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

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Abstract

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.

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Acknowledgements

The authors are grateful to Faraj Said (National Oil Corporation, Tripoli, Libya), Ahmed I. Asbali (Arabian Gulf Oil Company, Benghazi, Libya), Ramadan Aburawi and Bourima Belgasem (Libyan Petroleum Institute, Tripoli, Libya) for their scientific support and guidance during the project work in Libya. We also would like to thank the logistics team, provided by Bashir Grenat, for assistance in the field. The administrative and logistical support of staff at the Libyan Petroleum Institute is gratefully acknowledged. Richard Pearce (University of Southampton, UK) is thanked for providing the XRD data. The consortium of oil and gas industry subscribers is thanked for its financial support to the CASP project work in Libya. We are very grateful to Christopher Tiratsoo (Beaconsfield, UK) and Stephen Vincent (CASP, Cambridge, UK) for commenting on this work during pre-review.

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Authors and Affiliations

  1. CASP, West Building, Cambridge, CB3 0DH, UK

    Guido Meinhold

  2. Department of Sedimentology and Environmental Geology, Geoscience Center, University of Göttingen, Goldschmidtstrasse 3, 37077, Göttingen, Germany

    Guido Meinhold

  3. Department of Earth Sciences, Queen’s Building, Royal Holloway University of London, Egham, TW200EX, UK

    Daniel P. Le Heron

  4. Libyan Petroleum Institute, Gergarish Road, P.O. Box 6431, Tripoli, Libya

    Mohamed Elgadry

  5. Mellitah Oil & Gas B.V., Dat Al Imad Complex Tower 5 Floor 13, P.O. Box 91651, Tripoli, Libya

    Yousef Abutarruma

Authors
  1. Guido Meinhold
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  2. Daniel P. Le Heron
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  3. Mohamed Elgadry
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  4. Yousef Abutarruma
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Correspondence to Guido Meinhold.

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Meinhold, G., Le Heron, D.P., Elgadry, M. et al. The search for ‘hot shales’ in the western Kufra Basin, Libya: geochemical and mineralogical characterisation of outcrops, and insights into latest Ordovician climate. Arab J Geosci 9, 62 (2016). https://doi.org/10.1007/s12517-015-2173-0

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