The effects of minerals on reservoir properties in block 3A and 2C, within the orange basin, South Africa.

Abstract

The reservoir quality of the Orange Basin, offshore South Africa is known to be immensely impaired by the presence of authigenic minerals. The collective effects of burial, bioturbation, compaction and chemical reactions between rocks, fluid and organic matter conclusively determined the quality of reservoirs within the Orange basin. The aim of this study was to provide information on the quality of reservoirs within the Orange Basin. Data used to conduct this study include wireline logs (LAS format), well completion reports and core samples from potential reservoir zones of wells K-A2, K-A3 and K-E1. To accomplish the aim, petrophysical parameters were calculated, such as porosity, permeability and water saturation. Besides, depositional environments were identified using gamma ray log and core logging techniques. Thirdly, petrographic studies were supporting techniques in understanding how various minerals and diagenetic processes play a role in reservoir characterisation. Geophysical wireline logs (Gamma ray, Resistivity, Bulk density and Caliper) allowed for the estimation of the three main reservoir properties; namely: porosity, water saturation and permeability. The porosity calculations revealed a range of 3-18% for well K-A2, 2%-13% for well K-A3 and 3%-16% for well K-E1. The permeability’s ranged from 0.08-0.1 mD and 0.001-1.30 mD for K-A3 and K-E1, respectively. Thus, the findings of the petrophysical evaluation of the wells in Interactive Petrophysics indicated that the reservoir intervals of wells K-A2, K-A3 and K-E1 are of poor to good quality. Based on the core analyses, the depositional environment is mostly shallow marine, specifically tide dominated for well K-A2, sandstone channel for well K-A3 and intertidal environment for well K-E1. These environments were confirmed by XRD, revealing glauconite as the prominent mineral.