Chemotaxonomy of Oil Shales

Alison N. Olcott¹, Craig P. Marshall², and Tim I. Eglinton^1
1Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institute, Woods Hole, MA
²Chemistry, The University of Sydney, Sydney, NSW, Australia

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. With the exception of cannel coals, which contain plant and fungal matter as well as algal debris, there are three types of oil shales whose identifiable microfossils are traditionally thought to derive from specific algae. Oil shales that were deposited in a marine environment and contain the algal 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, not all organic-walled microfossils preserved in these oil shales may derive from the three species, and some might not be algae at all. It is of crucial importance to unravel the biological precursor organisms in order to understand the true origins of oil shales, both to understand the interplay of ecology and the microbiota through time, as well as to characterize the vast reservoirs of organic carbon locked in the rock record. The application of novel microanalytical techniques allows an exploration of biologic compounds, both extractable and recalcitrant, preserved in the rocks, and combining these new data with careful studies of geological context enable more robust assessments 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.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas