The search for 'hot shales' in the western Kufra Basin, Libya: geochemical and mineralogical characterisation of outcrops, and insights into latest Ordovician climate

Across the Saharan platform, mudrocks of latest Ordovician-Silurian age (the Tanezzuft Formation) are a major source rock interval for Palaeozoic petroleum systems, but source rock quality is variable and difficult to predict. In the Kufra Basin of southern Libya, evidence for organic enrichment in this formation is scarce. This paper presents the results of a spectral gamma-ray study of siliciclastic sedimentary rocks in Jabal Eghei at the western margin of the basin. The study spans the Ordovician-Silurian interval together with overlying Mesozoic strata and was conducted at outcrop using a hand-held gamma-ray spectrometer. Element concentrations of potassium (K), uranium (U) and thorium (Th) were collected from mudrock and sandstone successions to identify natural background values of these elements for different formations. Uranium contents were used to identify possible 'hot shales' with source rock potential. This study shows that sandstones and mudrocks are clearly discriminated by their K, U and Th contents. Most sandstones have <0.3 % of K, <4 ppm of U and <10 ppm of Th. In contrast, the mudrocks show values of >0.3 % of K, >4 ppm of U and >10 ppm of Th. Based on gamma-ray spectrometry, the Tanezzuft Formation is divisible into lower, middle and upper members. There is a significant difference in K content between the three members. Exposures of the lower member show K contents less than 0.5 %; those from the middle member are between 1.5 and 3.6 %; and the upper member has K contents between 0.5 and 1.5 %. Notably, U values >10 ppm are restricted to the upper member, indicating the presence of 'warm shales' in Jabal Eghei. There is a progressive increase of Th/K ratios from the basal part of the Tanezzuft Formation towards the upper member. The decrease in K content and increase in Th/K ratios between the middle and the upper members of the Tanezzuft Formation can be explained by a mineralogical change, confirmed by x-ray diffraction analyses on representative samples. The total volume of clay minerals is 64 % higher in the upper member than in the middle member, with kaolinite predominant. These changes in composition suggest a change towards a more weathered sediment source, which was probably caused by increased continental weathering due to climate change from icehouse to greenhouse conditions at the Ordovician-Silurian transition.