Impacts of diagenetic alterations on siliciclastic sediments of the pletmos basin: implications for reservoir quality

The Pletmos sub-basin of Outeniqua Basin, South Africa hosts heterogeneous siliciclastic reservoirs sediments deposited under a complex tectonic setting and its diagenetic processes remain largely undefined. This study focuses on Cretaceous reservoir sediments, aiming to evaluates the impact of diagenetic alterations on the reservoir quality. Despite the basin’s recognition as a prolific hydrocarbon prospect, no detailed investigation into the diagenetic evolution of its reservoir rocks is available. This is the first integrated Petro-sedimentological characteristics in the basin, combining petrographic and petrophysical data derived from conventional core samples of 100% recovery and well logs data from three exploratory wells. The siliciclastic reservoirs are primarily composed of moderately to poorly sorted feldspathic litharenites and lithic arkoses sandstones deposited in fluvial environment. Two major porosity–permeability trends are identified; due to primary intergranular pore networks and secondary porosity generated by mineral dissolution. High effective porosity is attributed to the coexistence of preserved primary pores and timely development of secondary porosity, supporting enhanced fluid migration and reservoir quality. Diagenetic processes include early mechanical compaction, quartz and clay cementation, calcite and iron oxide precipitation, and extensive feldspar dissolution into kaolinite. Authigenic chlorite coatings, variations in quartz and calcite cementation are especially influential in porosity distribution. These changes reflect eogenetic and mesogenetic phases, which significantly modify petrophysical properties. The reservoir porosity ranges between 13.5 and 16.3%, primarily controlled by grain compaction, quartz, calcite cementation, and mineral dissolution. Diagenesis directly influences reservoir quality, making this study a valuable reference for reservoir assessment, exploration, and regional diagenetic comparison.