Abstract
The Cretaceous sandstone in the Bredasdorp Basin is an essential potential hydrocarbon reservoir. In spite of its importance as a reservoir, the impact of diagenesis on the reservoir quality of the sandstones is almost unknown. This study is undertaken to investigate the impact of digenesis on reservoir quality as it pertains to oil and gas production in the basin. The diagenetic characterization of the reservoir is based on XRF, XRD SEM + EDX, and petrographic studies of 106 thin sections of sandstones from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1 and E-D3 in the basin. The main diagenetic processes that have affected the reservoir quality of the sandstones are cementation by authigenic clay, carbonate and silica, growth of authigenic glauconite, dissolution of minerals and load compaction. Based on the framework grain–cement relationships, precipitation of the early calcite cement was either accompanied or followed up by the development of partial pore-lining and pore-filling clay cements, particularly illite. This clay acts as pore choking cement, which reduces porosity and permeability of the reservoir rocks. The scattered plots of porosity and permeability versus cement + clays show good inverse correlations, suggesting that the reservoir quality is mainly controlled by cementation and authigenic clays.
Highlights
Reservoir quality is one of the main controls on prospectivity during petroleum exploration.During petroleum exploration, remote sensing techniques and geophysical methods are useful in identifying the presence of sandstones
Several researchers including [3,4,5,6,7] have reported that diagenetic changes or modifications in clastic rocks have a momentous impact on reservoir quality by altering their original porosity and permeability, which in turns affects the reservoir quality
The classification of the Bredasdorp sandstones is based on the classification schemes
Summary
Remote sensing techniques and geophysical methods are useful in identifying the presence of sandstones. They do not give any information or help to locate sandstones with high porosity and permeability that have been preserved from major diagenetic changes and porosity-loss [1,2]. It is very crucial to have a thorough understanding of the factors controlling reservoir quality so as to assist with the appraisal of the economic feasibility of petroleum discoveries. A thin section petrography is a crucial and important tool used for investigating the types and timing at which the diagenetic processes have affected porosity and permeability of clastic rocks [8]. The economic importance of certain sandstone units as Minerals 2020, 10, 757; doi:10.3390/min10090757 www.mdpi.com/journal/minerals
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