Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 1–11, 2015 Organic geochemical characteristics of the Mississippian black shales from Wardie, Scotland Agata Trojan 1 , Maciej J. Bojanowski 2 , Marek Gola 1 , Oliwia Grafka 1 , Leszek Marynowski 3 and Euan N. K. Clarkson 4 1 Institute of Geochemistry, Mineralogy & Petrology, Faculty of Geology, University of Warsaw, Z ˙ wirki i Wigury 93, 02-089 Warsaw, Poland. Email: a.majewska@twarda.pan.pl 2 Institute of Geological Sciences of the Polish Academy of Sciences, Twarda 51/55, 00-818 Warsaw, Poland. 3 Faculty of Earth Sciences, Silesian University, Be ˛dzin ´ska 60, 42-200 Sosnowiec, Poland. 4 School of Geosciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, Scotland, UK. ABSTRACT: Coal and hydrocarbons have been exploited from the Carboniferous rocks of the Midland Valley for over 200 years. This work characterises organic matter from the Mississippian black shales of the Midland Valley from Wardie, Scotland. Biomarker analysis allowed the estima- tion of the degree of microbial transformation of organic matter, type of kerogen and thermal maturity during hydrocarbon generation. Parameters based on the biomarker indicators confirm a generally mixed type II/III kerogen. However, some samples contain mostly terrestrial organic matter, whilst others contain predominantly marine organic matter, which shows that the sedimen- tary environment varied greatly throughout the basin. The presence of gammacerane suggests water column stratification and anoxic conditions. Organic matter was much better protected from post- depositional alteration within the concretions, where higher TOC (total organic carbon) and TS (total sulphur) contents occur, than in the surrounding sediments. This can be induced by very early diagenetic formation of these concretions which protected organic matter from late diagenetic degradation. Estimated values of vitrinite reflectance (Rc, Rcs) show that the sedimentary rocks reached the catagenesis stage. Most samples exhibit maximum organic matter maturation temperatures of around c60–90  C. However, stable isomers of phenyldibenzo[b,d ]thiophene detected in some samples indicate that in some cases post-depositional hydrothermal activity affected maturation of organic matter increasing temperatures to as high as c174  C. KEY WORDS: Biomarkers, concretions, hydrothermal alteration, Midland Valley, thermal maturation, West Lothian Oil-Shale Formation Biomarkers, also called molecular fossils (Peters et al. 2005a), are organic compounds containing a molecular structure very similar to that of their source organisms. Some important geo- logical issues can be explained by their occurrence and mutual proportions; such as characterisation of sedimentary con- ditions, type of deposited organic matter (OM) and degree of thermal transformation of kerogen (Didyk et al. 1978; Huang & Meinshein 1979; Tissot & Welte 1984; Radke et al. 1986; van Aarsen et al. 1992; Simoneit 1997; Otto & Simoneit 2001; Killops & Killops 2005; Peters et al. 2005a). However, an original biomarker assemblage may be overprinted by sec- ondary processes, such as interaction with hot hydrothermal fluids, weathering or biodegradation. Such secondary alterations may have an influence on biomarker parameters. Therefore, it is crucial to know how these alterations affect organic matter. The Carboniferous rocks of the Midland Valley have been an important source of energy resources for over 200 years. Exploitation of coal and hydrocarbons has fuelled the indus- trial development of Scotland. These deposits are particularly related to the Mississippian (Lower Carboniferous) West Lothian Oil-Shale Formation, which is the biggest hydrocarbon reservoir in the estimated resources of the Midland Valley (Monaghan 2014). This area is covered by the sedimentary rocks cut by numerous igneous intrusions. Two types of intru- sions were distinguished: alkaline-dolerite (Carboniferous) and quartz-dolerite (Permian). George (1992) showed that the Permian quartz-dolerite dykes and sills had a much greater influence on the thermal maturity of organic matter in the sedi- ments of Midland Valley than did the older alkaline-dolerite sills. He also indicated that the thermal maturity of organic matter varies depending on the distance from the igneous in- trusions. This present study concerns the Mississippian organic carbon-rich shales with siderite concretions from the Midland Valley, known as the oil shales from Wardie, Scotland. Boja- nowski et al. (2012) revealed that the thermal maturity of organic matter from these rocks was not solely controlled by burial, but also by hydrothermal activity related to the igneous intrusions. The purpose of this present paper is: (1) to under- stand how precipitation of early diagenetic concretionary cements affected organic matter enclosed in concretions in comparison to the surrounding sediments; (2) to detect differences between the rocks influenced by hydrothermal fluids and biodegrada- tion and those that did not undergo these alterations; and (3) to evaluate the influence of these processes on the thermal 6 2015 The Royal Society of Edinburgh. doi:10.1017/S1755691015000225