Petrography and stable isotope geochemistry of cretaceous sandstones, Orange Basin, South Africa

Abstract

Some siliciclastic rock samples of the Orange Basin were subjected to diagenetic and reservoir quality evaluation; 33 core and side wall core samples from 8 wells within the genetically related packages (systems tracts); lowstand systems tract, transgressive systems tract and highstand systems tract were collected for this study, to determine the prevailing conditions at deposition and post deposition of the sediments within the age bracket of Barremian to Campanian. The samples were subjected to petrographic and mineralogical analyses including bulk stable isotope analysis of oxygen and carbon. From the petrography of the samples, the basin shows complex diagenetic histories which include compaction, cementation/micritization, dissolution, overgrowth of quartz, and fracturing. The Eh–pH shows that the cements in the area of the basin under investigation were precipitated under weak acidic and slightly alkaline conditions. The δ18O isotope values of the samples range from −1.648 to 16.180 % whilst δ13C isotope values range from −25.86 to −6.95 %, which is indicative of terrestrial biomass contribution. The plot of δ18O versus δ13C shows that the sediments were deposited in shallow marine temperate conditions with some Barremian and Aptian samples more marine. There is no significant variation of δ13C isotope values across the stratigraphic sequences, which could be an indication that the burial diagenesis does not have significant effect on geochemical pattern of the δ13C isotope in the sandstones but contribution from land-derived materials. The authigenic minerals precipitated blocked the grain interspaces and interlayers and with continued burial, compaction impeded the development of secondary porosity resulting in the poor reservoir quality. The origins of the cementing materials were both autochthonous and allochthonous.