--> The Qualitative Assessment of Fractured Basement Reservoir Potential in Pletmos Basin, Republic of South Africa.
[First Hit]

2018 AAPG International Conference and Exhibition

Datapages, Inc.Print this page

The Qualitative Assessment of Fractured Basement Reservoir Potential in Pletmos Basin, Republic of South Previous HitAfricaNext Hit.

Abstract

The hydrocarbon potential of naturally fractured basement reservoir in Pletmos basin has been ignored even after flow of gas to surface was proven almost 50 years ago. The aim of this study is to integrate different datasets with varying resolution and quality to assess the potential of fractured basement formed by Table Mountain quartzite of the Cape Supergroup. The Pletmos basin is located off-shore the south coast of South Previous HitAfricaNext Hit. It was formed during the Mesozoic opening of the South Atlantic Ocean. The basin is characterized by a 200-300-km-long extensional fault, the Superior Fault, which dips north and defines a tilted block, the Superior High. The successful integration of vintage data will be achieved by application of new technologies and analogues. In the Pletmos basin, the basement was intersected by four vertical exploration wells. All these wells were designed to intersect basement as a secondary target based on the interpretation of 2D seismic data. The only well that has proven hydrocarbon flow to the surface is Ga-A1. Offset wells Ga-A4 and Ga-Q2 reported strong gas peaks while drilling in the basement. The interpretation was carried out using 1130 km2 of 3D PSTM seismic data acquired and processed in 1999/2000. It is suggested that attributes analysis can be used to identify areas with increased fracture density. These areas qualitatively correlated good with the conventional logs (caliper, gamma, and sonic) that are known to indicate fracture occurrences and intensity. The combination of conventional well logs, gas readings and seismic attributes has improved confidence to identify fractures in the basement. However, without unconventional logs these results remained quantitatively uncalibrated. The core data and dip meter logs were very limited in terms of the availability and quality. Thin-sections analysis have shown that there is no primary porosity in the basement quartzite, but only secondary (fracture) porosity and therefore, gas in the basement can only be stored in the fracture networks. In the Pletmos Basin, porosity was calculated from sonic logs even though this method is known to over-estimate fractured porosity but analogues from different parts of the world of the same geology were used to constrain this parameter. The current integration study has proven that hydrocarbon potential of the basement appears encouraging and might open the close by basins to new targets.