HYDROCARBON GENERATION AND MIGRATION FROM BARREMIAN – APTIAN SOURCE ROCKS, NORTHERN ORANGE BASIN, OFFSHORE WESTERN SOUTH AFRICA: A 3D NUMERICAL MODELLING STUDY

A 3D numerical modelling workflow was applied to the Barremian—Aptian source rock interval in a shelfal to lower slope area of the northern Orange Basin, offshore western South Africa. The main objective was to investigate the timing of hydrocarbon generation and migration. Hydrocarbon migration has previously been investigated in the south of the basin by relating gas escape features with structural elements as seen on seismic sections, but migration pathways are still poorly understood. The modelling study was based on data from three exploration wells (AO-1, AE-1 and AF-1) together with 42 2D seismic sections totalling 3537 km in length, and a 3D seismic cube covering an area of 750 sq. km.

Modelled formation temperatures increase from north to south in the study area and were consistent with downhole temperatures at well locations. However, there is variation between measured and modelled values of vitrinite reflectance (VR), especially in the Turonian and Cenomanian intervals. The measured VR is lower than the modelled VR within the Turonian section in the north of the study area, suggesting that erosion has affected the thermal maturity of the sediments. However, in the Cenomanian interval, the measured VR is higher than the modelled VR. Uplift, increased erosion in the hinterland and sediment transport to the coastal areas resulted in Cenomanian progradation of the Orange Basin fill. This together with a heat flow pulse resulted in increased thermal maturities in the study area.

Modelling results show that hydrocarbon generation began in the central part of the study area by 116 Ma and reached a peak in the Late Cretaceous (65 Ma). Hydrocarbon migration began at about 110 Ma with an expulsion efficiency of 0.77. At the present day, ∼100% transformation of reactive kerogen into hydrocarbons has taken place in the central part of the study area, with random gas migration within Cenomanian and Albian reservoirs. Modelled oil migration likely influenced by hydrodynamic factors is down-dip (westwards), towards deeper-water, more distal parts of the basin.

Gas saturation on a reactivated listric fault, which was ∼100% saturated at 93 Ma, declined to ∼15% by 65 Ma. This decrease in gas saturation is linked to uplift of the African margin in the Late Cretaceous which resulted in fault reactivation and re-migration of gas.

Despite the uncertainties which are associated with petroleum systems modelling, the study provides an insight into hydrocarbon migration in the northern part of the Orange Basin and contributes to the de-risking of future oil and gas exploration in this area.