--> Abstract: Nanopetrography, Provenance and Pore Water Geochemistry of the Lower Cretaceous Sandstone in Orange Basin, South Africa, by Oluwaseun A. Fadipe, Akinsehinwa Akinlua, Paul P. Carey, and Adeniyi S. Adekola; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Nanopetrography, Provenance and Pore Water Geochemistry of the Lower Cretaceous Sandstone in Orange Basin, South Africa

Oluwaseun A. Fadipe1; Akinsehinwa Akinlua2; Paul P. Carey1; Adeniyi S. Adekola1

(1) Earth Science, University of the Western Cape, Cape Town, South Africa.

(2) Geology Department, Obafemi Awolowo University, Ile Ife, Nigeria.

The reservoir characterization of the Lower Cretaceous sandstone Orange basin was investigated using nanopetrography, and pore water geochemistry. These sediments are continental and siliciclastic in nature. The sediments within the Orange basin may be as old as the Jurassic, but the oldest recorded sediment date back Hauterivian.

This study considered two wells within the concession with special attention to the Albian age. The methods used for the execution of this study includes the description and calibration of spot cores with conventional standard logging record responses, wireline log interpretation using sequence stratigraphy approach, petrographic (SEM, HR-TEM) and geochemical (pore water geochemistry and XRF) analyses.

High resolution transmission electronic microscopy (HR-TEM) and Scanning electronic microscopy (SEM) investigations of chlorite grain coatings from the different reservoirs zone indicate a progressive change in both texture and arrangement of chlorite and illite particles from the contact with the detrital substratum to the centre of the pore. Nanopetrographic investigations near the detrital quartz-chlorite coating interface demonstrate that inhibition of the quartz cement is due to the limitation of the epitaxial growth of quartz to the inter-particular space at the base of the chlorite coating.

Pore water analyses reveals that most of the reservoir interval spans between acidic and slightly alkaline medium, the integration of petrographic analysis with pore water data explains the cement succession within the reservoir zone mixing with acid water derived from dewatering of interbedded organic rich mudstones. The acid produced during the breakdown of this organic matters are presumed to have mixed with sandstone pore fluid due to further compaction of muds leading to reduction of initial alkalinity.

The results obtained from geochemical analysis indicate that the sandstones are Fe-rich belonging to the terrigenous class of sands (greywacke and litharenite) while chemical index of alteration (CIA) in the studied wells reveals an average CIA values of 62.23 %, and 61.65 % in both wells. The plots of SiO2/Al2O3 indicate that the reservoir sediments are within the transition zone of mature and immature nature of greywacke while the lithologic-diagenetic model of the study area rocks reflects that these clay minerals have an influence on the effective porosity and consequently on the fluid saturation.