High-resolution 3D fabric and porosity model in a tight gas sandstone reservoir: A new approach to investigate microstructures from mm- to nm-scale combining argon beam cross-sectioning and SEM imaging Guillaume Desbois a, ⁎, Janos L. Urai a , Peter A. Kukla b , Jan Konstanty c , Claudia Baerle d a Structural geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstrasse 4–20, D-52056 Aachen, Germany b Geological Institute, RWTH Aachen University, Wüllnerstr. 2, D-52056 Aachen, Germany c Wintershall Holding AG, Friedrich-Ebert-Straße 160, 34119 Kassel, Germany d Wintershall Holding AG, Erdölwerke Barnstorf, Rechterner Straße 2, 49406 Barnstorf, Germany abstract article info Article history: Received 6 January 2011 Accepted 6 June 2011 Available online xxxx Keywords: argon beam cross-sectioning SEM tight gas reservoir porosity rotliegend sandstone The development of new technologies to enhance tight gas reservoir productivity could strongly benefit from a better resolution and imaging of the porosity. Numerous methods are available to characterize sandstone porosity. However, imaging of pore space at scales below 1 μm in tight gas sands remains difficult due to limits in resolution and sample preparation. We explored the use of high resolution SEM in combination with argon ion beam cross sectioning (BIB, Broad Ion Beam) to prepare smooth, and damage-free, true-2D surfaces of tight gas sandstone core samples from the Permian Rotliegend in Germany, to image porosity down to 10 nm. The quality of cross-sections allows measuring porosity at pore scale, and describing the bulk porosity by defining different regions with characteristic pore morphology and pore size distribution. Serial cross sectioning of samples produces a 3D model of the porous network. We present a model of fabric and porosity at 2 different scales: the scale of sand grains and the scale of the clay grains in the intergranular volume. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Producing gas economically from unconventional sources is a great challenge. Tight gas reservoirs, with their low permeability and low porosity, have recently received much interest because of the very large reserves, which could be produced with suitable technology. Tight gas reservoirs are found throughout the world and occur in all common types of reservoir rocks (Law and Curtis, 2002; Haines, 2005; Holditch, 2006; Littke et al., 2008). For improved recovery of these tight gas reservoirs, it is essential to quantify porosity at the pore scale. However, because pores in tight gas reservoirs are usually below 1 μm, this is difficult because of technical and preparation limits. Thus, the fundamental information about pore morphologies in tight gas reservoirs is missing, although pore morphology and connectivity is the key to correlate porosity and permeability data. Traditional techniques for characterization of different aspects of porosity in sandstone are X-ray computer tomography (micro-CT) scanning (Honarpour et al., 2003; Tono and Ingrain, 2008), magnetic resonance imaging (MRI) (Pape et al., 2005), particle size analysis, point counting based on petrographic thin sections, environmental scanning microscopy (ESEM) (Tiab and Donaldson, 1996), X-ray diffraction (XRD) (Ward et al., 2005), X-ray fluorescence (XRF) (McCann, 1998), confocal scanning laser microscopy (CSLM) (Menendez et al., 2001) and mercury porosimetry (Favvas et al., 2009). The combination of these complementary methods is considered to give a robust characteri- zation of reservoir properties of sandstones (e.g. Baraka-Lokmane et al. 2009) and in sandstones this has produced a growing body of litera- ture of 3D pore models (with a resolution of a few micrometers) and based on these, numerical models of fluid flow through porosity, which accurately predict bulk properties such as Darcy Permeability (Sholokhova et al., 2009; Tölke et al., 2010). There is a considerable body of literature on the evolution of porosity by compaction and diagenetic processes (Gaupp et al., 1993; Schöner et al., 2008; Zwingmann, 2008) and it appears that to investigate porosity at nm scale in tight gas reservoirs, SEM imaging is the most direct approach (Nadeau and Hurst, 1991; Ziegler, 2006). This method, however, is limited by the poor quality of the inves- tigated surfaces (mainly broken or mechanically polished surfaces including the decoration of porosity by colored resin embedment), which make observations and interpretation difficult (Lanson et al., 2002; Schöner and Gaupp, 2005). The recent development of Argon ion source milling tools which produce polished cross-sections of exceptional high quality offers a new alternative for high resolution SEM imaging of porosity at nano- scale (Desbois et al., 2009; Loucks et al., 2009; Holzer and Cantoni, Journal of Petroleum Science and Engineering 78 (2011) 243–257 ⁎ Corresponding author. Tel.: + 49 241 80 94352; fax: + 49 241 92358. E-mail address: g.desbois@ged.rwth-aachen.de (G. Desbois). 0920-4105/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2011.06.004 Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol