Characterization of pore systems in seal rocks using Nitrogen Gas Adsorption combined with Mercury Injection Capillary Pressure techniques Mayka Schmitt a, * , Celso P. Fernandes a , José A.B. da Cunha Neto a , Fabiano G. Wolf b , Viviane S.S. dos Santos c a Porous Media and Thermophysical Properties Laboratory (LMPT), Mechanical Engineering Department, Federal University of Santa Catarina, LMPT-UFSC, Campus Universitário Trindade, 88040-900 Florianópolis, SC, Brazil b Mobility Engineering Center, Federal University of Santa Catarina, 89219-905 Joinville, SC, Brazil c Leopoldo Américo Miguez de Mello Research and Development Center (CENPES-Petrobras), 21941-598 Rio de Janeiro, RJ, Brazil article info Article history: Received 5 August 2011 Received in revised form 23 August 2012 Accepted 6 September 2012 Available online 23 September 2012 Keywords: Seal rocks Microstructure characterization Nitrogen Gas Adsorption Mercury Injection Capillary Pressure Permeability abstract Porous microstructure parameters of seal rock samples originating from different depths within Bra- zilian geological formations were correlated to empirical models which predict the intrinsic perme- ability. Mercury Injection Capillary Pressure (MICP) and Nitrogen Gas Adsorption (N 2 GA) were applied in combination as complementary techniques; MICP to obtain the porosity values and the size distribution of meso- and macropores, and N 2 GA associated with the Brunauer, Emmett and Teller (BET) theory to determine the specific surface area (S o ). The Barret, Joyner and Hallenda (BJH) theory was applied to find the size distribution of the micro- and mesopores. The combination of the MICP and N 2 GA curves showed that the samples analyzed present a polymodal pore size distribution (PSD) and a total porosity ranging from 0.33 % to 10.44 %. The S o values measured by N 2 GA were higher than those calculated by MICP, due to the majority of the samples having a mean pore size of 20e1000  A. The intrinsic permeability could also be predicted applying the measured parameters, S o , PSD curves and total porosity in the CarmaneKozeny and SerieseParallel models. The ranges of permeability values obtained were 4.09  10 24 e4.96  10 21 m 2 and 9.48  10 27 e9.14  10 22 m 2 , respectively. These results were compared with values reported in the related literature and those obtained for four samples submitted to pressure pulse decay permeability (PDP) tests. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Seal rocks are geological formations with extremely low porosity and permeability overlying oil or gas reservoirs, which constitute a barrier against the volumetric flow of hydrocarbons into the upper layers. Although a seal rock can be considered as a seal to hydrocarbons, it is erroneous to regard it as a completely impermeable layer (Li et al., 2005). Seal rock, also called cap rock, is a crucial and sometimes overlooked factor in the evaluation of a potential hydrocarbon accumulation. Together with source rocks, reservoir rocks and overburden rocks, they represent one of the four essential elements of petroleum systems (Hao et al., 2000). The retention of hydrocarbons by overlying seals is controlled by the capillary entry pressure, the permeability and the extent of diffusive losses (molecular transport) through the fluid-saturated pore space (Schlömer and Krooss, 1997). Today, data on the permeability of cap rock find wide usage in the context of petro- leum and gas exploration and also in the storage of anthropogenic CO 2 in the subsurface region, e.g. saline aquifers or exploited gas reservoirs (Hildenbrand et al., 2002). In recent studies, the permeability of seal lithologies was found to range from 4300 down to 0.1 nD (nD ¼ nanoDarcy; 1 nD ¼ 10 9 Darcy ¼ 10 21 m 2 ) (Schlömer and Krooss, 1997). Yang and Aplin (2007) measured the vertical permeability of 30 natural mudstones and obtained values ranging from 160 to 0.24 nD. Mallon and Swarbrick (2008) reported values of 148 to 1 nD for the permeability of chalk samples measured using the transient pulse decay procedure. Cap rocks are formed in the process of sedimentary rock accu- mulation and so the physical and chemical parameters of these rocks change under the influence of lithogenic processes. In general, the microstructure of cap rocks is highly complex and anisotropic and contains very small-diameter pores (in the order of angstroms). Also, in clastic rocks, like sandstones, pores are observed between compacted grains and in the form of micro- and * Corresponding author. Tel.: þ55 (48) 3721 7709; fax: þ55 (48) 3721 7615. E-mail address: maykadole@yahoo.com.br (M. Schmitt). Contents lists available at SciVerse ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2012.09.001 Marine and Petroleum Geology 39 (2013) 138e149