--> Diagenetic Alterations Within Stratigraphic Settings of Orange Basin, Southwestern Atlantic, Adekola, Adeniyi S.; Akinlua, Akinsehinwa; Fadipe, Oluwaseun A, #90100 (2009)

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Diagenetic Alterations Within Stratigraphic Settings of Orange Basin, Southwestern Atlantic

Adekola, Adeniyi S.1
 Akinlua, Akinsehinwa2
 Fadipe, Oluwaseun A.3

1Earth Sciences, University of the Western Cape, Cape Town, South Africa.
2Earth Sciences, University of the Western Cape, Bellville, South Africa.
3Earthn Sciences, University of the Western Cape, Bellville, South Africa.

The reservoir rock in the Orange Basin consistently showed poor porosity and permeability characteristics. The reservoir of the basin being siliciclsatic is not expected in real sense to be so. In unravelling the problem associated with the reservoir rock, data sets obtained from Petroleum Agency of South Africa (PASA) includes 783.63km digital seismic reflection sections (2D), digital wireline logs, base map, sidewall and core samples were obtained for the analyses. The digital seismic and wireline logs were analysed using specialized software like PETREL 2008, Kingdom 8.2, and FastTracker. Core and sidewall core samples collected from 9 wells of interest at various parasequence settings were subjected to multi-mineral and elemental analyses which include X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Electron Dispersive Spectrometry (EDS) to establish the diagenetic alterations within the settings. The XRD and SEM results within the Lowstand, Transgressive, and Highstand System Tracts (LST, TST and HST) show overgrowth of quartz and authigenic precipitated minerals such as montmorillonite which graded to chlorite in most of the settings. The EDS and XRD results in the LST show deposition of sediment in a reducing condition because of the presence of pyrite, nickel and vanadium in well K_B1 at depth 1950m suggesting marine influence. At same depth in well K_B1 there is complete replacement of aluminium by chromium. The presence of phosphorous in well A_C3 at depth 3597.5m in EDS result maybe due to secretion of the element from shell of marine invertebrate. The TST setting shows a redox environment because of the pyrite and hematite in XRD result existing in almost at equilibrium. The HST setting show a little reducing condition but with more oxidising condition because of the abundance of hematite which may be due to exposure of the sediments to the atmospheric condition. The massive overgrowth of quartz and authigenic mineral precipitation must have been responsible for the poor porosity and permeability experienced across the basin.

 

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil