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  • Journal article
    Abdulkarim MA, Muxworthy A, Fraser A, Sims M, Cowan Aet al., 2022,

    Effect of hydrocarbon presence and properties on the magnetic signature of the reservoir sediments of the Catcher Area Development (CAD) region, UK North Sea

    , Frontiers in Earth Science, Vol: 10, Pages: 1-20, ISSN: 2296-6463

    This paper presents a detailed study investigating the effect of hydrocarbon presence on magnetic mineral diagenesis in sediments from the Catcher Area Development (CAD) region, UK North Sea, between 1,000 and 1,500 m (True Vertical Depth Sub-Sea). Magnetic analysis of core samples from hydrocarbon fields of the region and nearby dry-well sandstones (background) was carried out to determine if their signatures can serve as a proxy for understanding petroleum reservoir systems. From the background samples, nanometric and micron-sized magnetite, hematite and titano-iron oxides, were identified. Hydrocarbon presence in the reservoir sediments was found to diminish the iron-oxide signature and favour the precipitation of hexagonal pyrrhotite, siderite and potentially vivianite, lepidocrocite, greigite and paramagnetic iron sulphides. Hexagonal pyrrhotite was found at the oil-water transition zones. This relationship is possibly related to biodegradation at this interface. Siderite was found in increased abundance at shallower depths within the reservoir, which we attribute to hydrocarbon vertical migration and biodegradation. The interbedded shales also experienced significant magnetic mineral diagenesis that depended on its proximity to the hydrocarbon plume. These findings suggest that mineral magnetism can be applied to the identification of oil-water transition zones, reserve estimation, production planning and the determination of hydrocarbon migration pathways. It also suggests that mineral magnetic methods can be used to estimate the timing of hydrocarbon migration.

  • Journal article
    Roberts AP, Heslop D, Zhao X, Oda H, Egli R, Harrison RJ, Hu P, Muxworthy AR, Sato Tet al., 2022,

    Unlocking information about fine magnetic particle assemblages from first-order reversal curve diagrams: Recent advances

    , Earth-Science Reviews, Vol: 227, ISSN: 0012-8252

    The magnetic domain state of a material determines its magnetic recording capability and magnetic properties. Constraining the domain state of magnetic components within complexly mixed natural magnetic mineral assemblages is challenging because most bulk magnetic methods do not enable component-specific domain state identification. First-order reversal curve (FORC) diagrams are the most diagnostic tool for this important endeavour. Over the last 20+ years, an extensive framework has been developed for FORC diagram interpretation. Recent years have been fertile and key developments are highlighted here. New FORC measurement types provide enhanced domain state diagnosis, including recognition of vortex state signatures and their importance in rock magnetism. FORC diagrams are also indicative of the dominant magnetic anisotropy type in a material, with multi-axial, in addition to uniaxial, anisotropy signatures recognised increasingly. A fundamental challenge in FORC data processing is to avoid emphasizing noise at the expense of signal or distorting a FORC distribution by excessive smoothing. Selection of an optimal FORC distribution that avoids over- or under-smoothing is now possible with machine learning approaches. A further new FORC measurement protocol enables identification of magnetically viscous particles and can assist in separating signals due to magnetic mineral mixtures. Furthermore, FORC unmixing for large sample sets now enables quantitative separation of magnetic mineral mixtures. Splitting of the FORC signal into remanent, induced, and transient magnetization components, each of which provides information about magnetic domain state fractions in a sample, holds potential for future single sample unmixing.

  • Journal article
    Abdulkarim M, Muxworthy A, Fraser A, Neumaier M, Hu P, Cowan Aet al., 2022,

    Siderite occurrence in petroleum systems and its potential as a hydrocarbon-migration proxy: a case study of the Catcher Area Development and the Bittern area, UK North Sea

    , Journal of Petroleum Science and Engineering, ISSN: 0920-4105
  • Journal article
    Roberts AP, Zhao X, Hu P, Abrajevitch A, Chen Y-H, Harrison RJ, Heslop D, Jiang Z, Li J, Liu Q, Muxworthy A, Oda H, O'Neill H, Pillans BJ, Sato Tet al., 2021,

    Magnetic domain state and anisotropy in hematite (alpha-Fe2O3) from first-order reversal curve diagrams

    , Journal of Geophysical Research. Solid Earth, ISSN: 2169-9356
  • Journal article
    Muxworthy A, Baker E, 2021,

    ThellierCoolPy: A cooling-rate correction tool for paleointensity data

    , G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, Pages: 1-8, ISSN: 1525-2027

    We report a new approach of implementing cooling-rate corrections in absolute ancient magnetic field intensity (paleointensity) studies. Nearly all methods of determining paleointensity estimates rely on rocks having recorded a thermoremanent magnetization (TRM), on cooling from above the rock’s constituent minerals’ Curie temperature. Typically paleointensity estimates are made by comparing natural TRM, with a TRM induced in the laboratory; however, TRM intensity has long been reported to be dependent on cooling rate. Natural cooling rates are impractical in laboratories. We have developed a new cooling-rate correction method and corresponding software (ThellierCoolPy), that directly corrects the unprocessed paleointensity data, using first-order reversal curve data collected on a sister sample. This site tailored cooling-rate correction has a unique correction for each temperature step within the paleointensity data set. This new method differs from previous approaches which apply a blanket cooling-rate correction independent of the material properties of the sample. Paleointensity data from historical lavas from Parícutin, Mexico, are used to demonstrate the new software. For this data set, it is shown that cooling time of 1 million years yields a reduction of the paleointensity of ∼7%. The software is available for download.

  • Journal article
    Badejo SA, Muxworthy A, Fraser A, Stevenson G, Zhao X, Jackson Met al., 2021,

    Identification of magnetic enhancement at hydrocarbon/water contacts

    , American Association of Petroleum Geologists (AAPG) Bulletin, Vol: 105, Pages: 1973-1991, ISSN: 0149-1423

    Identifying the depths of the hydrocarbon-fluid contacts in a reservoir is important for determining hydrocarbon reserves and production planning. Using core samples from the Tay sandstone reservoir in the Central North Sea, we show that thereis a magnetic enhancement at the hydrocarbon-fluid contacts, that is detectable both through magnetic susceptibility measurements and magnetic hysteresis measurements. We observed this magnetic enhancement at both gas-oil and oil-water contacts, that have been independently identified using non-magnetic methods; we did not consider gas-water contacts in this study. We demonstrate that this magnetic enhancement is due to the precipitation of new nanometric iron oxide (magnetite) and iron sulphide (greigite)phases. The magnetic enhancement may be caused by diagenetic changes or preferential biodegradation at the top of the oil column during early filling and at the oil water contact. Our findings have the potential to be used to identify paleo-hydrocarbon-fluid contact in both structurally modified fields and failed wells. The technique can also be used to infer the fill history of a basin and calibrate petroleum systems models. Magnetic susceptibility measurements have the advantage that they can easily and quickly be measured in the field on whole core-material.

  • Journal article
    Badejo SA, Muxworthy AR, Fraser A, Neumaier M, Perkins JR, Stevenson GR, Davey Ret al., 2021,

    Using magnetic techniques to calibrate hydrocarbon migration in petroleum systems modelling: A Case Study from the Lower Tertiary, UK Central North Sea

    , Geophysical Journal International, Vol: 227, Pages: 617-631, ISSN: 0956-540X

    Magnetic minerals form or alter in the presence of hydrocarbons, making them a potential magnetic proxy for identifying hydrocarbon migration pathways. In this paper, we test this idea by magnetically measuring core samples from the Tay Fan in the Western Central Graben in the Central North Sea. In a companion paper, 3-D petroleum systems modelling has been carried out to forward model migration pathways within the Tay Fan. Rock magnetic experiments identified a range of magnetite, maghemite, iron sulphides, siderite, goethite and titanohematite, some of which are part of the background signal, and some due to the presence of hydrocarbons. Typical concentrations of the magnetic minerals were ∼10–200 ppm. Importantly, we have identified an increasing presence of authigenic iron sulphides (likely pyrite and greigite) along the identified lateral hydrocarbon migration pathway (east to west). This is likely caused by biodegradation resulting in the precipitation of iron sulphides, however, though less likely, it could alternatively be caused by mature oil generation, which subsequently travelled with the migrating oil to the traps in the west. These observations suggest mineral magnetic techniques could be a rapid alternative method for identifying the severity of biodegradation or oil maturity in core sample, which can then be used to calibrate petroleum systems models.

  • Journal article
    Badejo S, Fraser A, Neumaier M, Muxworthy A, Perkins Jet al., 2021,

    3D Petroleum Systems Modelling as an exploration tool in mature basins: A study from the Central North Sea, UK.

    , Marine and Petroleum Geology, ISSN: 0264-8172
  • Journal article
    Hu P, Oda H, Zhao X, Harrison R, Heslop D, Sato T, Muxworthy AR, Roberts APet al., 2020,

    Assessment of magnetic techniques for understanding complex mixtures of magnetite and hematite: the Inuyama red chert

    , Journal of Geophysics Research - Solid Earth
  • Journal article
    Chang L, Hong H, Bai F, Wang S, Pei Z, Paterson GA, Heslop D, Roberts AP, Huang B, Tauxe L, Muxworthy Aet al., 2020,

    Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments

    , Geophysical Journal International, ISSN: 0956-540X
  • Journal article
    Zhang Y, Muxworthy A, Jia D, Zhang Y, Chen Z, Wang M, Zhigang Let al., 2020,

    Fluid migration and widespread remagnetization in the Dabashan fold and thrust belt, China

    , Journal of Geophysical Research. Solid Earth, Vol: 125, ISSN: 2169-9356

    To better understand the fluid migration in orogenic zones and associated chemical remagnetization, we have conducted a detailed magnetic, petrographic, and strontium isotope study in an important orogenic belt of China, the Jurassic Dabashan fold and thrust belt. This belt formed by the continued collision of the North and South China blocks after the Late Triassic closure of the Paleo‐Tethys Ocean. Samples were collected in a variety of rock units of Ediacaran to Permian age, in both the thrust and the fold belts. Paleomagnetic analysis indicates that all the samples were remagnetized and carry a Middle‐Late Jurassic paleo‐direction. Rock magnetic data and scanning electron microscopy observations found that the proposed remagnetization is carried by framboidal magnetite, which likely formed by the replacement of pyrite. The pervasive nature of the chemical remagnetization in these units and belts and its temporal and spatial association with the orogeny suggest that it resulted from the alteration of orogeny‐induced fluids. Sr‐isotopic analysis of the units that are thought to be remagnetized suggests that the sediments in the thrust belt were altered by externally derived evolved fluids, whereas the Permian samples in the fold belt were altered by internal pore fluid mixing during the orogeny. Together with the lithological and structural features, we conclude that the external orogenic fluids migrated preferentially along thrust faults and unconformities but were blocked by layers of low‐permeability gypsum. Our results help to constrain the origin of widespread remagnetization in South China.

  • Journal article
    Heslop D, Roberts AP, Oda H, Zhao X, Harrison RJ, Muxworthy AR, Hu P, Sato Tet al., 2020,

    An automatic model selection‐based machine learning framework to estimate FORC distributions

    , Journal of Geophysical Research: Solid Earth, Vol: 125, Pages: 1-16, ISSN: 2169-9313

    First‐order reversal curve (FORC) distributions are a powerful diagnostic tool for characterizing and quantifying magnetization processes in fine magnetic particle systems. Estimation of FORC distributions requires the computation of the second‐order mixed derivative of noisy magnetic hysteresis data. This operation amplifies measurement noise, and for weakly magnetic systems, it can compromise estimation of a FORC distribution. Previous processing schemes, which are based typically on local polynomial regression, have been developed to smooth FORC data to suppress detrimental noise. Importantly, the smoothed FORC distribution needs to be consistent with the measurement data from which it was estimated. This can be a challenging task even for expert users, who must adjust subjectively parameters that define the form and extent of smoothing until a “satisfactory” FORC distribution is obtained. For nonexpert users, estimation of FORC distributions using inappropriate smoothing parameters can produce distorted results corrupted by processing artifacts, which can lead to spurious inferences concerning the magnetic system under investigation. We have developed a statistical machine learning framework based on a probabilistic model comparison to guide the estimation of FORC distributions. An intuitive approach is presented that reveals regions of a FORC distribution that may have been smoothed inappropriately. An associated metric can also be used to compare data preparation and local regression schemes to assess their suitability for processing a given FORC data set. Ultimately, our approach selects FORC smoothing parameters in a probabilistic fashion, which automates the derivative estimation process regardless of user expertise.

  • Journal article
    Døssing A, Riishuus MS, MacNiocaill C, Muxworthy AR, Maclennan Jet al., 2020,

    Late Miocene to late Pleistocene geomagnetic secular variation at high northern latitudes

    , Geophysical Journal International, Vol: 222, Pages: 86-102, ISSN: 0956-540X

    We report a palaeomagnetic study of Icelandic lavas of late Miocene to late Pliocene age to test the geocentric axial dipole hypothesis at high northern latitudes. Cores were sampled from 125 sites in the Fljótsdalur valley in eastern Iceland, and hand samples were taken for 17 new incremental heating 40Ar/39Ar age determinations. 96 per cent of the cores were oriented using both a Brunton compass and a sun compass. Comparison of the magnetic and sun azimuths reveals deviations of ±5°, ±10° and ±20°, respectively, for 42, 16 and 3 per cent of the data points, indicating that core sampling intended for palaeosecular variation (PSV) studies at high northern latitudes should be oriented by sun. A total of 1279 independent specimens were subjected to AF- and thermal-demagnetization for palaeodirectional analysis, and well-grouped site mean directions were obtained for 123 sites of which 113 were found to be independent sites. Applying a selection criteria of k > 50 and N ≥ 5 (Nmean = 9.5), we obtain a combined grand mean direction for 46 normal and 53 reverse (for VGPlat > ±45°) polarity sites of declination = 5.6° and inclination = 77.5° that is not significantly different from that expected from a GAD field. The corresponding palaeomagnetic pole position (VGPlat = 86.3°N, VGPlon = 21.2°E, dp/dm = 4.0°/4.3°) is coincident with the North Pole within the 95 per cent confidence limits. An updated age model is constructed based on the 40Ar/39Ar ages, showing that the majority of the Fljótsdalur lavas fall within 2–7 Ma. We combine the Fljótsdalur data with existing data from the nearby Jökuldalur valley. The 154 palaeodirections are well-dispersed between 1 and 7 Ma and constitute a high-quality data set for PSV analysis. Our results partly support previous conclusions of a generally higher dispersion during reverse polarity intervals. However

  • Journal article
    Valdez Grijalva M, Nagy L, Muxworthy A, Williams W, Roberts AP, Heslop Det al., 2020,

    Micromagnetic simulations of first-order reversal curve (FORC) diagrams of framboidal greigite

    , Geophysical Journal International, Vol: 222, Pages: 1126-1134, ISSN: 0956-540X

    Greigite is a sensitive environmental indicator and occurs commonly in nature as magnetostatically interacting framboids. Until now only the magnetic response of isolated non-interacting greigite particles have been modelled micromagnetically. We present here hysteresis and first-order reversal curve (FORC) simulations for framboidal greigite (Fe3S4), and compare results to those for isolated particles of a similar size. We demonstrate that these magnetostatic interactions alter significantly the framboid FORC response compared to isolated particles, which makes the magnetic response similar to that of much larger (multidomain) grains. We also demonstrate that framboidal signals plot in different regions of a FORC diagram, which facilitates differentiation between framboidal and isolated grain signals. Given that large greigite crystals are rarely observed in microscopy studies of natural samples, we suggest that identification of multidomain-like FORC signals in samples known to contain abundant greigite could be interpreted as evidence for framboidal greigite.

  • Journal article
    Abubakar R, Muxworthy A, Fraser A, Sephton MA, Watson JS, Heslop D, Paterson G, Southern Pet al., 2020,

    Mapping hydrocarbon charge-points in the Wessex Basin using seismic, geochemistry and mineral magnetics

    , Marine and Petroleum Geology, Vol: 111, Pages: 510-528, ISSN: 1873-4073

    This study reports a multidisciplinary approach to determining hydrocarbon charge-points and migration in the Wessex Basin, southern England. Geochemical analysis of reservoir core material (Bridport Sandstone and Inferior Oolite) using gas chromatography-mass spectrometry (GC-MS), suggests that the oil in the Wessex Basin is from a single source, and that small variations in environmentally sensitive biomarkers are likely due to small differences in maturity or depositional conditions during the formation of the oil over millions of years. Using seismic data, basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall of the Purbeck fault into the Sherwood Sandstone reservoir at Wytch Farm. One of these potential pathways is represented by cores termed Creech and the other Bushey Farm. To try to distinguish between the two potential pathways, cores were studied using mineral magnetic techniques. The magnetic signature was characterised using low-temperature (<50 K) magnetic measurements; this is because much of the magnetic signature was dominated by nanoparticles < 30 nm, which are thermally activated at room temperature and magnetically “transparent”. Wells that contained considerable amounts of hydrocarbons were dominated by nanometric magnetite (<30 nm). Such particles are small enough to migrate with the oil, through pore spaces, which are of the order ~100 nm. Wells located at the fringes of large hydrocarbon accumulation had enhanced pyrrhotite-dominated magnetic signals. Of the two potential migration pathways, the mineral magnetic results suggest that the oil migrated through Creech rather than through Bushey Farm.

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