Heterogeneity characterization and genetic mechanism of deepwater fine-grained sedimentary rocks during icehouse period : a case study from the Cline Shale in the Midland Basin, West Texas




Peng, Junwen

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Despite being the most abundant sedimentary rocks and having global economic significance, many uncertainties remain concerning the petrographic, sedimentological, and bulk geochemical signatures of fine-grained sedimentary rocks (i.e., mudrocks) in deep-water environments. A systematic and comprehensive petrographic (chapters 2 and 3), sedimentological (chapter 4), and geochemical (chapter 5) studies of the deep-water organic-rich Upper Pennsylvanian Cline Shale, in the Midland Basin, were conducted in this study to elucidate general principles that can inform synthesis depositional and diagenetic models for deep-water fine-grained sedimentary systems. Multiple high-resolution petrographic techniques applied to mudrocks suggest that primary grain assemblages of the Cline Shale are complex and include both intrabasinal and extrabasinal grains. Total Organic Carbon (TOC), porosity, and permeability of the Cline Shale are proportional to the intrabasinal grains content, whereas rock strength is related to the content of intergranular clay-size microquartz cement (originating from biosilicoeus allochems). The composition of primary grain assemblages is believed to further determine the potential diagenetic pathways and evolution of mechanical and petrophysical rock properties. Detailed sedimentological analysis of three continuous cores from the Cline Shale along the dip direction allows us to define seven lithofacies that stack in a repeated pattern, constituting ~8 to 20 m thick composite cycle sets. The apparent cyclicities recorded in the Cline Shale are interpreted to be controlled by high-amplitude glacioeustatic sea-level fluctuations during a period where the earth was in a ice-house state. High-resolution bulk geochemical analysis revealed significant changes in trace-element enrichment pattern and other bulk geochemical signatures in different lithofacies of the Cline Shale. Such variations in geochemical data were believed to be caused by the variations in benthic redox conditions, primary marine productivity, and hydrographic circulation in the basin, which was interpreted to be associated with high-amplitude and high-frequency glacioeustatic oscillations. This dissertation established a high-resolution imaging protocol for shale petrography (chapters 2 and 3) and confirmed a high-resolution energy-dispersive X-ray fluorescence geochemical technique in mudrock elemental composition detection (chapter 5)


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