Sequence Stratigraphic and Diagenetic Alterations within the Silisiclatic Reservoir Deposits of Orange Basin, Southwestern Africa Margin.

Abstract

The Orange Basin in South Africa of South Atlantic Ocean consistently has shown poor reservoir quality. The reservoir been a silisiclastic deposit normally should have shown good to very good quality. The poor porosity and permeability characteristics of the reservoir across the five blocks in the basin necessitated this work. In finding the cause of the problems in the basin, some wells were subjected to sequence stratigraphic and diagenetic analyses. This was done using the digital wireline logs and 2D seismic sections, sidewall core and core samples acquired form Petroleum Agency of South Africa (PASA). The wireline logs were interpreted and broken to parasequence sets. The seismic section also interpreted based on unconformities, reflection truncations and terminations to obtain sequence stratigraphic framework. Core and sidewall core samples were collected within parasequence settings: Highstand HST, Lowstand LST and Transgressive system tracts to look at the effect of diagenetic alterations within the framework. The cores samples were subjected to: Thin sectioning, XRD, SEM, and EDS analyses to reveal the deposition and post deposition history of the basin. It was shown that authigenic minerals and quartz over growth is responsible for the poor reservoir quality of the reservoirs.