Application of an Integrated 3D Crust-Scale Model to Constrain Petroleum System Evolution in the Western Bredasdorp Basin (Southern Offshore of South Africa)

Sonibare, Wasiu A.¹; di Primio, Rolando; Anka, Zahie; Scheck-Wenderoth, Magdalena; and Mikes, Daniel
¹[email protected]

The southern South Africa documents a complex margin system that has undergone both continental rifting and dextral strike-slip movements. Here, we focus on the Western Bredasdorp Basin constituting the proximal section of the larger Bredasdorp sub-basin, which is the westernmost of the five southern South Africa offshore Mesozoic sub-basins. Associated lithospheric thinning and break-up processes as well as the subsequent thermal relaxation resulted in the ~7 km Upper Jurassic to Cenozoic sedimentary fill of the study area.

Integration of 1200 km of 2D seismic-reflection profiles, well logs, cores and gravity data yields a consistent 3D crust-scale structural model which forms the starting model for a 3D numerical simulation of basin-scale petroleum systems calibrated by borehole temperature and vitrinite reflectance data. This 3D crust-scale model is achieved on the base of isostatic approach, 3D gravity and 3D thermal modelling and consists of 10 seismo-stratigraphic layers (representing the syn-rift to post-rift successions) underlain by an upper crustal pre-rift meta-sedimentary layer and a denser lower crust with geometric information and distribution of physical properties.

Three syn-rift (pre-Valanginian and Hauterivian) and three post-rift (Aptian, Barremian and Turonian) source rock intervals are modelled for hydrocarbon generation. Basal heat-flow incorporating radiogenic heat production from the crustal model gives a straightforward calibration of the present-day maturity. In order to further constrain thermal maturity within the basin, vitrinite reflectance data are employed from which three phases of paleo-heatflow peaks are predicted as critical events controlling hydrocarbon generation, migration, accumulation and leakages. The first heatflow elevation is the early Cretaceous syn-rift rapid and short-lived subsidence typical of a pull-apart basin mechanism. Whereas the second and third heatflow peaks are proposed to be the late Cretaceous mantle-related hotspot transit and the Oligo-Miocene margin uplift and erosion probably due to Tertiary magmatic intrusions and volcanism in Southern Africa, respectively. These basin critical moments temporally correlate with increased hydrocarbon generation and losses and decrease in hydrocarbon reservoiring, whist the Oligo-Miocene event contributed the most by an order of magnitude to this occurrence.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013