Was one of the main reasons Hitler attacked the Soviet Union due to oil?

Oil was cited as one of the reasons, it is hard to judge whether it was "one of the main reasons" or not. But this was not the oil of Caucasus, this was Romanian oil. The problem was that after wrestling a part of Romanian territory in 1940, Soviet army was in close striking distance from Romanian oil fields, and the Germans felt vulnerable.This point is explicitly stated in the German declaration of war:

http://www.jewishvirtuallibrary.org/adolf-hitler-declaration-of-war-on-the-soviet-union-june-1941

Now, there is a strong evidence that Soviet Union was preparing to attack Germany earlier or later, only the timing is disputed. So the oil question was certainly one of the considerations.

Speaking of Caucasus oil, this was certainly not a priority in 1941. The decision to move to Caucasus was made much later, after the German defeat near Moscow. And the main reason was not taking this oil for Germany but depriving the Soviet Union of Caucasian oil.

Germany wanted Russia for its resources: Caucasian oil, Donets basin minerals, Kazakh chrome, and Ukrainian wheat, among others.

In 1940, German occupied Europe had twice the population of the United States, with its combined GDP not far behind America's. It had, however, about 15% of the world's industrial capacity, or less than half of the U.S. total.

The Soviet Union had about half of the population of German-occupied Europe, a fraction of its GDP, but about the same industrial capacity.

If Germany could take over the Soviet Union, it could have three times the population as the U.S., about the same GDP, with industrial capacity approaching that of the U.S.Germany would then have power to rival that of the U.S. This would especially be true if Japan could be raised to act as a counterweight to Britain, America's other major ally, e.g. by taking India away from Britain and giving it to Japan.

Source For Industrial Production: The Rise and Fall of the Great Powers, Paul Kennedy.

Source For GDP: Contours of the World Economy, 1–2030 AD by the British economist Angus Maddison.

It's widely believed that the reason for Hitler's invasion of the Soviet Union was due to the oil fields in the Caucasus

Oil certainly was an important consideration for Hitler, but Stalin was already selling him all the oil he would ask for.

Which, according to many historians, was the fact he prioritized his armies south rather than to capture Moscow.

He prioritized it to deny USSR the Baku oil (170MB/y). Getting Baku oil for Germany was a pipe dream _{^{(OTOH, Hitler was famous for pipe dreams - the relevant one involved Rommel from North Africa meeting List from Caucasus in Palestine and sending an expeditionary force from there to India)}}. Getting North Caucasus oil (50MB/y) was more realistic, but less lucrative.

Weren't the oil fields in Romania enough for Hitler?

Nope.

Germany consumed 44MB/y in 1938 (peace time!) while it got at most 13MB/y from Romania during the war.

They were already engaged in Northern Africa, wouldn't it be an easier target to keep the occupation there and possibly capture other Middle Eastern territories

North African oil fields were much less developed at that time (e.g., oil was discovered in Libya only in 1959). Also, extracting the oil requires infrastructure and transporting it to Europe over the Mediterranean - which would have been impossible against the RN and RAF based on Gibraltar, Malta and Egypt.

... [why] attack the Soviet Union?

1. USSR was an existential ideological enemy for Hitler, somewhat like "Carthago delenda est".
2. It was also supposed to be the source of Lebensraum.
3. It was also a military threat (especially to the Romanian oil fields!) - there was no doubt that it would attack at the first opportunity.
4. It was also perceived as the last hope of Britain - Hitler assumed that when USSR is destroyed, Britain would sue for peace.

See

1. Oil campaign of World War II
2. The Role of Synthetic Fuel In World War II Germany
3. What goods did Germany trade during World War II, and with whom?