--> --> Abstract: Reconstructing Gas Migration Pathways in the Southern Orange Basin, Offshore South Africa, Using 3d Modelling, by Gesa Kuhlmann, Rolando di Primio, Katja K. Hirsch, Dave van der Spuy, and Brian Horsfield; #90082 (2008)

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Reconstructing Gas Migration Pathways in the Southern Orange Basin, Offshore South Africa, Using 3d Modelling

Gesa Kuhlmann1, Rolando di Primio1, Katja K. Hirsch1, Dave van der Spuy2, and Brian Horsfield1
1GFZ Potsdam, Potsdam, Germany
2Petroleum Agency, Parow, Cape Town, South Africa

Gas chimneys and gas leakage features such as mud volcanoes are observed frequently along the western margin of southern Africa. In the southern Orange Basin it is thought that the observed massive gas migration is fed by an active underlying hydrocarbon system.

Here we present a 3D petroleum systems model that covers the passive margin evolution from Early Cretaceous rift initiation to the establishment of a shelf margin. The modelling is based on interpreted 2D seismic, borehole data including sedimentological and geochemical analyses as well as structural, geophysical and heat-flow data. The model implements proven source rocks of Aptian age, and an inferred Cenomanian-Turonian source. After calibration against known temperature constraints hydrocarbon generation and migration has been modelled to explain the initiation, duration and spatial distribution of gas accumulation and leakage within and throughout the sedimentary column.

The Albian source rock started hydrocarbon transformation 90 to 80 Ma ago in the centre of the basin and between 75 to 65 Ma ago towards the distal part of the basin. A Tertiary episode (<15 Ma) of enhanced transformation is related to the prograding depositional wedge at that time. The modelling results indicate highest generation rates at 75 Ma, a strong decrease up to 15 Ma followed by a slight increase up to present day west of the Cretaceous shelf break accounting for the location of present day generation. The location of gas generation in the outer basin compared to the gas leakage sites, however, implies subsequent migration towards the inner part of the basin.

Our modelling constrains the migration pathways, timing and duration of the gas leakage and gives a basin-scale quantification of thermogenic gas contributions into the hydrosphere and atmosphere as a function of geologic time.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery