Imaging Nanoscale Pores in the Mississippian Barnett Shale of the Northern Fort Worth Basin

Reed, Robert¹, Robert Loucks² (1) University of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX

The Barnett Shale (calcareous and siliceous mudstones) is a shale-gas system in which the rock is the source, reservoir, and seal. Five core analyses show porosities ranging from 0.1 to 7.6% and permeabilities ranging from 0.0006 to 0.35 millidarcys. NMR and capillary-pressure analyses indicate that the pores present are in the size range of micro- and nanopores.

Several problems prevent standard SEM techniques from providing insight into pore geometry. At the nanoscale, polished thin sections of mudstones are rough surfaces that present problems of interpreting which voids are natural pores and which are induced holes. Because epoxy impregnation of nanopores is difficult, absence of epoxy in pores is not diagnostic for recognizing induced pores.

Ion-beam milling provides a small, very low relief surface without any of the topography related to differential hardness seen on normally polished surfaces. SEM imaging of this surface allows unambiguous identification of pores. A few intragranular and intergranular micropores in the range of 500 nm in diameter are present in Barnett mudrock samples. These are commonly associated with shell material or with siltier areas of the rock. Nanopores, commonly elliptical in cross section, range in size from 500 to 20 nm in diameter. The clustered sets in particular are commonly associated with carbonaceous grains (intragranular) or carbon-rich matrix material. Maturation of organic material to hydrocarbon liquids may create some of the pores. Visible pores are not sufficient to account for measured porosities, but the smallest pores (<20-nm diameter) may be obscured by conductive sample coating.