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 d 18 O isotope values of the samples range from-1.648 to 16.180 % whilst d 13 C isotope values range from-25.86 to-6.95 %, which is indicative of terrestrial biomass contribution. The plot of d 18 O versus d 13 C 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 d 13 C 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 d 13 C 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.

Shale-gas potential from Cretaceous succession in South Africa’s Orange Basin: Insights from integrated geochemical evaluations

“…Kudu gas field was explored for more likely prospects in 1974 (Brown 1997). In 1997, Oribi plus Oryz fields produced fewer than 5,000 barrels per day for domestic consumption (Adekola and Akinlua 2013). The AK2 well yields 30 million ft 3 gas with some 600 barrels of condensate at a flow rate of 2200 psi (Berge et al 2002).…”