While not a full answer, I just want to quickly mention two very relevant facts with regards to oil in Siberia:

1. Most, if not all oil fields in Siberia wasn't discovered until after WWII. During the war, the Soviets were getting most of their oil from Baku, northern Caucasus mountains and the northern coast of the Caspian Sea. Geological surveys of Siberia in hopes of finding oil were only just taking place and the oil industry there was in very very nascent state.

2. Most of the oil in Siberia is in western part of it - thousands of miles away from the Pacific Ocean. The Japanese would have had to go through vast stretches of land, very difficult terrain and climate far harsher than even the Germans experienced in European Russia to get there. Also if the Russians had decided to destroy the Trans-Siberian Railroad during their retreat, there would be literally no way to get there.

So getting oil from Siberia was a non-starter.

Lets start with the easy one first: why didn't they focus on taking Siberia?

Simple, the Soviets walloped the Japanese Kwantung Army in 1939 in a months-long campaign known as the Battle of Khalkhin Gol. After that, the Japanese signed a nonaggression pact with the USSR.

Now, the longer one: why didn't they focus all their efforts upon taking China?

Because America didn't want Japan to be in control of all of China. Throughout the 30s, American (and other western) leaders kept on trading with Japan despite Japanese incursions into China, but actually attempting to conquer all of China was a step too far. Japanese actions in China and Indochina are what provoked the US to lead an embargo on Japan, depriving them of oil, iron, rubber, and other supplies necessary for their war effort. Japan had to secure more oil to continue fighting. The most obvious target was Dutch East Indies, which was an oil producing region. However, Japanese leaders feared that America would go to war with Japan even if the Japanese only attacked the Dutch and maybe the British (Malaya had resources too), and given the US's position in the Philippines, the US going to war with Japan would throw a wrench in any attempt to conquer the East Indies. Therefore, Japanese leaders settled on a preemptive attack at Pearl Harbour - basically thinking that if war was inevitable, they might as well start it on their own terms.

(Oh geez, first post. I hope I do it right. D:)

Full disclosure: my area of specializaton is Korea during Japanese rule rather than imperial Japan itself, so if anyone wants to correct/disagree with anything I say, please do!

One thing to consider is that Japan came to the empire game quite late, and had to contend with all of the other growing empires (particularly US, which was also empire-building in Southeast Asia). Japan was trying to modernize and build a nation on par with the "Western" powers, and so it was necessary from a global relations standpoint to have every incursion into new territory seen as "legitimate." This meant that military action and occupation couldn't be taken willy-nilly, and a lot of Japan's empire-building was ideological rather than purely military. Any military incursion had to be justified as some sort of necessary, modernizing mission to gain international support.

I'll try to find a copy online, but there's a really interesting piece that was published in foreign newspapers in English around 1905(ish), written by Ito Hirobumi, that justifies the Korean occupation in the language of international relations at the time. Basically, Japan couldn't have an empire if the US and others really tried to stop them, so it was important to play nice with the imperial neighbours.

Case in point: Japan did have troops in Sibera, but withdrew when it became clear that the Americans were getting nervous (1920-22).

As for China, Japan had wars with China twice during this period. The First Sino-Japanese War led to the annexation of Korea (a Chinese vassal state), and the Second Sino-Japanese War sort of bled into WW2, which Japan lost, along with most of its territories. That effectively put a stop to any incursion into China. Or anywhere else, really.

I'm not really sure what you mean by: "the only reason Japan invaded [China?] initially was due to the Mukden Incident." The Mukden/Manchurian Incident was orchestrated by the Japanese to justify "liberating" Manchurians from Chinese rule, and took place in 1931, nearly 40 years after the first Sino-Japanese War (in which Japan gained Taiwan and briefly the Liaodong peninsula until France threw a fit - again, other empires were cramping Japan's style). So it isn't a reason for invading northeastern China, it was a pretext. The benefit of Manchuria arguably had more to do with securing already-existing railway lines to get people/supplies further north, since getting a navy up there is difficult when ports are frozen half the time, and building your own infrastructure gets expensive.

So basically, had WW2 not ended Japanese empire entirely, Japan would have eventually intensified their efforts on China and Siberia.

Unfortunately, I'm not near my office and can't give page number citations. Alexis Dudden's book Japan's Colonization of Korea has a section on how Japan used the existing language of empire to justify its colonial holdings, which iirc is where I got the letter from Ito Hirobumi. But maybe not. I'll double-check when I can. Oguma Eiji's A Genealogy of 'Japanese' Self-images is also useful. It goes into the more ideological aspects of Japanese empire.

TLDR: There was a focus on China and Siberia as eventualities, but it made other imperial powers nervous, and then WW2 happened.