A core-based approach to petrophysical flow zonation in the Southeastern Bredasdorp basin, offshore South Africa

Abstract

This study integrates petrophysical models (Flow Zone Indicator (FZI), Winland r35, and Stratigraphy Modified Lorenz Plot) with geochemical techniques to classify flow zones and assess their storage and flow capacities of clastic reservoirs in the southeastern Bredasdorp Basin, Offshore South Africa, using data from three wells. A notable aspect of this study is the linkage of petrophysical flow units with mineral composition, as determined through X-ray diffraction (XRD) and X-ray fluorescence (XRF) data. Results revealed four flow zones: moderate, low, very low, and tight. The moderate flow zone has the best rock type, with permeability ranging from 10 to 100 mD, porosity from 10 to 20%, and FZI from 3 to 5 microns. The tight flow zone exhibited the least rock quality, characterized by permeability < 1 mD, porosity < 6%, FZI < 1 micron, and r35 < 1 micron. The mineralogy results revealed the impact of mineral types on the flow zones and flow capacities. This study has shown that illite mineral content (< 1.5%), plagioclase (< 4.0%), and aluminium (< 3.5%) belong to fair and good quality reservoir rock types, petrophysical rock type (PRT), which are (PRTs 2 and 3). In contrast, illite mineral contents (> 1.5%), plagioclase (> 4.0%), and aluminium (> 3.5%) are characteristic of poor and impervious reservoir rock types (PRTs 4 and 5). Consequently, using illite, plagioclase, and aluminium content as proxies for reservoir quality extends conventional rock typing into the geochemical realm. The absence of PRT1 in our study may be due to the impact of intense diagenesis, which affects facies distribution and petrophysical properties. This combined approach is a valuable addition to standard flow unit analysis, and it is less common in earlier studies of the Bredasdorp Basin.