PROVENANCE, DIAGENESIS AND RESERVOIR QUALITY OF THE LOWER CRETACEOUS SANDSTONE OF THE ORANGE BASIN, SOUTH AFRICA

摘要

The Orange Basin, offshore South Africa, formed as a result of Gondwana break-up and rifting during the late Jurassic to early Cretaceous periods followed by drifting apart of the African and South American plates. A number of wells have been drilled over the last three decades in the Orange Basin, focussed primarily in water depths of less than 500m with gas/condensate and one oil discovery in the syn-rift succession illustrating a working petroleum system. Lower Cretaceous (Albian age) sandstones of the Orange Basin conformably overlie Aptian age sedimentary rocks including potential source rocks for Albian reservoirs, although well testing has yet to demonstrate commercial volumes of hydrocarbons. This work examines sandstones sampled from well A-W1 and attempts to clarify sediment provenance and post-depositional diagenetic modification of these potential reservoir sands through a combination of analytical techniques including thin section petrographic characterization, X-ray diffractometry (XRD), X-ray fluorescence (XRF) analyses (major element analysis after Al-Harbi and Khan, 2008), high-resolution transmission electron microscopy (HR-TEM) and scanning electron microscopy (SEM). The data generated from these analytical techniques have been utilized to interpret the diagenetic and geochemical variation and development of the Albian sands of this well as characteristic of the potential play area. Albian age sandstones have a detrital mineralogy dominated by quartz, K-feldspar and mica and range from greywacke to litharenites. Diagenetic modifications include feldspar and lithic fragment dissolution, compaction and reduction of the depositional porosity through grain rearrangements, rotation and fragmentation of grains and cementation (primarily quartz and carbonate). Relatively early formation of authigenic chlorite, followed quartz cementation occurred in an environment where the movement of formation waters was relatively unrestricted.