Structure and evolution of basin and petroleum systems within a transformrelated passive margin setting : data-based insights from crust-scale 3D modelling of the Western Bredasdorp Basin, offshore South Africa

[摘要] ENGLISH ABSTRACT: This study investigates the crustal structure, and assesses the qualitative and quantitative impacts of crust-mantle dynamics on subsidence pattern, past and present-day thermal field and petroleumsystem evolution at the southern South African continental margin through the application of amulti-disciplinary and multi-scale geo-modelling procedure involving both conceptual andnumerical approaches. The modelling procedure becomes particularly important as this margindocuments a complex interaction of extension and strike-slip tectonics during its Mesozoiccontinental rifting processes. Located on the southern shelf of South Africa, the WesternBredasdorp Basin (WBB) constitutes the focus of this study and represents the western section ofthe larger Bredasdorp sub-basin, which is the westernmost of the southern offshore sub-basins. Tounderstand the margin with respect to its present-day structure, isostatic state and thermal field, acombined approach of isostatic, 3D gravity and 3D thermal modelling was performed by integratingpotential field, seismic and well data. Complimenting the resulting configuration and thermal fieldof the latter by measured present-day temperature, vitrinite reflectance and source potential data,basin-scale burial and thermal history and timing of source rock maturation, petroleum generation,expulsion, migration and accumulation were forwardly simulated using a 3D basin modellingtechnique. This hierarchical modelling workflow enables geologic assumptions and their associateduncertainties to be well constrained and better quantified, particularly in three dimensions.At present-day, the deep crust of the WBB is characterised by a tripartite density structure (i.e. preriftmetasediments underlain by upper and lower crustal domains) depicting a strong thinning that isrestricted to a narrow E-W striking zone. The configuration of the radiogenic crystalline crust aswell as the conductivity contrasts between the deep crust and the shallow sedimentary coversignificantly control the present-day thermal field of the study area. In all respects, this present-dayconfiguration reflects typical characteristics of basin evolution in a strike-slip setting. For instance,the orientations of the deep crust and fault-controlled basin-fill are spatially inconsistent, therebyindicating different extension kinematics typical of transtensional pull-apart mechanisms. As such,syn-rift subsidence is quite rapid and short-lived, and isostatic equilibrium is not achieved,particularly at the Moho level.Accompanied syn-rift rapid subsidence and a heat flow peak led to petroleum preservation in thebasin since the Early Cretaceous. Two additional post-rift thermal anomalies related to the LateCretaceous hotspot mechanism and Miocene margin uplift in Southern Africa succeeded the syn-riftcontrol on maturation. This thermal maturity of the five mature source rocks culminated in fourmain generation and three main accumulation phases which characterise the total petroleum systemsof the WBB. The Campanian, Eocene and Miocene uplift scenarios episodically halted sourcematuration and caused tertiary migration of previously trapped petroleum. Petroleum loss related tothe spill point of each trap configuration additionally occurs during the Late Cretaceous-Paleoceneand Oligocene-Early Miocene. The timing and extent of migration dynamics are most sensitive tothe geological scenario that combined faulting, intrusive seal bypass system and faciesheterogeneity. In fact, for models that do not incorporate facies heterogeneity, predicted past andpresent-day seafloor leakage of petroleum is largely underestimated. This complex interplay ofgeneration and migration mechanisms has significant implications for charging of petroleumaccumulations by multiple source rocks. Due to early maturation and late stage tertiary migration,the syn-rift source rocks particularly Mid Hauterivian and Late Hauterivian source intervalssignificantly control the extent of petroleum accumulation and loss in the basin.Lastly, the modelled 3D crustal configuration and Mezosoic to Cenozoic thermal regime of theWBB dispute classic uniform lithospheric stretching for the southern South African continentalmargin. Rather, this PhD thesis confirms that differential thinning of the lithosphere related to atranstensional pull-apart mechanism is the most appropriate for accurately predicting the evolutionof basin and petroleum systems of the margin. Also, the presented 3D models currently representthe most advanced insights, and thus have clear implications for assessing associated risks in basinand prospect evaluation of the margin as well as other similar continental margins around the world.

[发布机构] Stellenbosch University