Organic matter accumulation in the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, from sequence stratigraphic analysis and geochemical proxies

Abstract

We present a model for organic carbon accumulation in the Upper Devonian Duvernay Formation of the Western Canada Sedimentary Basin, relating total organic carbon (TOC) concentrations to a high resolution geochemical and organic petrologic database and comparing these to a core-based sequence stratigraphic interpretation. Many previous source rock studies have suggested that organic matter is enriched in transgressive systems tracts or at maximum flooding surfaces, an observation that is commonly attributed to an assumed relationship between water depth and oxygen levels. Our data set enables us to associate total organic matter content and organic assemblages with particular systems tracts and to examine how the organic assemblages change across the basin, tested against proxies for redox conditions and bioproductivity to relate relative sea level to specific mechanisms for organic accumulation.The Duvernay Formation comprises three third-order depositional sequences, superimposed on a second-order late transgressive systems tract and early highstand systems tract. Sequences are composed almost entirely of transgressive systems tracts and highstand systems tracts, with a lowstand systems tract only at the base of the upper cycle that marks the second-order sea level turnaround. Depositional facies generally vary from bioturbated carbonate-rich siltstones to siliceous mudstones from the platform margins to basin center. Organic petrologic analysis records predominantly amorphous organic matter and solid bitumen, with much less abundant vitrinite and inertinite. Organic matter type indicated by Rock-Eval analysis shows that in most cases, high total organic carbon (TOC) content is associated with better quality organic matter (high hydrogen index) and vice versa, indicating more reducing conditions or higher bioproductivity were responsibility for organic enrichment. However, near carbonate reefs, hydrogen index is uncorrelated with TOC, indicating that here, dilution by carbonate minerals was the primary control on organic enrichment. Across most of the basin, highest TOCs are recorded just above the second-order maximum flooding surface. Secondary peaks in TOC are recorded above the maximum flooding surface in the upper third-order cycle and in the transgressive systems tract of lower third-order cycle. Low TOC values are recorded in the lowstand systems tract and in the middle and upper third-order transgressive systems tracts almost up to the maximum flooding surfaces.Throughout much of the basin, proxies for relative sea level, restriction of water masses (Mo/TOC), redox conditions (Mo/Al and S/Fe) and bioproductivity (biogenic silica) coincide. These relationships demonstrate that organic carbon accumulation resulted from influxes of nutrient-rich upwelled water during high sea level; thus, TOC values are highest in the upper part of transgressive systems tracts and lower highstand systems tracts. Anoxia typically developed as a result of bioproductivity and enhanced organic matter accumulation but was not itself a trigger for organic sedimentation. Near carbonate reefs, however, varying sea level regulated carbonate sedimentation rate and organic matter dilution, and bioproductivity had less impact on organic matter concentrations; thus, TOC was high when carbonate deposition was low during transgressions, which forced carbonate reefs to backstep, limiting carbonate sedimentation in deeper water area.