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Characterizing the Development of North American Source Rock Reservoirs from the Ordovician-Jurassic: A Proxy-Based Multivariate Geochemical Approach


Source rock reservoirs (SRRs) are continuous-type hydrocarbon (HC) systems characterized by fine-grained facies that contain adequate organic matter (OM; typically >2 wt%) to generate and retain significant HC accumulations. Though SRRs may be deposited in a range of settings, OM content and quality is largely controlled by the interplay of three factors; a source, flux to the sediment, and efficiency of preservation. Much work on this topic has been done using modern analogs to develop models for application to the ancient record. From these studies, it is clear that O2-deficient conditions favor preservation of qualitatively- and quantitatively-enhanced OM, with appreciable productivity and OM flux to the sediment required to establish and maintain these conditions through OM remineralization. Yet, the relative dominance of these factors remains disputed. It is important here to consider that the Earth system has varied dramatically over geological time. Thus, it is necessary to examine SRR development not only relative to contemporary systems, but also within the context of Earth’s redox evolution. Here, we present detailed multi-proxy sedimentary geochemical studies of major Paleozoic and Mesozoic North American SRRs to elucidate their depositional conditions. This is the first study focused on SRR geochemistry to use Fe-speciation - presently the only independent proxy able to distinguish anoxic conditions as ferruginous (HS-limited) or euxinic (H2S-replete, Fe-limited) - coupled with total organic carbon (TOC) and redox-sensitive trace element proxies. Further, we apply multivariate statistics (random forest analysis) to define the relative importance of redox setting vs. productivity for a more complete view of the (bio)geochemical factors at play in the development of these units as SRRs. We show that no single factor (i.e., preservation, production, sedimentation) predominates control of these units across geologic time. Rather, our results suggest that while deposition under euxinic conditions characterizes most major SRRs, there is evidence for intervals of persistent ferruginous anoxia in the Paleozoic, and deposition in these settings does not preclude development of a SRR. We propose that beyond a broad framework for redox evolution in which to place these observations, universal models should be abandoned in favor of more basin- or formation-specific perspectives, to aid in both exploration of new plays and enhanced production of current SRRs.