Chemotaxonomy of Oil Shales

Oil shales are often divided into one of six different categories, depending on the environment of deposition and the state of preservation of the organic matter. Of these six types, five are thought to be derived from algal material. Furthermore, three of the types are thought to derive from specific types of algae: oil shales that were deposited in a marine environment and contain the cyst Tasmanites punctatus are tasmanites, oil shales that formed in a lacustrine environment and contain the remains of the green algae Botryococcus braunii are torbanites while lacustrine rocks containing the remains of Gloeocapsomorpha prisca are kukersites. However, here we show that not all organic-walled microfossils preserved in these oil shales derive from the three species, and some might not be algae at all. It is of crucial importance to determine what types of organisms formed an oil shale deposit, not only to understand the interplay of ecology and biota through time, but to ascertain the best way to unlock these vast reservoirs of organic carbon locked in the rock record. The application of novel microanalytical techniques, including chemical oxidation of the organic material and infrared spectroscopy, to these organic-rich rocks allows an exploration of biologic compounds, both extractable and recalcitrant, preserved in the rocks. Once combined with careful studies of geological context, these data enable a more robust assessment of the organisms responsible for the formation of these deposits, as well as the environment in which they once dwelled. We demonstrate that, while the traditional oil shale classification seems straightforward, it is, however, a vast oversimplification and has lead to the propagation of a great number of misconceptions.