The main reason for a lack of effort in Darfur and Zimbabwe is a bit more complex than merely being for want of oil reserves (both Darfur and Zimbabwe are thought to have reserves of strategic minerals that would make an invasion even more attractive than oil). As it stands the British want to go into Zimbabwe and the French into Darfur. As both regimes are backed by China and an attack on these regimes would likely draw a response, possibly even war, it is up to both countries to convince their main ally, the USA, to take that risk. The USA would then have to consult those allies it would rely on in a war against China- Australia, Japan, Phillipines and Singapore. I don't know about the other countries, but the general consensus in Australian policy circles is that neither situation is worth risking a war with China- especially as we could expect no help from either Britain or France in such a war.
You could argue it is a small risk but it is there nonetheless; China relies on Sudan for much of its oil imports. As a strike against Sudan would invariably involve targetting infrastructure it could be seen by the Chinese as an attempt to destabilise their economy under the guise of humanitarian action.
It will never happen for Darfur because the consensus view here is that while regrettable, there is too much moral ambiguity attached to the Darfurese themselves. There was a reasonable compromise peace worked out to end the Sudanese civil war between Muslim North and Christian South that the animist Darfurese could have used diplomatically to broaden their own freedoms. Instead they initiated a conflict, and the atrocities.
Zimbabwe is even more complex because of the position of South Africa; it is difficult to justify risking a war with China if the South Africans do not see a problem with the situation.