Quantitative analysis of the permian sediments of Karoo Basin of South Africa using combined petrophysical and x-ray analytical techniques

Abstract

This study quantifies the petrophysical parameters and determines the impact of mineralogy on the petrophysical parameters of the Permian sediments of the main Karoo Basin, South Africa. Conventional well logs from KDD-01 well, thirty-two and forty-two samples are investigated for mineral and composition analyses using X-ray diffraction and fluorescent techniques, respectively. Three intervals of Abrahamskraal Formation (500–513m), dolerite sill that intruded the Abrahamskraal Formation (513–530m), and Waterford Formation (1014–1048m) are investigated. The results show that the average porosity of the Abrahamskraal Formation is 2.9%, the dolerite interval has an average value of 1.8%, whereas the Waterford Formation has an average value of 2.1%. The average permeability for the Abrahmaskraal Formation is 0.05mD, dolerite 0.04mD, and the Waterford Formation is 0.08mD, respectively. No hydrocarbon was detected. The low values of petrophysical properties may be associated with the pore network, which is poorly connected, intense precipitation of secondary minerals, and pore infilling during burial diagenesis. Quartz mineral dominates in the Abrahamskraal Formation, with an average value of 47% and 61% in the Waterford Formation.In comparison, plagioclase is the main mineral constituent in the dolerite, with an average of 44%. Based on the combined petrophysical and geochemistry classification scheme, the formations evaluated belong to the type III pore structure. The type III pore structure is a poor and ineffective reservoir, with rock porosity usually less than 6% and a permeability of less than 0.15mD. However, regarding the mineral elemental ratio, the Abrahamskraal and the Waterford Formations meet the type II pore structure criteria, while the dolerite is type III. This paper has produced a helpful rock classification scheme based in the Karoo Basin, South Africa, using petrophysical and geochemical data for the first time.