Integrated Fractured Basement Play Fairway Definition

Abstract

Basement can comprise tight sediments, metamorphic rocks or igneous bodies and is generally considered unprospective. Yet the the first well drilled offshore in the Pletmos sub-basin in South Africa in 1968, Ga-A-1, discovered gas in the fractured quartzite basement of the Table Mountain Group (TMG) Formation. New understanding from modern and reprocessed seismic data, complemented by outcrop studies and potential field data analysis, has enabled the identification of this play fairway along the regional Cape Fold Belt (CFB) trend and across to its conjugate in Argentina. Prolific fractured basement reservoirs have been recognized globally, from Vietnam’s Southern Margin (Bach Ho Field, discovered in 1975) to the recent Atlantic Margin Lancaster field discovery. The application of modern seismic technology apears to be able to resolve some of the inherent complex technical issues, suggesting significant volumes of undiscovered hydrocarbons hidden by a failure to identify and intersect an active fracture system which can now be hunted down. The fractured quartzites comprising the Ga-A discovery reservoir are charged from an adjacent Jurassic lacustrine source from the Southern Pletmos sub-basin and Superior Graben . A structural closure at basement level can have upside from back-fill of the fracture system (Hurricane Energy’s Lancaster model), in addition to a down-dip oil leg to a gas cap. 2D seismic data reprocessed in 2016 were analysed and integrated with satellite-derived gravity anomaly data, to map the fractured quartzite basement play fairway in the Pletmos and other sub-basins. A revised plate reconstruction for the region has been proposed which identifies a direct continuation of the Cape Fold Belt offshore into the southern portion of the Orange Basin and further westwards, suggesting the development of a fractured basement play along the northern Argentinian margin. The fractured basement play has been overlooked globally, despite being associated with large resources and significant upside potential. The re-discovery of this play offshore South Africa is possible from a platform that integrates all available data from onshore geology, wells, seismic and potential field data. Such work has resulted in the identification of the continuation of the pre-Atlantic opening Cape Fold Belt and potential fractured basement play fairway in the Outeniqua and Orange Basins of South Africa, and the conjugate Colorado Basin of Argentina.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018