A Systematic Evaluation of Fault Seal Integrity in the southern Pletmos Basin, offshore South Africa: A 3D Multidisciplinary Modelling Approach
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The syn-rift succession encompasses the primary exploration target in the southern Pletmos Basin. Several fault-bounded structural traps that contain gas accumulations have been discovered within this succession. Likewise, ubiquitous residual gas shows have been encountered in most drilled wells. Yet, the impact of faults on fluid flow is poorly understood. Therefore, this study aspires to predict, and where possible, quantify fault seal integrity and sealing capacities of some of the major prospect-bounding faults. A multi-disciplinary research strategy was employed in order to fulfil the study objectives. Fault mapping and geo-cellular modelling using geostatistical algorithms were undertaken to provide the basic geometric and structural input for more advanced fault seal analysis applications. Juxtaposition analysis was carried out to identify zones with a high probability to seal (or leak) and as the first-order tool for predicting fault seal potential. Threshold pressures, hydrocarbon column heights, cross-fault permeability and transmissibility were used to estimate the sealing capacities of the faults. In addition to juxtaposition and customary fault-rock properties, the study also analysed parameters that can be deemed to be representative of cross-fault fluid flow (i.e. effective cross-fault permeability and transmissibility; ECFP and ECFT). Finally, modelling of the geo-history facilitated the validation of the properties that underpinned fault seal analysis studies. The Ga-Q and proposed Ga-K prospects along with their main bounding faults formed the foci of the fault seal analysis results. The analysed faults showed excellent initial sealing potential due to either favourable juxtaposition or shale gouge development. Nonetheless, predicted hydrocarbon column heights and threshold pressures were low suggesting that the seal integrity of the analysed faults is predisposed to failure. In addition, high predicted fault permeability and transmissibility values signify the presence of open and permeable fracture networks within the fault zones. Thus, it is proposed that the faults are very likely to have leaked during hydrocarbon migration and filling of traps resulting in empty or under-filled hydrocarbon reservoirs.