Permeability of the Mercia Mudstone: suitability as caprock to carbon capture and storage sites P. J. ARMITAGE 1,2 , R. H. WORDEN 1 , D. R. FAULKNER 1 , A. R. BUTCHER 3 AND A. A. ESPIE 2 1 Earth and Ocean Sciences, University of Liverpool, Liverpool, UK; 2 BP Exploration, Sunbury, UK; 3 FEI Europe, Eindhoven, The Netherlands ABSTRACT The Upper Triassic Mercia Mudstone is the caprock to potential carbon capture and storage (CCS) sites in porous and permeable Lower Triassic Sherwood Sandstone reservoirs and aquifers in the UK (primarily offshore). This study presents direct measurements of vertical (k v ) and horizontal (k h ) permeability of core samples from the Mercia Mudstone across a range of effective stress conditions to test their caprock quality and to assess how they will respond to changing effective stress conditions that may occur during CO 2 injection and storage. The Mercia samples analysed were either clay-rich (muddy) siltstones or relatively clean siltstones cemented by carbonate and gypsum. Porosity is fairly uniform (between 7.4 and 10.7%). Porosity is low either due to abundant depositional illite or abundant diagenetic carbonate and gypsum cements. Permeability values are as low as 10 À20 m 2 (10nD), and therefore, the Mercia has high sealing capacity. These rocks have similar horizontal and vertical permeabilities with the highest k h /k v ratio of 2.03 but an upscaled k h /k v ratio is 39, using the arithmetic mean of k h and the harmonic mean of k v . Permeability is inversely related to the illite clay content; the most clay-rich (illite-rich) sam- ples represent very good caprock quality; the cleaner Mercia Mudstone samples, with pore-filling carbonate and gypsum cements, represent fair to good caprock quality. Pressure sensitivity of permeability increases with increasing clay mineral content. As pore pressure increases during CO 2 injection, the permeability of the most clay-rich rocks will increase more than carbonate- and gypsum-rich rocks, thus decreasing permeability heteroge- neity. The best quality Mercia Mudstone caprock is probably not geochemically sensitive to CO 2 injection as illite, the cause of the lowest permeability, is relatively stable in the presence of CO 2 –water mixtures. Key words: caprock, carbon sequestration, clay minerals, diagenesis, Mercia Mudstone Formation, pore throat radius, siltstone Received 15 April 2014; accepted 19 February 2015 Corresponding author: Richard H. Worden, Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK. Email: r.worden@liv.ac.uk. Tel: +151 794 5184. fax: +151 794 5196. Geofluids (2016) 16, 26–42 INTRODUCTION The permeability of reservoir-grade rocks has traditionally been much better studied than the permeability of fine- grained sedimentary rocks, not least as it is easier to make direct measurements of the permeability of reservoirs than sealing lithologies. However, there is a new imperative to understand the flow properties of low-permeability rocks as they are integral to the operation of engineered carbon capture (CO 2 sequestration) projects that are being devel- oped (Baines & Worden 2004). Carbon capture and stor- age (CCS; also known as geological sequestration of CO 2 ) in depleted petroleum reservoirs and saline aquifers is considered to be a viable option for reduction of CO 2 emissions to the atmosphere (Holloway 1997) and has been investigated in field trials such as Weyburn in Canada (Riding et al. 2003), Sleipner in the North Sea (Torp & Gale 2003) and in a Carboniferous reservoir in North Africa (Armitage et al. 2011). Potential has been identified to store CO 2 in Lower Tri- assic Sherwood Sandstone Group (SSG) reservoirs and saline aquifers onshore and offshore UK, including in the East Irish Sea Basin and Southern North Sea Basin (Williams et al. 2014). In the East Irish Sea Basin, there has been careful study of the potential for CCS within existing petroleum fields and also in structures that do not contain petroleum but could feasibly trap sequestered CO 2 (Gough & Shackley 2006; Gough et al. 2006; Heinemann et al. 2012). A storage capacity of 1047.6 million tonnes has been calculated for sequestered CO 2 in the East Irish Sea © 2015 John Wiley & Sons Ltd Geofluids (2016) 16, 26–42 doi: 10.1111/gfl.12134