Your right its not oil but natural, its a very shallow anoxic layer of sediment. Anoxic = no oxygen. This is where other (micorscopic)organisms turn to breaking down compounds to derive energy, sometimes a bad/sulphur smell and that distinctive colour. Anoxic layers are found in sediment that doesn't get disturbed / turned over very often to allow aeration either through physical means (waves etc.) or biological (worms etc.).