Relationship Between Organic Matter and Authigenic Illite/Smectite in Devonian Black Shales, Michigan and Illinois Basins, USA
Victoria C. Hover, Donald R. Peacor, Lynn M. Walter, 1996. "Relationship Between Organic Matter and Authigenic Illite/Smectite in Devonian Black Shales, Michigan and Illinois Basins, USA", Siliciclastic Diagenesis and Fluid Flow: Concepts and Applications, Laura J. Crossey, Robert Loucks, Matthew W. Totten, Peter A Scholle
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Organic-rich Late Devonian Antrim and New Albany Shales are important hydrocarbon source rocks in the Michigan and Illinois Basins. These shales have been investigated using STEM/AEM and SEM techniques to clarify textures and mutual genetic relations between the low to moderately mature organic matter (R0 = 0.45–0.6 %) and authigenic illite-rich clays.
The Antrim and New Albany Shales contain up to 15 wt % TOC dominated by the marine algae Tasmanites. Organic matter forms an interconnected network within the clay-rich matrix with no detectable intergranular pore space even at the STEM scale. The clay-rich matrix is principally illite-rich mixed-layer I/S. Crystal sizes, compositions, defect states and presence of smectite interlayers are consistent with authigenesis of illite from precursor smectite with subsequent preservation of that immature, diagenetic illite after neoformation.
Textural relations verify that gas transport from the Antrim Shale source occurs via desorption from organic matter and diffusion through the interconnected organic and authigenic illite matrix to open fractures. In both the Antrim and New Albany Shales, plastic deformation of organic material entrains illite crystals, suggesting that illite formation preceded or was concurrent with thermal maturation of organic matter. Based on existing thermal maturity data, illite authigenesis occurred prior to Late Pennsylvanian/Early Permian time at depths less than ˜2000 m and temperatures less than ˜120°C. In contrast to the Gulf Coast Tertiary sequence, fluids derived from smectite dehydration were probably not a significant driving force for primary hydrocarbon migration from these intracratonic basin shales as a result of the presence of relatively thin shale sequences and normal pressures during burial compaction.
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Siliciclastic Diagenesis and Fluid Flow: Concepts and Applications
Research in the area of siliciclastic diagenesis has historically incorporated advances in related disciplines such as petrography and petrophysics, mineralogy, geochemistry, organic geochemistry, stratigraphy and basin analysis, and more recently, fluid flow. While the collection of papers in this publication covers a broad range of topics, an underlying theme is the importance of fluid flow in diagenetic processes. The mineralogy, texture and geochemistry of authigenic minerals provide constraints for fluid flow models, while formation waters provide modern snapshots of pore fluid evolution. Separated into two sections (Part I: Concepts and Part II: Applications), conceptual and practical applications are both represented.