Sedimentary organic matter characterization of the Whitehill shales (Karoo Basin, South Africa): an integrated quantitative approach using FE-EPMA and LA-ICP-MS

Gelu Costin, Annette E. Götz, Katrin Ruckwied

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    Abstract

    Permian black shales of South Africa's Karoo Basin have been classified as potential unconventional gas resources, the Whitehill Formation of the southern basin parts being the main target for future shale gas exploration and production. Here, we present a novel approach of SOM characterization integrating routine palynofacies analysis and high-resolution BSE imaging, modal mineral analysis, quantitative carbon analysis, EPMA trace element analysis, WDS mapping of carbon particles and carbon peak shift method using field emission microprobe as well as LA-ICP-MS trace element analysis. Black shales of the Whitehill Formation intersected in two deep boreholes of the southern Karoo Basin were studied: (1) massive carbonaceous siltstone (borehole KZF-1, southwestern basin) and (2) laminated shale made of alternating silty and clayey–silty laminae (borehole KWV-1, southeastern basin). Palynofacies indicates an outer shelf setting in the southwestern part (KZF-1) and a stratified basin in the southeastern part (KWV-1). The two shale types reveal striking differences in mineralogy, major and trace element concentrations, and shape and texture of organic particles. The EPMA trace element data of SOM show that the concentration of the elements in the organic particles does not correlate with the chemistry of the black shale when SOM is thermally untransformed. However, in the case of thermally affected and mobilized SOM, the organic acid-rich fluids liberated during the transformation and deformation of SOM can differentially dissolve chemical elements from the surrounding minerals. The LA-ICP-MS data show that the Whitehill shales are enriched in S, Mn, Mo, Ag, Cd, Re, Bi and U relative to the standard marine mud MAG-1. Shales from the southeastern basin (KWV-1) are strongly depleted in Cu, Zn, As and Ba. The granulometric and mineralogical properties of the shales suggest an outer shelf setting with low-energy sediment fallout and sediment reworking, consistent with the paleoenvironmental interpretation based on palynofacies analysis. Different burial depths are inferred from different organic textures: KZF-1 shales show organic particles which preserve organic textures, randomly distributed throughout the siltstone, whereas KWV-1 shales show deformed, thermally transformed and mobilized organic particles, located in between distinct laminae. WDS quantitative element mapping allows for identifying various carbon concentrations within the organic particles. This variability of carbon at grain scale may indicate different carbon–hydrogen molecule speciations. The observed peak shift of carbon in EPMA on different textural types of SOM allowed a correlation between the peak shift and the degree of SOM maturation, indicating that this method potentially works as a vitrinite reflectance proxy. Ultimately, the evaluation of the diagenetic conditions of organic-rich shales from different parts of the basin based on textural criteria of the detrital material and organic particles combined with the carbon peak shift in different organic particles contributes to the assessment of the hydrocarbon potential.
    Original languageEnglish
    Pages (from-to)29-42
    Number of pages14
    JournalReview of Palaeobotany and Palynology
    Volume268
    Early online date29 May 2019
    DOIs
    Publication statusPublished - Sep 2019

    Keywords

    • FE-EPMA
    • LA-ICP-MS
    • SOM
    • black shales
    • Karoo Basin
    • South Africa

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