The Japanese at this point have a total of 41 divisions in existance, so the Kwantung Army represents about 25% of the entire Japanese Army strength when Germany invades the Soviet Union.

Soviet:
On June 22 1941 the Soviets had in the Far East, Siberian, Central Asian and, Trans-Bakal military districts a total of 5 Armies, 16 Rifle corps, and 1 cavalry corps. There were a total of 28 Rifle divisions, 4 Cavalry divisions, 14 Border guard regiments, 3 Rifle Brigades and, 1 Airborne Brigade.
In mechanized units the Soviets had in the same districts:
2 Mechanized corps with 4 Tank Divisions and 2 Mechanized divisions in them. Additionally, there was 1 additional mechanized division, 1 mechanized brigade and , 3 motorcycle regiments in these districts.
The Soviets had about a 5 to 1 advantage in aircraft and about a 7 or 8 to 1 advantage in artillery.
On 1 July 42 the composition was:
7 Rifle corps, 4 cavalry corps, with 52 Rifle divisions, 30 Rifle Brigades and, 23 Cavalry divisions. Mechanized units included: 2 Tank divisions, 19 tank brigades, 2 motorized rifle brigades and, 1 mechanized brigade.
The Soviets still had about the same preponderance of artillery and aircraft they had a year before.

There is little doubt that had Japan attacked the Soviets they would have found themselves in very dire straights within a month or two of opening hostilities. If the US placed an embargo on Japan for increasing hostilites, a very likely proposition, Japan would have been badly hurt economically and in no position to open a war against the US or South East Asia.