The most important thing to understand about petroleum is that it is formed from dead plankton. Tiny creatures live near the top of the ocean. When they die, their bodies settle down to the bottom. If there are other creatures living on the bottom, they eat the organic material that rains down from up above, and bacteria consume whatever doesn’t get eaten by larger creatures. In order for those bottom-dwelling organisms to stay alive, there has to be enough oxygen. As long as the bottom waters have enough oxygen, all the dead organic stuff falling down from the surface waters gets consumed.
But if the bottom waters don’t have enough oxygen, there are no creatures living on the bottom to eat the organic material, and it accumulates. The organic stuff is layered in the sediment and gets buried. When enough sediment piles up on top of the organic layers, the temperature starts to rise. The combination of pressure of the pile of sediment and the increased temperature within the earth cause the organic material to change into petroleum trapped in the sediment. This process takes many millions of years. Most of the petroleum we drill today formed at least 50 million years ago.
So the essential condition for creating petroleum is an ocean basin where the bottom waters have no oxygen. Oxygen only gets into ocean water in two ways: either plants produce it in shallow well-lit water, or oxygen gets mixed into the water by wave action near the surface. Either way, it is surface waters that have the most oxygen. Sometimes the surface water can get carried down to the bottom if it is colder or saltier (and therefore heavier) than the water underneath. But in narrow ocean basins that are cut off from other ocean basins, the surface water does not mix with the bottom water, and the bottom water has little or no oxygen.
Ocean basins are usually narrow at two times: when they have just formed, and when they are being squeezed out of existence.
Diverging boundaries and new ocean basins.
When a new ocean basin opens at a diverging plate boundary, it will be long and narrow. There will not be established currents that move water sideways or vertically. This means that the water is pretty stagnant with well-defined vertical layers. The surface waters do not get carried to the bottom, so the bottom waters lack oxygen. This is a perfect environment for organic material to settle and accumulate, and then be buried in the sediment.
As the ocean basin gets wider, this buried organic material is left behind on the edges of the continents. So we should expect that the edges of continents – now called passive margins, since they aren’t plate boundaries anymore – are good places to look for petroleum. But it can be an expensive place to look for petroleum because it involves offshore drilling, sometimes in deep water.
Collisions and narrowing ocean basins
Ocean basins can also be long and narrow when they are closing up. This happens when continents collide, and the ocean in between then is squeezed shut. It can also happen when smaller chunks of land, such as fragments of continent (like Indonesia) or volcanic island chains (like Japan or Hawaii) run into a continent at a subduction zone. The ocean floor between the continent and the island chain gets eaten up at the subduction zone, and that section of ocean gets thinner and thinner until the island chain collides with the continent.
Either way, whether it is a big collision (like when Africa hit North America to create the Appalachian Mountains) or a smaller collision when an island chain hits a continent, the narrow bit of ocean between the landmasses will not have much vertical mixing of water, and the bottom will become oxygen-poor. Organic material gets layered into the sediment and becomes petroleum. When the collision happens, the rocks containing petroleum get pushed upward. So the best place to find petroleum in a collision zone is next to the high mountains formed by the collision. This becomes an excellent place to look for cheap oil, because it’s easy and less expensive to drill for petroleum on land.