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Occurrence of Organic-Matter Pores in Sub-1.0% Vitrinite Reflectance Mudrocks: Examples From the Devonian New Albany Shale, the Mississippian Barnett Shale and the Cretaceous Eagle Ford Formation


There is controversy about the development of organic-matter (OM) pores in lower maturity organic-rich mudrocks, with some studies concluding OM pores are not formed until the gas window is reached (vitrinite reflectances > 1.3%). However, we have observed many samples in which OM pores are present in sub-1.0% vitrinite reflectance mudrocks from the Devonian New Albany Shale of the Illinois Basin (siliceous-argillaceous lithologies), the Mississippian Barnett Shale of the Fort Worth Basin (dominantly siliceous lithologies) and the Cretaceous Eagle Ford Formation from South Texas (dominantly calcareous lithologies). In these same rocks, OM pores are generally absent or rare at vitrinite reflectances less than 0.75%. Further controversy exists over being able to differentiate maturation-related OM pores from pores that remain when mobile organic matter incompletely fills pre-existing interparticle or intraparticle pores due to the presence of other fluid phases. Such pores tend to be smooth walled and larger than maturation-related OM pores and this mechanism typically forms only one or two pores in the center of an area of OM. Three cores were sampled in the Barnett, one core was sampled in the New Albany and four cores were sampled in the Eagle Ford. All samples were in the 0.75 to 1.0% vitrinite reflectance thermal maturity range. Pores were examined using a field-emission scanning electron microscope on surfaces prepared with broad-ion-beam milling using Ar-ions. OM pores observed are typically less than a micrometer in diameter. OM pore shapes are highly variable from nearly circular to highly elongate in outline. OM pore abundances are highly variable ranging from a scattered few to densely concentrated spongy patches. Differences in pore size, shape, and abundance may be related to underlying differences in the composition and structure of the original OM. OM pores are overall lower in abundance in sub-1.0% vitrinite reflectance mudrocks than in more mature mudrocks. However, heterogeneity of pore development is such that nonporous to highly porous organic matter is found across a range of thermal maturities. Variation occurs both within and between samples.