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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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