Variation of the coal stratigraphy and characterization of the Soutpansberg Coalfield, Limpopo Province, South Africa

Abstract

The future of energy in South Africa depends on coal as it is one of the cheapest and most affordable sources of energy; however, some of the coal is uneconomical to mine due to the thickness and depth of the coal seams. For many years the coal resources of the Soutpansberg Coalfield remained untapped and limitedly researched and with the coal resources running out in the other coalfields, the Soutpansberg Coalfield remained the bright coalfield of tomorrow in South Africa. Coal seams in the Soutpansberg Coalfield occur within the Madzaringwe Formation. Three coal basins have been identified in this coalfield, are the Venda-Pafuri, Tshipise and Mopane. Sedimentological basin analysis of the coal stratigraphy and characterisation of the Soutpansberg Coalfield in the Makhado area, Limpopo Province was investigated within the farms of Rissik, Fannie, Duel, and Lurkin. The main purpose of the study was to establish the variation of the coal stratigraphy and how coal influenced the stratigraphy within the area. Fieldwork involved core logging and core sampling from the different farms within the Makhado coal area, while laboratory work included petrographic studies, investigation of the physical and chemical properties of coal. Core logging revealed the occurrence of coal zones where coal seams were intercalating with mudstones, but rarely with siltstone that formed the footwall of coal. Rarely was coal intruded by dolerite dyke as was the case along borehole W6610001. Core logging further revealed the sedimentary structures in the lithologies and the depth at which different lithologies were intersected. From these sedimentary structures, the environment of coal deposition was deduced supported by the geochemical analysis of major oxides and trace elements. Correlation of boreholes along the strike showed that the shale and mudrock were the predominant rocks within the coal horizon leading to the conclusion that these were the coal host rocks. Whole rock geochemical analysis was undertaken, using X-Ray Fluorescence Spectroscopy in order to establish the rock types and their trace element contents. The collected samples were analysed at the University of Venda, Department of Mining and Environmental Geology Laboratory, Siza Coal Services in Secunda and Council for Geoscience. The study indicated that the coal seams of the Soutpansberg Coalfield were deposited within the floodplain of a mixed-load fluvio-deltaic (fluvial and braided) systems. This sedimentary channel has been the major influence on the development of the coal seams. Locational changes in sedimentary facies above and below the coal seams within the study area has caused variations in the rates of compaction and subsidence which influenced the coal basin morphology and the coalification pattern. These two parameters (variations in the rates of compaction and subsidence) controlled the coal quality parameters, and coal seam thickness as well as the coal composition. The study confirmed that coal quality and thickness vary markedly from place to place in this coalfield due to varying local depositional environments. Most of the drill holes intersected mainly 3 coal seams, although in some cases either 2 or 5 seams were intersected. The thickest coal seam (borehole F578002) was 8 m. It was concluded that the coal was sub-bituminous to bituminous coal rank class (medium-volatile bituminous coal rank class). The coal had low moisture content ranging from 0.7-0.8%, and ash content ranging from 21.4-32%. The fixed carbon and volatile matter values of the coal samples ranged from 42.5 to 50.4% and from 25.2 to 27.4% respectively. The carbon and hydrogen were the main principal combustible elements in coal, however; carbon is the predominant one based on weight, constituting about 5.3% (the lowest) to 70.3% (highest) of the total. Due to the nature and thickness of the coal seams we conclude that this coal was economical to be mined and can be used for electricity generation and in cement grinding plants. A graph of coal gas concentrations over combustion time showed that methane (CH4) and carbon dioxide (CO2) had high gas concentrations, amounting to 1.75% and 1.70% respectively.