Lithostratigraphic modelling and framework of the Karoo coal and non-bearing strata in the Waterberg coalfield, South Africa

Abstract

The Waterberg Coalfield is located 340 km in the north-western part of South Africa, in Limpopo Province, north-west of Johannesburg, north of the Waterberg Mountains. It extends westwards into Botswana, joining the Kalahari Karoo Basin. The study involved 3D lithostratigraphic facies modelling, interpretations of coal seam geometry, structural interpretations, and coal and non-coal seam grid analyses (thickness and quality) at the Waterberg Coalfield. This approach presents additional information on the characteristics of the coalfield, supporting findings and theories proposed in previous research work, by bringing three dimensional visualization and interpretation. Sparsely distributed boreholes (757), comprising lithological units, defined coal and non-coal zones with laboratory sample analyses were used as the building blocks of the 3D lithostratigraphic facies model using GEOVIA MinexTM 6.1.1 geological modelling software. The coal analyses were displayed and processed in ESRI® ArcGISTM 10.2 software. Using these industry-recognised software applications, processing, interpretation, and presentation of the geological data was achievable. Coal and non-coal zone thicknesses, quality distributions, cross-sections depicting geometries of the sedimentary strata, and stratigraphy of the Waterberg Coalfield were created and interpreted. The bituminous coal analysed is highly shaley and of a low grade; with raw qualities on weighted average - Calorific Value: 9.77MJ/kg; Inherent Moisture: 2.06%; Ash: 45.63%; Volatile Matter: 21.59%; Fixed Carbon: 25.49%; Total Sulphur: 0.93% and Phosphorus: 0.02%. The model reveals that the Waterberg Coalfield experienced more structural disturbance than initially postulated. Post depositional, delineated upthrow and downthrow faults 1 to 7, occurring much deeper into the Waterberg Coalfield, were identified from the cross-sections and isopach maps. Multiple step faults and folds occur throughout the coalfield, subdividing it into deeper central blocks and shallower outer edge blocks.