TWO- AND THREE-DIMENSIONAL INVESTIGATION OF PORE CHARACTERISTICS AND VARIATION IN CARBONATE-RICH MUDROCKS: INSIGHTS INTO THE CONTROLS OF PORE ARCHITECTURE AND DEVELOPMENT

Abstract:

In comparison to “conventional” carbonate rocks, carbonate-rich mudrocks are generally understudied. These rocks have been shown to be different from siliciclastic mudrocks due to susceptibility to post-depositional diagenetic overprint such as dissolution and cementation. The goal of this study is to investigate the pore architecture in carbonate-rich mudrocks and highlight differences to conventional carbonates and other types of mudrocks. Micropore architecture of carbonate mudrock samples was investigated from three different formations including the Mississippian Meramecian and Kinderhookian in the Anadarko Basin of Kansas and Oklahoma, and one sample from the Upper Jurassic to Lower Cretaceous Vaca Muerta Formation of the Argentinean Neuquén Basin. Backscattered electron (BSE) and secondary electron (SE) images of FIB cross-sectioned the surfaces show complex pore geometry with variations observed among the formations. Observations of both 2D and 3D cross-sections indicate that interparticle, intercrystalline, intramineral, intraparticle, and organic matter pores associated with clay, carbonate, quartz, pyrite, and kerogen are the most prevalent components. In the BSE images, samples with high TOC, show kerogen ubiquitously filling the interparticle pore space between inorganic siliciclastic grains. Open pores of variable connectivity are observed in both the kerogen and inorganic matrix with the size, shape, and number of pores varying among the investigated samples. Estimates of organic matter and pore volume percentages of the reconstructed carbonate mudrock volumes range from 0 to 90.0% for the kerogen and 0.2 to 2.3% for pores. Estimates of pore-size distributions suggest that although pores of less than 100 nm in diameter dominate in number, they do not necessarily dominate in total volumetric contribution in some samples. The combination of the 2D SEM images and 3-D reconstructions reinforce the variability and complexity of the pore system and their microstructures in these rocks.