Durham E-Theses Petrographic and Geochemical Analysis of the Carboniferous (Namurian) Holywell Shale of northeast Wales NEWPORT, LEO,PHILIP How to cite: NEWPORT, LEO,PHILIP (2016) Petrographic and Geochemical Analysis of the Carboniferous (Namurian) Holywell Shale of northeast Wales, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/11467/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk 2 Petrographic and Geochemical Analysis of the Carboniferous (Namurian) Holywell Shale of northeast Wales THIS THESIS WAS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY by LEO PHILIP NEWPORT 2015 DEPARTMENT OF EARTH SCIENCES DURHAM UNIVERSITY Abstract he potential of the Holywell Shale as a source-rock reservoir was explored using a T multidisciplinary approach which focused on understanding the depositional setting of the Holywell Shale within the UK Namurian (mid-Carboniferous) Pennine Basin. The deposition of the Holywell Shale was examined to understand the supply and preservation of organic matter and detrital input into the basin both temporally and spatially. Both outcrop and borehole material was sampled and analysed using organic geochemical (e.g. total organic carbon, carbon isotopes, and RockEval™), inorganic geochemical (e.g. X-ray fluorescence) techniques in combination with detailed petrographic analysis. Organic matter content of the Holywell Shale was highly variable (0.1 wt % to 10.3 wt %), with an average 2.0 wt % and predominantly Type III kerogens present. Carbon isotopes revealed a change in supply of organic matter to the basin from the strongly marine influenced Lower Holywell Shale (-31.1 ‰ to -25.6 ‰, average -28.8 ‰; containing some Type II kerogen) to the terrestrial dominated Upper Holywell Shale (-28.0 ‰ to -22.4 ‰, average -24.5 ‰; containing some Type IV kerogens). Trace element and RockEval™ pararmeters indicate that the Holywell Shale was deposited under predominantly oxic conditions with some periods of hypoxia. This resulted in relatively poor preservation of organic matter (low HI values < 301 mg/g and high OI values < 121 mg/g). There is no clear relationship between organic matter source, quantity and quality within the Holywell Shale. The highest organic matter quantity with greatest preservation occurs within fossiliferous, clay-rich lithofacies associated with marine highstands. Although, the small-scale variability and heterogeneity of both organofacies and lithofacies mean that the reservoir quality of the Holywell Shale is inherently difficult to predict. Contents Abstract .............................................................................................................................. List of Figures ................................................................................................................ vii Chapter 2: ................................................................................................................... vii Chapter 3: ................................................................................................................... vii Chapter 4: .................................................................................................................. viii Chapter 5: ................................................................................................................... xii Chapter 6: ................................................................................................................... xv Chapter 7: .................................................................................................................. xix List of Tables................................................................................................................. xxi Declaration and Copyright Statement ........................................................................... xxi Acknowledgements ..................................................................................................... xxiv Chapter 1: ........................................................................................................................ 1 Introduction ..................................................................................................................... 1 1.1: Background ........................................................................................................... 2 1.2: Aims ...................................................................................................................... 3 1.3: Thesis outline ........................................................................................................ 5 Chapter 2: ......................................................................................................................... 7 Geological Background .................................................................................................... 7 2.1: Introduction ........................................................................................................... 8 2.2: Characteristics of shale gas systems ...................................................................... 8 i 2.2.1: Organic Matter Quantity ................................................................................. 8 2.2.2: Organic Matter Type....................................................................................... 9 2.2.3: Organic Matter Maturity ............................................................................... 11 2.2.4: Mineralogy .................................................................................................... 12 2.2.5: Porosity ......................................................................................................... 12 2.2.6: Permeability .................................................................................................. 12 2.3: Determining depositional environment using organic and inorganic geochemical analyses ...................................................................................................................... 13 2.3.1: Using carbon isotopes to determine organic matter source .......................... 13 2.3.2: Determining the source of organic matter from RockEval™ pyrolysis ........ 15 2.3.3: Determining the depositional environment from inorganic and petrographic techniques. .............................................................................................................. 15 2.4: Carboniferous basins, structure, and palaeogeography ....................................... 17 2.5: Carboniferous climate ...................................................................................... 19 2.6: North Wales stratigraphy ................................................................................. 21 Chapter 3: ...................................................................................................................... 26 Methods .......................................................................................................................... 26 3.1: Sample collection ................................................................................................ 27 3.1.2: Outcrop sampling.......................................................................................... 27 3.1.3: Borehole sampling ........................................................................................ 29 3.2: Sample Preparation ............................................................................................. 32 3.2.1: Powder Preparation ....................................................................................... 32 ii 3.2.2: Thin Section Preparation .............................................................................. 32 3.3: Total organic carbon (TOC), total nitrogen (TN) & stable isotope analysis ....... 34 3.4: Kerogen Type & Maturity ................................................................................... 35 3.4.1: Thermogravimetric analysis (TGA) coupled with mass spectrometry ......... 37 3.5: Qualitative bulk powder X-ray diffraction .......................................................... 38 3.6: X-Ray Fluorescence spectroscopy ...................................................................... 40 3.7: Optical Thin Section Analysis ......................................................................... 41 3.8: Scanning Electron Microscopy (SEM) ............................................................... 41 3.8.1: Energy Dispersive X-ray Spectroscopy (EDS)............................................. 43 3.8.2: Broad Ion Beam (BIB) milling ..................................................................... 47 3.8.3: Focused Ion Beam (FIB) .............................................................................. 48 Chapter 4: ......................................................................................................................