In a nutshell, you're a long way from a sediment source. Most sediments by volume come from erosion of the continents or other landmasses. There's a much highe

In a nutshell, you're a long way from a sediment source. Most sediments by volume come from erosion of the continents or other landmasses. There's a much higher rate of deposition if you're near a continent (e.g. river deltas). Additionally, oceanic crust is always very young geologically. It's continually formed and destroyed (subducted). The ocean basins haven't had much time to fill up with sediment, because oceanic crust is essentially a conveyor belt. ----- So the short answer is: You're depositing things very slowly on something that doesn't have much time for sediments to build up. ----- In a bit more detail, the sediments you get in the middle of the ocean are either the initial volcanic-related sediments that were deposited as the oceanic crust formed, or they're limited to "pelagic rain" (things near the surface dying and very fine particles of clay) and some chemical sediments (things that precipitate directly out of water). Not only that, but a lot of the things that die and sink will dissolve below about 2 Km water depth (see Carbonate Compensation Depth). Therefore, deepwater depositional rates are very, very slow. There's a very important exception, though -- Carbonates. If you start in shallow water (say, due to a small sliver of continental crust), organisms in the photosynthetic zone can produce huge amounts of carbonate sediments just from their shells (e.g. coral reefs). Under the right conditions, carbonates can keep growing and keep pace with the subsiding crust. A good example of this is the Bahamas. You have several kilometers of sediment made of nothing but what grew there and died. There are some very significant oil fields in these sorts of "isolated platforms". However, you need fairly unique conditions to get oil in the sort of isolate carbonate platforms that develop far away from land. In most cases you'd find out in the ocean basins, you won't have a source rock capable of generating hydrocarbons beneath the pile of sediments that the carbonates produce. Instead, the source rock is conveniently sitting out there in the deep water where it's not buried deep enough to produce oil. There's one other exception, and that's when you're near a volcano. Volcanoes will happily produce lots of volcanic-related sediments. However, volcaniclastic sediments almost never make good reservoirs (there are exceptions) or seals, so even if you did generate oil in a narrow band around a seamount, it would either leak out (no seal) or be trapped in something that you can't get it out of (bad reservoir). And there's the longer answer :)