The fate of mercury (Hg) during thermal maturation of sediments and its implications for interpreting the geological record

How does Hg behave during thermal maturation of sediments (heat-driven reactions that alter the composition of organic matter, e.g., conversion of sedimentary organic matter to petroleum or cracking of oil to gas) and what are the implications in terms of Hg mobility in sediment cores.

 Aims of the Project

To understand the effects of sediment thermal maturation on potential Hg fingerprints for large igneous volcanism in the sedimentary record and make predictions about which hydrocarbon reservoirs are most likely to contain high Hg concentrations.

Project Description

Volcanic activity associated with the emplacement of Large Igneous Provinces (LIPs) has been linked to the majority of Phanerozoic extinctions and episodes of major environmental change. Recent work has shown the potential of enhanced mercury (Hg) in organic-rich sedimentary archives to record the fingerprints of particularly subaerial LIP volcanism and to investigate links with environmental change events in the geological record (e.g., Sanei et al., 2012; Percival et al., 2015). However, there remain many open questions regarding the application of this proxy. For example, little is known about how the Hg contents of different types of sediments are influenced by varying types of organic matter (e.g. terrestrial vs marine OM) and degrees of thermal maturity. Understanding these processes is important in terms of unlocking the potential of the Hg proxy for LIP volcanism in the sedimentary record but also has direct implications for the petroleum industry where Hg is considered a contaminant in hydrocarbon fields where it is co-produced with hydrocarbons in highly varying concentrations. Knowledge about the presence and level of Hg in these hydrocarbon streams is important because it can determine, amongst other things, decisions of processing facility design (e.g., the inclusion of costly removal units) to remove Hg pollution.

This project will take a 2-fold approach to investigate this problem: firstly, making new measurements on cores from the same basins, covering the same depositional period but displaying different thermal maturities; secondly, developing pyrolysis experiments tracking the effects of sediment maturation on Hg in the laboratory.

Sediment measurements: Via collaboration with industry partners (Shell co-supervisor and Erdem Idiz) we have access to 3 cores (kerogen Type I/II) from the Lower Saxony Basin, Germany, that sample the same stratigraphy (the Lower Toarcian Posidonia Shale, but have been buried to varying depths and exhibit different thermal maturities. These cores all cover the interval of the Toarcian Oceanic Anoxic Event (T-OAE), shown to be associated with a global Hg excursion (Percival et al., 2015). The student will make Hg measurements (Lumex analyzer) on all 3 cores to understand the evolution of the Hg signal with respect to thermal maturity of these sediments. Hg measurements will be made on bulk core, extracted core residues and the soluble (extractible) organic fractions. In addition, the student will have access to advanced analytical techniques such as FTIR and FT-ICR-MS to characterize the nature of organic-Hg ligands in OM-soluble fractions during their placement with Shell. These measurements will be complemented by similar measurements from Cenomanian–Turonian cores from the Eagle Ford Formation, Texas, USA, deposited in the Maverick Basin in the Western Interior Seaway, which record Oceanic Anoxic Event 2. Scaife et al. (2017) suggest the presence of a small Hg/TOC excursion. These cores display relatively low TOC during the OAE interval and are dominated by kerogen Type III, making an illuminating contrast to the high-TOC, Type I/II-dominated black shales of the T-OAE from Europe. Other OAE 2 targets are possible: Tarfaya, Demerara Rise (IODP core), Bonarelli Level of central Italy, Black Band of Lincolnshire, UK.

Significant contextual work (detailed sedimentological characterization, mineralogy, XRF, major/trace elements, stable Mo, U, Cd and Zn isotopes (responsive to sulphide burial), TOC, organic/inorganic C-isotope stratigraphy, Rock-Eval data, extract analysis – including biomarkers/organic petrographic characterisation of in situ OM) exists for all these cores (Ruhl et al., in prep., Dickson et al., in prep., Ricardo Celestino, Exeter PhD thesis), making them ideal targets to study Hg behaviour during sediment maturation.

Pyrolysis experiments: Rock sample aliquots (~2 g) will be sealed in glass tubes and heated at 200, 300, 400, 500 °C for each of 2 days, 1 week and 1 month. Both anhydrous and hydrous runs will be undertaken. Hg measurements will be made of both the starting samples and the pyrolysis products. The latter will be separated into the bitumen fraction (soluble OM) and kerogen (insoluble residual OM) by Soxhlet extraction for 48 h with 3:1 dichloromethane/methanol (e.g., Rooney et al., 2012; Alex Dickson innovated this methodology while in Oxford, now at Royal Holloway he will be a key collaborator). Starting materials must be thermally immature and contain measurable Hg. Candidate samples will include thermally immature Lower Saxony core organic-rich shales, other suitable sections also covering the T-OAE (e.g., Sancerre, France), the Eagle Ford sections and Hg-rich coal and shales. The student will measure indicators of thermal maturity and other sedimentary characteristics where they are not already known. They will employ similar analytical techniques as those applied to the Lower Saxony cores to characterize the nature of organic-Hg ligands. This pyrolysis methodology will assess the metal content and nature of binding of Hg by organic matter by progressively cracking the kerogen into extractable lower molecular weight components, thereby avoiding the harsh acid demineralization techniques normally employed.

These studies will improve our understanding of the Hg proxy for LIP volcanism and why some global hydrocarbon reservoirs are more Hg-rich than others, contributing to the body of knowledge empowering cleaner energy production.

Methods to be used

Measurements of sediment cores and pyrolysis experiments. Some travel to field sites and core repositories for sampling will be involved.

Specialised skills the student will need

Some chemistry/geochemical laboratory experience is an advantage

Please contact Tamsin Mather tamsin.mather@earth.ox.ac.uk

References

Rooney, A.D., Selby, D., Lewan, M.D., Lillis, P.G., Houzay, J.-P. (2012) Evaluating Re–Os systematics in organic-rich sedimentary rocks in response to petroleum generation using hydrous pyrolysis experiments, Geochimica et Cosmochimica Acta 77, 275–291.

Sanei, H., Grasby, S., Beauchamp, B. (2012) Latest Permian mercury anomalies, Geology 40, 63–66.

Scaife, J.D., Ruhl, M., Dickson, A.J., Mather, T.A., Jenkyns, H.C., Percival, L.M.E., Hesselbo, S.P., Cartwright, J., Eldrett, J.S., Bergman, S.C., Minisini, D. (2017) Sedimentary mercury enrichments as a marker for submarine Large Igneous Province volcanism: evidence from the Mid-Cenomanian Event and Oceanic Anoxic Event 2 (Late Cretaceous), Geochemistry Geophysics Geosystems 18, 4253-4275.

Percival, L.M.E., Witt, M.L.I., Mather, T.A., Hermoso, M., Jenkyns, H.C., Hesselbo, S.P., Al-Suwaidi, A.H., Storm, M.S., Xu, W., Ruhl, M. (2015) Globally enhanced mercury deposition during the end-Pliensbachian extinction and Toarcian OAE: A link to the Karoo-Ferrar Large Igneous Province, Earth and Planetary Science Letters 428, 267-280.

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