Morphological Analysis of an Organic-rich Shale: Implication for Potential Gas Energy Generation, Witbank Coalfield, South Africa

The morphology of an organic-rich shale is crucial to understanding the macerals capable of generating potential gas in shale materials and storage structure system. Generation of potential gaseous hydrocarbon has been attributed to organic materials using proliferous approach [1, 2]. Due to the increasing demand for energy consumption, it has become imperative to identify the organic materials which are prone to generate potential gas and their subsequent storage in pore spaces of shale materials using morphological approach. This approach provides a cost effective and faster way compared to the conventional proliferous approach. The proliferous approach involves pyrolysis of the shale materials [3] using Rock-Eval analysis. As such, this study aimed to characterise the dominant macerals and pore storage system in an organic-rich shale materials from the Witbank coalfield in South Africa. The petrographic microscopy and scanning electron microscopy were used to examine macerals and pore structures of the studied organic-rich shale materials. The materials were collected from the Tuli Basin of South Africa and the petrographic investigation of the samples involved crushing to about 1mm with a minimum of fines. The particles were set in epoxy resin under vacuum (24 hours), and the blocks were subsequently ground to achieve a polished surface, as per ISO 7404 part 2, using a Struers Tegraforce polisher. The polished blocks were assessed petrographically using a Zeiss AxioImager M2M petrographic microscope at a magnification of x500 under oil immersion at the Department of Geology in the University of Johannesburg. The Fossil Hilgers system was used for the vitrinite reflectance determination (based on ISO 7404 part 3), and the system was calibrated in non-polarised reflected white light against a YAG standard (0.900 reflectances in oil of refractive index 1.518). A semi-automated point counting stage was used for the maceral group analysis (conducted in accordance with ISO 7404-4). Maceral and mineral group results are reported on a volume % (vol%) basis, and all macerals are included in the count (not determined by calculation). Microphotographs were taken to illustrate key features in each sample under white light and are not necessarily representative of the results. A Zeiss EVO MA15 scanning electron microscope (SEM) equipped with a tungsten filament and a Bruker energy dispersive X-ray (EDX) spectrometer were used for the morphological analysis. The system was operated under a high system vacuum, approximately 7.03 to 007 mbar. The electron beam was generated with an accelerating voltage of 20 kV and a probe current of approximately 2nA. The sample was sprinkled onto a carbon tape substrate and carbon-coated before being loaded into the SEM. A secondary electron detector was used to produce images at 300x, 600x, and 900x magnifications. Some of these particles are out of focus, owing to the three-dimensional depth loss of information - focusing one-part results in the other being out of focus on some of these images, particularly at the high magnifications. The maceral composition of an organic-rich shale is determined using polished blocks observed under non-polarised reflected white light. The result reveals that maceral group mainly comprised of vitrinite group as shown in Figure 1a. The vitrinite group predominates most of the studied samples (10.0 to 35.0 vol%), indicating a preponderance of organic input from vascular tissues of higher plants such as xylem, cells, and phloem tissues from the terrestrial environment. The elongated vitrinite band in Figure 1b indicates an intense thermal diagenetic alteration. Apart from the vitrinite group, Figure 1b shows the presence of liptinite group maceral (25.0 vol%) stemming from algal or bacteria precursors derived from the lacustrine or marine depositional environment. The incorporation of organic matters derived from marine environment suggests sea marine ingression caused by eustatic sea-level rise. (a) microphotographs of vitrinite (v) macerals (b) microphotographs of liptinite (L) and pyrite (p) The representative scanning electron microscopy-Energy Dispersive X-ray (SEM-EDX) images of the studied carbonaceous shales depicts a combination of organic-matter, pore structures reflecting carbonate dissolution morphologies (Figure 2). The images revealed conspicuous intragranular pores within and between the organic matter. The bubble-like pore structures located within organic matter as organic-matter pores reflect both gaseous hydrocarbon generation source and storage reservoir. SEM image of sampled shale showing organic-matter pore structures. Morphological analysis of organic-rich shale is a veritable and cost effect approach to characterize organic-rich shale which are prone to generate potential gas. It reveals predominance of vitrinite materials which can produce potential gas at suitable thermal condition. In addition, the organic-matter-type dominate the pore storage system for the potential gas [5].