Calculating Organic Richness in Calcareous Mudstones Using the DeltaLogR Technique: Implications for Interpreting the Burial of Organic Carbon During Extreme Climate Events in the Geologic Past

DeltaLogR is a petrophysical method for evaluating organic richness of marine mudstones. It has been applied to evaluations of source rock quality in petroleum systems and enabled regional evaluations of carbon burial related to paleoclimate events. Such studies have improved our understanding of how climate influences the global carbon cycle. However, DeltaLogR was devised under an assumption that mudstones are homogenous in composition and structure. Attempts to apply this method to heterogeneous calcareous mudstones demonstrate mixed results. In this study, we evaluated the DeltaLogR response of Cenomanian–Turonian-aged calcareous mudstones from two cored wells in the Western Interior Seaway and, as a result, two primary factors are recognized as limitations of DeltaLogR. Establishing a baseline representative of the composition of the study interval is key, as both resistivity and density vary with changing mineralogy. Thus, proper baselines were established by identifying intervals of minimal separation between the logs where geochemical data demonstrates composition similar to the study interval. A second limiting factor characteristic of these calcareous mudstones is the occurrence of cemented limestones within limestone/marlstone cycles. Limestones can generate a false positive DeltaLogR response due to resistivity increasing at a greater proportion to density. However, utilization of a gamma ray minimum enables identification of such lithologic units. Application of these corrections in DeltaLogR improves calculations of organic content and enhances understanding of the accumulation of organic carbon in such lithologies that are characteristic of pelagic marine environments during extreme climate events.