--> --> Comparing and Contrasting the Onshore and Offshore Components of the Gamtoos Basin from Petrography, Lithostartigraphy and Facies Analysis, South Coast of South Africa

2018 AAPG International Conference and Exhibition

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Comparing and Contrasting the Onshore and Offshore Components of the Gamtoos Basin from Petrography, Lithostartigraphy and Facies Analysis, South Coast of South Africa

Abstract

The Gamtoos Basin is one of the five Outeniqua sub-basins on the south coast of South Africa known for its hydrocarbon potentials. The basin is one of the only two on the south coast present both onshore and offshore with a thickness of up to 4 km. The onshore geology is divided into lithologies of the Uitanhage Group which comprises the late-Jurassic to early-Cretaceous Enon Formation and the Early to mid-Cretaceous Kirkwood Formation. The Enon Formation comprises massive red-grey conglomerate units at the base overlaid by alternating layers of red, grey and white sandstones, siltstones and mudstones of up to 0.48 km in thickness. The overlaying Kirkwood Formation comprises red-white alternating layers of sandstones, siltstones and mudstones at the base and massive grey-red layers of conglomerates at the top adding up to 2.2 km in thickness.The Enon Formation shows a general upward fining lithostratigraphic sequence whereas the Kirkwood Formation shows a general upward coarsening lithostratigraphic succession. The following facies were encountered across the entire basin: dark grey thin bedded and laminated sandstones, light grey massive bedded sandstones, light grey wavy laminated sandstones, light grey to white massive bedded mudstones and red to grey convolute bedded sandstones. The following facies were observed onshore: red massive bedded mudstones, red wavy laminated mudstones, red tabular cross-bedded sandstones, grey lenticular sandstones and grey to red massive bedded conglomerates. The following facies were observed offshore: grey intraclastic sandstones, dark grey massive bedded mudstones, grey bioturbated sandstones, grey bioclastic sandstones and grey fine grained pelletic mudstones. Petrographic analyses of both onshore and offshore samples revealed the presence of organic intraclasts from the following rock types: fine grained siltstones, fine grained mudstones, lithic arenites, quartz arenites, quartz wackes and pelletic wackes. It also showed the presence of asphalt from fine grained siltstones, grey intraclastic sandstones, grey thinly bedded and laminated sandstones and grey massive sandstone facies (occasionally). Asphalt, organic carbon rich micro-fractures, coal lenses, macroscopic organic carbon rich intraclasts, glauconite and bioturbation were only observed offshore. Red lithologies, massive conglomerates, load casts and lenticular sandstones were only observed onshore.