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Geochemical and lithological controls on a potential shale reservoir : carboniferous Holywell Shale, Wales.

Newport, Leo P. and Aplin, Andrew C. and Gluyas, Jon G. and Greenwell, H. Christopher and Gröcke, Darren R. (2015) 'Geochemical and lithological controls on a potential shale reservoir : carboniferous Holywell Shale, Wales.', Marine and petroleum geology., 71 . pp. 198-210.

Abstract

The Holywell Shale is part of the Carboniferous Bowland Shale Formation, identified as the main potential shale gas system in the UK. Here, we report geochemical and petrographic data from five outcrops of the Lower and Upper Holywell Shale across northeast Wales. At outcrop, the Holywell Shale is immature to early oil mature and has total organic carbon (TOC) values ranging between 0.1 and 10.3 wt %, with a mean of 1.9 wt %. Carbon isotope data clearly differentiate terrestrial and marine organic matter and show that both occur throughout the Holywell, with terrestrial sources (Type III/IV) dominating the Upper Holywell and marine sources dominating the Lower Holywell (Type II/III). Trace element data indicate that bottom waters were oxygenated, resulting in poorly preserved organic matter, supported by C/N and HI data. A range of silt- and clay-rich lithofacies occur, which show no relationship to either the amount or type of organic matter. We interpret the data in terms of a mixed supply of terrestrial and marine organic matter to marine depositional environments in which there was sufficient hydrodynamic energy to transport fine-grained sediment as bed load. The resulting mudstones exhibit a range of sedimentary textures with millimetre- to centimetre-scale silt–clay bed forms which show almost no relationship to organic matter type and amount. The small-scale variability and heterogeneity of both organofacies and lithofacies means that the reservoir quality of the Holywell Shale is inherently difficult to predict.

Item Type:Article
Full text:(AM) Accepted Manuscript
Available under License - Creative Commons Attribution Non-commercial No Derivatives.
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1016/j.marpetgeo.2015.11.026
Publisher statement:© 2016 This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Date accepted:30 November 2015
Date deposited:23 November 2016
Date of first online publication:08 December 2015
Date first made open access:08 December 2016

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