An integrated approach of determining shale gas potentiality of carbonaceous shale of the Permian Tuli Basin, Limpopo Province of South Africa

Abstract

Shale gas displays signs of future potential for energy generation. Apart from the fluctuating prices of liquid fuel and energy resources, the recent national load shedding of electricity supply is probably the most obvious sign of the energy security crisis in South Africa. To expand energy security, there is a need for an energy mix to complement existing sources. Despite extensive studies on stratigraphic, sedimentology, and coal investigations, the source rock potential of organic-rich shale for gas generation remains elusive in the Permian Tuli Basin. As a result, this research aims to investigate the shale gas potentiality of the carbonaceous shale of Madzaringwe and Mikambeni Formations in the Tuli Basin of Limpopo Province, South Africa. A total of Twenty (20) representative core samples are selected to determine the whole rock major and trace elements using X-ray fluorescence (XRF) and Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) respectively. In addition to the mineralogical characterisation using X-ray diffraction (XRD), the functional groups of organic compounds in studied samples are identified using Fourier Transform Infrared Spectrometry (FTIR). Furthermore, the porosity networks of the shale samples are determined using the Scanning Electron Microscopy-Energy Dispersive X-Ray Analysis (SEM-EDX). Lastly, the total organic carbon content (TOC), Kerogen type and thermal maturity of the studied samples to generate hydrocarbon are determined using the Rock-Eval 6 programmed pyrolysis. The major oxides results indicate a varying amount of Al2O3 (19.37–20.32%), CaO (0.20-0.22%), Fe2O3 (0.85-0.94%), K2O (1.59-1.66%), MgO (0.25-0.28%), Na2O (0.12-0.15%), P2O5 (0.04%), SiO2 (50.37-51.90%), TiO2 (0.69-0.73%) in the studied Mikambeni samples. The compositional trend of other major components is comparable, except for the lower average value of loss-on-ignition (LOI), which averages 15.04% in the Madzaringwe shale, indicating a lower TOC content. The average LOI values of Mikambeni shales (25.02%) show a higher TOC content which correspond to the actual TOC test. The elemental ratio K2O / Al2O3 (0.08) of the studied samples showed values ≤ 0.5, which suggests a moderately mature shale since a significant amount of Al2O3 is typical of immature sediments. The LA-ICP-MS analysis reveals trace elements Ba (294.16-560.88 ppm), Zn (57.46-121.63 ppm), Zr (189.02–341.72 ppm), Rb (67.69-102.26 ppm), V (81.24-156.88 ppm), Sr (92.02-344.91ppm), Cr (47.50–86.66 ppm), Pb (11.25-35.22ppm) in all studied samples. The presence of Ba suggests the dissolution of mineral barite in the black shale by the action of sulphate-reducing bacteria while Zn indicates paleo-productivity of abundant organic matter. Furthermore, Mo concentration > 2.5ppm of Mikambeni, suggests a marine sediment input in the Mikambeni samples apart from terrigenous sources in all studied samples. The interpretation of the anoxic environment is consistent with the V/(V+Ni) average value of 0.84ppm which exceeds the 0.54 limit for anoxic conditions for all studied samples. The XRD analysis shows the presence of montmorillonite, mixed illite/smectite (I/S), illite, chlorite, and non-clay minerals such as dolomite, albite, microcline, pyrite, and quartz in the studied samples. The presence of pyrite suggests activities of sulphide-reducing bacteria on parent organic matters to generate biogenic gas. The presence of illite and chlorite suggests illitisation and chloritization at greater depths of Madzaringwe samples, suggesting a high thermal alteration level for sediments. The functional group of all studied samples shows infrared absorption peaks between 2800 and 3300cm-1 wavelength attributed to an aliphatic C-H stretching vibrations. The sp2 C-H hybridization found with absorption peak between 3000-3100 cm-1 wavelength indicates the aliphatic methane gas stretching of methyl and methylene vibration. Bubble-like porosity resulting from organic matter decomposition typifies most of the studied samples, suggesting a gaseous release from organic matters. However, isolated-irregular pattern of matrix-mineral pores are displayed apart from preferred-oriented linear pores exhibited by the micro-fracture pore types in all studied samples. The EDX elemental compositions indicate intragranular grains composed of quartz, feldspars, carbonate, and pyrite minerals to form interconnected matrix. Furthermore, a non-spherical, closely packed polyframboidal-pyrite is observed, having multiple presence of Fe contents on the EDX composition. Thus, this suggests an anaerobic condition that favours organic matter degradation that initiates methanogenesis. The Rock-Eval 6 programmed pyrolysis and TOC of the studied shale samples revealed a TOC content that exceeds threshold limit of 5.0 wt %, indicating an excellent source rock in both formations. The hydrocarbons produced from the thermal breakup of kerogen (S2) ranged from 101.64 to 122.75mg HC/grock with an average value of 115.7mg HC/g rock in the Mikambeni samples. The S2 content of Madzaringwe shale ranged from 15.25 to 16.47 mg HC/g rock averaging at 1.82 mg HC/g rock. The plot of Hydrogen Index (HI) against TOC indicates a mixed kerogen TYPE II-III for the Mikambeni samples and TYPE-III for the Madzaringwe samples, which is gas prone. The maximum temperature, Tmax, corresponding to the peak of hydrocarbon yield (S2) range between 430 to 434°C, averaging at 431.8°C thus indicating a thermally immature source in the Mikambeni samples. On the other hand, the Madzaringwe shale samples yield a thermally mature condensate wet-gas, with Tmax values (464 - 470°C) averaging at 467.2°C. The studied samples have an average Productivity Index (PI) value of 0.1 which indicates a moderately mature source, generating a mixed biogenic-thermogenic gas. As such, the petroliferous indicators of the studied Mikambeni shale generates thermal immature biogenic gas while Madzaringwe shale produces a matured condensate wet-gas in the Tuli Basin.