Abstract

This study uses a 3D petroleum system modelling approach to investigate the links between hydrocarbon generation and migration with observed gas chimneys and gas leakage features along the western margin of offshore South Africa. In addition we have investigated the impact and timing of mass failures and related sediment mass movement on the petroleum system and hydrocarbon maturation history within the southern Orange Basin.

Our 3D-model covers this passive margin’s evolution from Early Cretaceous drift initiation to the establishment of a present day continental margin, extending from the shelf to the deep marine domain. The model is based on interpreted 2D seismic profiles and borehole data including sedimentological and geochemical analyses as well as heat flow data. The model includes a proven source rock of Aptian/Albian age, and a second assumed Cenomanian-Turonian source. Several heat flow and uplift-erosion scenarios have been tested with the model to assess consistency with calibration data and present day surface heat flow values. After calibration against known well temperatures and vitrinite reflectance hydrocarbon generation and migration have been modelled to investigate the initiation, duration and spatial distribution of petroleum accumulation and leakage within and throughout the sedimentary column.

The main sedimentary depocentres of the Orange Basin developed from east to west, with a siliciclastic basin infill and aggradation during the Cretaceous and westward progradation during the Late Cenozoic. A Late Cretaceous episode of margin instability occurred in the north western part of the study area followed by a second phase of Late Cenozoic mass movement in the south-western part of the study area. At the location of the Cretaceous mass failure the increase in sediment load profoundly affected the underlying source rocks seen in modelled maturation and petroleum generation potential. Albian source rocks started hydrocarbon transformation 100 to 85 Ma ago (phases I to III) in the centre of the basin and between 75 to 65 Ma ago (phase IV and VI) towards the distal part of the basin. The highest generation rates occurred at 75 Ma, followed by a rapid decrease until 15 Ma and a slight increase in generation potential until present day (phase V) caused by enhanced Cenozoic sediment load west of the Cretaceous shelf break. This recent increase accounts for the location of present day gas generation, though it does not substantially affect the overall regional maturation history.

Today’s gas leakage features that have been observed in the shelf area (<400 m water depth) east of the slope failures are fed by an active hydrocarbon system. Compared to these gas leakage sites, the location of gas generation in the outer basin implies subsequent migration of the fluids towards the near-shore part of the basin. Our modelling constrains the migration pathways, timing and duration of the gas leakage and gives a basin-scale quantification of episodes with enhanced thermogenic gas generation and leakage.

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