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Laboratory measurements of porosity, permeability, and fluid saturation are routinely acquired from core samples to quantify petrophysical data for shale reservoir studies. However, these measurements do not provide information regarding the microstructural character of the mineral matrix, organic matter distribution, and pore network. New imaging techniques using focused ion beam scanning electron microscopy (FIB-SEM) were applied to a subset of prepared core samples from four wells to study the microstructural character of the Cenomanian-Turonian Eagle Ford Formation in the Mavrick Basin, south Texas, in the United States. The studied wells span a wide range of organic thermal maturity from the oil window to the dry-gas window. Sixteen digital rock physics (DRP) models were prepared from three-dimensional (3-D) SEM image volumes to quantify volumes of mineral matrix, organic matter, and pore space. The results of this study revealed organic matter pore type, pore size, and permeability vary by stratigraphic interval and the degree of thermal maturity. Pendular organic matter with bubble pores was more common in the oil window. Both pendular and spongy organic matter types with bubble and foam pores were common in the gas/condensate window. Organic matter pores from samples within the dry-gas window had smaller average pore diameters and lower permeability than samples within the oil window. Comparison of core porosity and permeability measurements derived from DRP models with laboratory core measurements showed broad similarities, indicating that despite the small analysis volume involved with FIB-SEM 3-D image volumes, the DRP models prepared in this study were generally representative of the larger core samples.

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