This thesis has been submitted in fulfilment of the requirements for a postgraduate degree (e.g. PhD, MPhil, DClinPsychol) at the University of Edinburgh. Please note the following terms and conditions of use: This work is protected by copyright and other intellectual property rights, which are retained by the thesis author, unless otherwise stated. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given. Geochemical Characteristics of Unconventional Gas Resources in the U.K. and the Applications for Gas Tracing Rory McKavney Thesis submitted for the degree of Doctor of Philosophy School of Geosciences University of Edinburgh 2018 Declaration I declare that all work in this thesis, unless otherwise referenced, is entirely my own. None of this work has been submitted for any degree or professional qualification other than that specified on the title page. Signed: ………………………………………………………………. Date: ………./………./………….. Please note the following terms and conditions of use: This work is protected by copyright and other intellectual property rights, which are retained by the thesis author, unless otherwise stated. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given i ii Abstract Unconventional gas extraction has caused controversy due to induced seismicity, inadequate disposal of waste by-products, and alleged incidents of shallow groundwater contamination. Determining the origin of shallow gases is problematic because methane and other hydrocarbons have numerous sources that may have overlapping geochemical characteristics, and few baseline measurements were taken prior to drilling. Additionally, hydrocarbons can be fundamentally altered by physico- chemical and microbial processes which can mask the original geochemical signatures. This project develops the understanding of natural tracers in UK unconventional gas resources and reports the results of major gas composition, stable carbon and hydrogen isotopes, radiocarbon content of methane, and novel noble gas (He, Ne, Ar, Kr, Xe) measurements from unconventional gas sites across the UK. Characterising different gas sources reveals processes within unconventional gas reservoirs, develops a baseline for future work, and allows the development of a geochemical ‘fingerprint’ which allows more accurate and precise determination of the different sources of methane. The Coal Bed Methane (CBM) field at Airth, Central Scotland, is characterised by a methane-dominated thermogenic gas based on δ13CCH4 and δDCH4 data and elevated ethane and propane contents. Gases show high helium concentrations (1105 ppm – 2984 ppm) with a 3He/4He isotopic ratio (0.18 Ra) which is uniform across the field, indicating a small (<4%) but resolvable mantle helium contribution not previously observed onshore in the UK. Also observed are elevated concentrations of excess 40Ar* (40Ar/36Ar = 371 – 1031), and enrichments of 20Ne/36Ar relative to the ratios normally expected in air saturated water which cannot be explained by simple solubility fractionation or mass-fractionation. A model is outlined where increasing dewatering of the wells as the field is developed for commercial extraction results in lower overall noble gas concentrations, a decrease in the air-derived inventory, and an increase in crustal and mantle components. This is hypothesised to occur via the progressive degassing and removal of dissolved air-derived components from formation waters from the more permeable cleat and fracture networks surrounding wellbores, leaving the remaining gases with a greater contribution from crustal-rich components sourced from within the coal matrix. iii Abandoned mine gases from Nottinghamshire and South Yorkshire coal mines showed variable methane concentrations (39 – 75%), with the remaining volume being mainly composed of a mixture of nitrogen and carbon dioxide. Correlation of N2 and 36Ar concentrations in some samples showed ingress of atmospheric air as a result of the suction applied to the mine to facilitate gas production, which was subsequently depleted of oxygen in the mine environment. Carbon dioxide was sourced in lower volumes from coal oxidation, variably mixed with higher concentrations formed from the chemical dissolution of carbonates in the acidic mine environment. Coal gases were characterised by a narrow range of δ13CCH4 and δDCH4 values of a primarily thermogenic origin, thermogenic levels of ethane and propane, and high levels of purely radiogenic helium (350 - 1506 ppm at 0.006 – 0.039 Ra). The stable isotope δ13CCH4 and δDCH4 measurements for unconventional gases typically overlapped those of other coal gases, North Sea gases, and landfill gas, which demonstrates this commonly used tool can be an ambiguous for gas source determination. Elevated levels of helium were found to be ubiquitous in all unconventional gases, and up to 3 orders of magnitude above the concentration found in air (5.24 ppm), which shows helium is an excellent tracer for unconventional gases. iv Lay Summary Natural gas for home and industry is extracted from well bores that penetrate deep into the Earth to reach trapped gas accumulations. The gas is stored in rocks such as sandstone that have open pore spaces between the sand grains, allowing the gas to flow freely through the rock into the well. New resources of ‘unconventional’ gas have been found, where gas is trapped in tiny pores in rocks such as shales, or adsorbed onto coal surfaces, and is less free to flow through the rock. These types of gas often require special techniques to make the gas flow, which might include “hydraulic fracturing”, also known as “fracking”. In America, hundreds of thousands of well bores have been drilled for this unconventional oil and gas. Some people worry that the fracking or the well bores might be causing methane natural gas to rise up through the rocks and contaminate groundwater near the surface. Methane gas in groundwater can come from lots of places, such as coal seams, swamps, or rotting rubbish dumps. The gas in water might be naturally occurring in an area, and not a result of human activity. However, simply measuring the amount of gas in water is not a good way to tell where it came from, so sometimes there can be problems in finding out why it is there, how to stop it, and if anyone is to blame. This project takes samples of these deep unconventional gases in the UK, and measures a chemical ‘fingerprint’. This fingerprint relies on the chemistry of the hydrocarbon gases, as well as from noble gases such as helium and neon, which exist in tiny amounts alongside the deep gases. These fingerprints could be used later on to see if they match any gas found in groundwater, and allow us to confirm if the gas has leaked from deep down or came from somewhere else. Samples were taken from Coal Bed Methane (CBM) well bores in Central Scotland, from abandoned mines near Yorkshire in England, and from two wells drilled into shale. These were found to be rich in helium. Because helium is so light, once it is in the atmosphere it escapes into space, and so the atmosphere has a very low concentration. The high helium concentrations in deep gases compared with the low concentrations in the air mean that even very small amounts of deep gas can be traced at the surface. The chemistry of the carbon and hydrogen contained within the methane was found to be in two narrow ranges for the Scottish and the English coal gases, which is useful for characterising and distinguishing between the gases. v Measuring the chemistry of the gases helps us to learn more about how unconventional gas formed, how it is stored, and how long it was been there. vi Acknowledgements This thesis is dedicated to my parents and family for their support and patience, for which I am deeply indebted. I’m glad I can now ascribe a sense of fulfilment to the end of both a rewarding and occasionally gruelling experience. Firstly, I would like to thank my primary supervisor Dr. Stuart Gilfillan for his guidance throughout the project. Thank you also to Dr. Domokos Györe and Prof. Fin Stuart for their immersive introduction to noble gas analysis, supervision and troubleshooting when analysing samples, and relieving me from the distraction of having windows in the lab. Terry Donnelly and Dr. Pauline Gulliver at SUERC are thanked for their sampling advice and support with stable isotope and radiocarbon analyses. Permission to take samples was kindly granted by Cuadrilla Resources Ltd., DART Energy Ltd. (now part of the INEOS group), Ground Gas Solutions Ltd. and Alkane Energy Ltd. I am also grateful for the funding provided by NERC and SEPA that made this project possible, and brought me to live in the beautiful city of Edinburgh. The team at Speck and Burke are thanked for going above and beyond to help me with the very basics of gas chromatography, when I couldn’t resolve the charts from my elbow and almost lost the PLOT. So many people made the experience wonderful and unforgettable, and I hugely appreciate the help many of you have given me. Thanks to everyone for helping me move, mull over ideas, being great friends, proofreading, organising geology trips (a.k.a.