Hydrocarbon source rock potential and elemental composition of lower Silurian subsurface shales of the eastern Murzuq Basin, southern Libya Guido Meinhold a, b, * , James P. Howard a , Dominic Strogen a, c , Matthew D. Kaye d , Yousef Abutarruma e , Mohamed Elgadry f , Bindra Thusu g , Andrew G. Whitham a a CASP, University of Cambridge, West Building, 181A Huntingdon Road, Cambridge CB3 0DH, United Kingdom b Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Sedimentologie/Umweltgeologie, Goldschmidtstrabe 3, D-37077 Göttingen, Germany c GNS Science, P.O. Box 30368, Lower Hutt 5010, New Zealand d OceanGrove Geoscience Ltd., 2 Woodland Cottage, Udny, Ellon, Aberdeenshire AB416SP, United Kingdom e Mellitah Oil & Gas B.V., Dat Al Imad Complex Tower 5 Floor 13, P.O. Box 91651, Tripoli, Libya f Libyan Petroleum Institute, Gergarish Road, P.O. Box 6431, Tripoli, Libya g Maghreb Petroleum Research Group, Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom article info Article history: Received 20 January 2012 Received in revised form 15 July 2013 Accepted 10 August 2013 Available online 19 August 2013 Keywords: Rock-Eval pyrolysis Whole-rock geochemistry Silurian Black shales Source rock Tanezzuft Formation Murzuq Basin Libya abstract A shallow borehole was drilled in lower Silurian (Rhuddanianelower Aeronian) Tanezzuft Formation siliciclastic sediments at the eastern margin of the Murzuq Basin. Shale samples were retrieved from the borehole to study their source rock potential and trace element geochemical composition. Thirty shale samples analysed from borehole CDEG-2a reveal three geochemically distinct intervals. Shales from the top (23e38 m) interval are weathered and have negligible organic content. Shales from the middle (38 e42 m) interval have moderate to good organic richness (0.6e1.7 wt.% TOC) with Hydrogen Index (HI) values up to 443 mg S2/g TOC. Shales from the bottom (42e51 m) interval have poor to moderate organic richness (0.07e0.8 wt.% TOC) with HI values up to 277 mg S2/g TOC. All samples have limited source rock potential and are immature to early mature for hydrocarbon generation. It is unlikely that they have ever been buried deeper than w2 km. Low TOC values (<2 wt.%) together with low concentrations of uranium (<10 ppm) indicate the absence of organic-rich black shale (‘hot shale’) source rocks in the studied interval. Inorganic geochemical redox proxies (Ce-anomaly, authigenic U, Th/U, V/Cr, Ni/Co, and V/Sc) indicate that the lower Silurian Tanezzuft Formation shales from borehole CDEG-2a were deposited under oxic bottom waters, explaining their relatively low TOC values. Deposition took place in a shallow marine environment, in a proximal position with regard to an early Silurian palaeo-shoreline. This combined Rock-Eval pyrolysis and whole-rock geochemical study provides new insights into the Silurian source rock distribution and hydrocarbon prospectivity in central North Africa. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The intracratonic Murzuq Basin is located in the southwest of Libya (Fig. 1) and has been the focus of gas and oil exploration, particularly since the discovery of the El Sharara (Block NC-115) and the Elephant (Block NC-174) oil fields in 1980 and 1997 respectively (Aziz, 2000; Davidson et al., 2000; Echikh and Sola, 2000; Lüning et al., 2000a; Hallett, 2002). A key risk for hydrocarbon explora- tion in southern Libya is the presence or absence of lower Silurian organic-rich black shales (‘hot shales’) of the Tanezzuft Formation (e.g., Lüning et al., 1999, 2000a, 2003a,b; Fello et al., 2006; Lüning and Fello, 2008; Belaid et al., 2010). These ‘hot shales’ form the major source rock for Palaeozoic-sourced hydrocarbons in North Africa (e.g., Boote et al., 1998; Lüning et al., 2000a,b; Craig et al., 2008). The genesis of such shales have been a topic of lively debate for several decades (e.g., Vine and Tourtelot,1970; Berry and Wilde, 1978; Tourtelot, 1979; Brumsack, 1986, 2006; Lüning et al., 2000a,b; Lüning et al., 2003a,b; 2005; Harris, 2005; Armstrong et al., 2005, 2009; Page et al., 2007; Loydell et al., 2009; Negri et al., 2009, and references therein). * Corresponding author. Geowissenschaftliches Zentrum der Universität Göttin- gen, Abteilung Sedimentologie/Umweltgeologie, Goldschmidtstrabe 3, D-37077 Göttingen, Germany. Tel.: þ49 551 3933455; fax: þ49 551 397996. E-mail address: guido.meinhold@geo.uni-goettingen.de (G. Meinhold). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.08.010 Marine and Petroleum Geology 48 (2013) 224e246