Revisiting paleoenvironmental analyses and interpretations of organic-rich deposits: The importance of TOC corrections

Abstract

Methods for reconstructing present-day TOC values to their original values are widely used in petroleum systems analysis. However, the importance of making these corrections appears to be underappreciated when TOC is used for paleoenvironmental interpretation. In this paper we demonstrate how differences between present-day TOC (TOCPD) values (e.g., as measured via LECO and similar methods) and those at the time of deposition (TOCO) can materially affect paleoenvironmental interpretations that use TOC as a variable. The difference between those two values depends on the thermal maturity of the deposits and the original kerogen type. We illustrate the effects of restoring and using TOCO values on samples from the Upper Cretaceous Eagle Ford Formation of South Texas and the Upper Devonian Lower Bakken Shale of North Dakota. Making this correction requires programmed pyrolysis (“Rock-Eval”) data rather than simple TOC measurements. We use plots of molybdenum and sulfur versus TOCPD and TOCO to illustrate the effects of restoring TOC, but any analysis that involves TOC as a variable will be affected. There are uncertainties associated with defining the original organic matter type, and other unknowns that can affect TOC reconstruction. As such we advocate presentation of a range of possible TOCO values, i.e., deliberately capturing uncertainty, rather than a single value. The method used to restore TOC has minimal impact on the interpretations, at least for the two methods employed herein.