Assessing low-maturity organic matter in shales using Raman spectroscopy : effects of sample preparation and operating procedure

Laser Raman spectroscopy is used to assess the thermal maturity of organic matter in sedimentary rocks, particularly organic-rich mudstones. However, discrepancies exist between quantified Raman spectral parameters and maturity values obtained by vitrinite reflectance. This has prevented the adoption of a standard protocol for the determination of thermal maturity of organic matter (OM) by Raman spectroscopy. We have examined the factors influencing the Raman spectra obtained from low-maturity OM in potential shale gas reservoir rocks. The inconsistencies in Raman results obtained are due to three main factors that are critically evaluated: (1) different operational procedures, including experiment setup and spectral processing methods; (2) different methods of sample preparation; (3) the analysis of diverse types of OM. These factors are scrutinized to determine the sources of inconsistency and potential bias in Raman results, and guidance is offered on the development of robust and reproducible analytical protocols. We present two new Raman parameters for un-deconvolved spectra named the DA1/GA ratio (area ratio of 1100–1400 cm−1/1550–1650 cm−1) and SSA (scaled spectrum area: sum of total area between 1100 and 1700 cm−1) that offer potential maturity proxies. An automated spreadsheet procedure is presented that processes raw Raman spectra and calculates several of the most commonly used Raman parameters, including the two new variables

Linking Redox Processes and Black Shale Resource Potential

Black shales, such as the Mississippian (~330 Ma) Bowland Shale Formation, are targets for unconventional hydrocarbon exploration in the UK and in equivalents across Europe. Despite this interest, global decarbonisation, by definition, will either require; (1) complete replacement of natural gas with renewables and nuclear power generation, or; (2) moderate to limited natural gas use globally or locally, for example as a ‘bridge fuel’, as a source for hydrogen via steam reformation, or coupled to carbon capture and storage (CCS) technology. Any of these scenarios will increase the demand for transition metals such as V, Co and Ni, key elements used for energy storage and as catalysts in steam reformation. Black shales in general can host ore-grade enrichments in these metals, although the exact resource potential of UK Mississippian black shales remains unresolved. We integrate comprehensive sedimentological and geochemical data from three sections through the Bowland Shale in the Craven Basin (Lancashire, UK) to explore the links between controls on hydrocarbon and metal prospectivity. The Bowland Shale at these sites is a highly heterogeneous and complex ~120 m thick succession comprising carbonate-rich, siliceous and siliciclastic, argillaceous mudstones. These sedimentary facies developed in response to a combination of high-frequency (~111 kyr) sea level changes, fault activity at the basin margins and linkage with the nearby prograding Pendle delta system. Palaeoredox proxies such as Fe-speciation, redox-sensitive trace elements and S isotope analysis from extracted pyrite (δ34Spy) demonstrate intervals associated with metal enrichment were deposited under anoxic and at least intermittently euxinic (sulphidic) bottom water conditions. Trace element enrichment ‘V scores’ (sum of V+Mo+Se+Ni+Zn in ppm) indicate the greatest enrichments in these key transition metals and non-metals are associated with deposition under strongly sulphidic conditions during marine transgressions. V scores in these intervals are often >400 ppm and sometimes >1000 ppm. These bulk enrichments are comparable to stratiform low-grade ores such as the Upper Mudstone Member of the Devonian Popovich Formation (Nevada, USA). Hosts for these metals likely include solid sulphides such as pyrite, organic matter and possibly phosphates or carbonates. Critically, a process of switching between ferruginous and euxinic conditions in anoxic porewaters, termed ‘redox oscillation’, is recognised by a distinctive redox-sensitive trace element enrichment pattern, particularly competition between V and Ni metalation. Redox oscillation operated during periods of reduced sea level, where an increased supply of reactive Fe to the basin promoted development of intermittently ferruginous conditions in bottom waters and early diagenetic porewaters. Therefore the distribution of many redox-sensitive elements through the Bowland Shale is predictable. If these elements can be efficiently extracted from the mineral or organic hosts, UK Mississippian black shales may represent a significant resource. This work also improves understanding of the potential for co-extraction of metals during hydraulic fracturing, or during remediation of waste water. Future work will seek to understand which minerals or organic compounds host these redox-sensitive trace elements

Text mining reveals ocean redox events

Text mining reveals ocean redox events J. Emmings1, 2*, J. Walsh1, D. Condon1, I. Ross3, S. Poulton4 1 British Geological Survey, Keyworth, Nottingham, UK 2 School of Geography, Geology and the Environment, University of Leicester, Leicester, UK 3 Center for High Throughput Computing, University of Wisconsin-Madison, Madison, WI, USA 4 School of Earth and Environment, University of Leeds, Leeds, UK The redox state of the oceans exerted a key control the evolution and diversity of life (e.g., Anbar, 2008) and the distribution of black shale resources through time. Here we implement the GeoDeepDive digital library and machine reading system (https://geodeepdive.org/) in order to delineate ocean redox events through geological time. At time of analysis, the GeoDeepDive library contained 10,661,918 published documents, including most content from publishers such as Elsevier and Wiley. We executed an algorithm in order to decompose sentences into speech and linguistic components using Stanford natural language processing (CoreNLP; Manning et al., 2014). This algorithm is used to define the changing proportion of pyrite-bearing rocks through geological time (sensu. Peters et al., 2017). 838 documents in the GeoDeepDive library contain target phases such as ‘pyrite concretions’, ‘pyrite nodules’ and ‘pyrite framboids’. 1154 target phrases were linked via an application programming interface (API) to 191 stratigraphic packages recorded in the Macrostrat database (Peters, 2006; Peters and Husson, 2018). Stratigraphic packages in North America, the Caribbean, New Zealand, the deep sea and parts of Central and South America are subdivided into ‘units’ containing lithological and environment of deposition attributes. Stratigraphic data also derive from the British Geological Survey and Geoscience Australia stratigraphic lexicons. Pyrite abundance is defined as the proportion of concretionary/nodular or framboidal pyrite-bearing lithostratigraphic packages. An alternative assessment, limited to the marine and sedimentary unit record, tested for potential bias due to changing rock type abundance through time. Manual assessment shows most pyrite mentions derive from the sedimentary rock record. Thus pyrite types are interpreted primarily as a proxy for bottom-water and/or pore-water conditions. Concretionary and nodular pyrite precipitate under advective or stagnant (poly)sulfidic conditions. Pyrite framboids precipitate from fluids supersaturated in reduced sulfur, a reaction that is catalysed by sulfate-reducing bacteria in the marine environment (e.g., Rickard, 2012). The pyrite record delineates the widely recognised key redox events, such as; the Great Oxidation Event (Holland, 2002); onset of ferruginous global ocean conditions at the start of the Neoproterozoic (Canfield et al., 2008), and; Phanerozoic ‘ocean anoxic events’ (OAEs), for example during the Permian-Triassic transition (Wignall and Twitchett, 1996) and Early Toarcian (Jurassic) OAE (Jenkyns, 2010). The ratio of pyrite concretion/nodule-bearing rocks versus framboid-bearing rocks may delineate fundamental changes to element cycling (iron, sulfur, redox-sensitive trace metals) in the marine environment. This is important for understanding hydrocarbon and mineral systems through geological time. Keywords: pyrite; shale; redox; machine; reading; GeoDeepDive; Macrostrat References Anbar, A.D. (2008) Elements and Evolution. Science 322, 1481-1483. Canfield, D.E., Poulton, S.W., Knoll, A.H., Narbonne, G.M., Ross, G., Goldberg, T. and Strauss, H. (2008) Ferruginous Conditions Dominated Later Neoproterozoic Deep-Water Chemistry. Science 321, 949-952. Holland, H.D. (2002) Volcanic gases, black smokers, and the great oxidation event. Geochimica et Cosmochimica Acta 66, 3811-3826. Jenkyns, H.C. (2010) Geochemistry of oceanic anoxic events. Geochemistry, Geophysics, Geosystems 11. Manning, C.D., Surdeanu, M., Bauer, J., Finkel, J., Bethard, S.J. and McClosky, D. (2014) The Stanford CoreNLP natural language processing toolkit, Proceedings of the 52nd Annual Meeting of the Association for Computational Linguistics, pp. 55-60. Peters, S.E. (2006) Macrostratigraphy of North America. The Journal of Geology 114, 391-412. Peters, S.E. and Husson, J. (2018) We need a global comprehensive stratigraphic database: here’s a start. The Sedimentary Record 16, 4-9. Peters, S.E., Husson, J.M. and Wilcots, J. (2017) The rise and fall of stromatolites in shallow marine environments. Geology 45, 487-490. Rickard, D. (2012) Chapter 6 - Sedimentary Pyrite, in: David, R. (Ed.), Developments in Sedimentology. Elsevier, pp. 233-285. Wignall, P.B. and Twitchett, R.J. (1996) Oceanic Anoxia and the End Permian Mass Extinction. Science 272, 1155-1158