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Stratigraphic Controls on Diagenetic Processes in Mudstones From the Upper Cretaceous of the North American Western Interior Seaway: Implications for Source Rocks and Unconventional Reservoir Quality


Diagenesis exerts a major control on the ultimate organic carbon contents and mechanical properties of mudstones by controlling organic carbon degradation pathways and cementation histories. Diagenetic processes therefore strongly impact hydrocarbon source and unconventional reservoir attributes. Recent studies suggest that in mudstones much diagenesis occurs prior to compaction and is intimately associated with processes occurring close to sediment water interface. Here we investigate how pre-compaction diagenetic processes are controlled by stratigraphic setting and how varying sediment starting compositions effect subsequent chemical transformations. Using our existing high-resolution stratigraphic framework erected for Cenomanian-Turonian aged sediments preserved in the Mid-Cretaceous Seaway as a natural laboratory the diagenesis within coeval sediment packages preserved in both proximal clastic detritus sediment rich locations and more distal production detritus rich settings were investigated. The data to inform this study were obtained using a combination of petrographic (optical and electron optical techniques) and geochemical techniques (XRD, total organic carbon (TOC)) methods. Mudstones collected from clastic detritus-rich environments are thin bedded siliciclastic medium to coarse mudstones with framework materials composed of quartz, and a matrix dominated by clay minerals. These mudstones are relatively depleted in biogenic debris, contain up to 2% (TOC) and contain framboidal pyrite and carbonate cements. The latter either infill the bulk of the intergranular volume close to stratal surfaces or former patchy cements infilling intragranular pore space. In contrast, those in production detritus-rich successions comprise thin-bedded calcareous medium to coarse mudstones, enriched in planktonic foraminifer (framework component), with up to 5% TOC, and some framboidal pyrite. These mudstones also contain significant volumes of calcite, kaolinite and silica cements that infill available pore space. The varying composition of these assemblages indicates that microbial degradation processes acting in pre-compaction pore waters strongly control diagenetic reactions that follow with key regional differences being generated by varying starting composition of the sediment (particular the varying availability of metastable biogenic detritus). Diagenesis is significantly influenced by stratigraphic setting and cannot be treated as a fixed variable.