Assemblages related to high organic carbon content in sediments are related taxonomically to assemblages in sewage outfall areas, where sediments generally have a high percentage of organic matter. Some species of genera Buliminella, Bulimina, Fursenkoina, Florilus, Nonionella, and probably Uvigerina are related to high organic carbon content in sediments. Florilus grateloupii and Fursenkoina pontoni are dominant in sewage outfall areas of the eastern Caribbean Sea. Fossil foraminiferal assemblages found by Batjes in oil-producing Miocene formations from Trinidad are similar to living assemblages in sediment with high organic carbon content. Some of these assemblages under certain conditions might be diagnostic of source rock for oil through its relation to the organic carbon in sediments, and they may be appropriate for the study of the development of oil basins.