AAPG Middle East Region Geoscience Technology Workshop, Rift Basin Evolution and Exploration

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Trap Geometries and Risks Through Syn-Rift Tectono-Stratigraphic Interactions: Examples from Producing Fields and Outcrop Analogues

Abstract

The scale and timing of faulting in syn-rift settings is critical in determining stratigraphic juxtapositions and hence seal potential. These factors also control a) length scales over which sedimentary facies and therefore reservoir properties vary within syn-rift megasequences, and b) the preservation or otherwise of both pre-rift and syn-rift strata in tilt-block crest structural trap targets. The interplay of fault geometries and fault timing is thus crucial in determining the probability of reservoir presence, whether through fault terraces and/or the basin axis containing sand-rich fairways or through fault block crests hosting carbonate build-ups, assuming appropriate environmental conditions. The aim of the presentation is to present examples of the range of outcomes that are possible in terms of reservoir development, preservation, connectivity and quality in combined structural-stratigraphic traps within the syn-rift. Examples will be presented based upon successful discoveries in the North Sea and upon comparable field analogues from the Gulf of Corinth Rift, Greece and from the Red Sea Rift. In risking siliciclastic reservoir presence and reservoir quality in syn-rift basin fills, attention needs to be given to understanding the distribution of sediment entry points across a basin margin and transport pathways around subaqueous fault blocks. Sediment grade and sediment transport process, and hence the extent and quality of sand fairways, will reflect the evolution of rift flank tectono-geomorphology and catchment area provenance. Whether a clastic depositional system is bedload-dominated or dominated by suspension load will strongly influence whether clastics bypass the basin margin or are preserved in downthrown, proximal hanging wall locations. Historical drilling success rates in, for example, the North Sea allow some analysis of stratigraphic/fault seal potential against major basin-bounding faults. Field analogues of proximal, footwall-derived fans in the Gulf of Corinth Rift and Red Sea Rift exemplify potential sedimentological controls on seal potential in such settings, which may boost hydrocarbon heights that might be supported by fault seal alone. There is a high probability that paralic sediments will be extensively developed during the initial marine transgression into a rift basin. However, the maintenance of paralic conditions and therefore the likelihood of developing useful reservoir volumes can be predicted to increase substantially in areas maintained at or close to base level. Thus, in settings without significant carbonate development, the margins of intrabasinal saddles or fault block hanging wall dipslopes become targets for combination traps containing beach and shoreface reservoir clastics (e.g. Beatrice Field, Corinth Isthmus outcrop analogue). There remains some uncertainty as to the extent to which these may connect to deepwater reservoir volumes downdip. In locations with potential for clastics starvation and hence significant carbonate production, such structural areas maintained within the photic zone may develop regressive carbonate ramps (eg. Musayr Limestone outcrop analogue, Red Sea Rift). Connectivity between dipslope carbonate ramps and deeper water clastics fairways will be influenced by local carbonate productivity and clastic sediment input rates set against rates of accommodation generation. Likelihood of connectivity is therefore likely to be higher in more proximal fault terraces where sediment flux is high close to the basin margin, and to decrease basinwards to more distal fault block locations. In clastics-filled syn- to early post-rift basins, axial sand fairways are proven to offer substantial reservoir volumes through stratigraphic entrapment (e.g. Buzzard, Scapa in the North Sea) or combined stratigraphic/structural entrapment (e.g. Miller, Tartan, Britannia). Characterization of whether a particular depocentre is a bypassed or partially bypassed fault terrace or the true basin floor is critical to being able to predict the lateral extent of the reservoir and the probability of the fault block being sand-rich or sand-poor. The present-day Gulf of Corinth Rift exemplifies the potential for lateral pinch-out of the sand fairway, but also for connectivity into laterally input slope apron fans or fan deltas. The possible updip extent and connectivity of thin sand stringers against the basin margin or onto intrabasinal fault block crests will be a product of the interplay of the rate of clastics flux versus accommodation generation and of the timing of block-bounding fault growth. Statistics will be reviewed for the proportion of structural, combination and stratigraphic traps proven on the UK Continental Shelf, statistics which are dominated by the mature North Sea province. This will be used to highlight the potential value of targeting syn-rift stratigraphic and combination traps but also the need to develop strong geological models encompassing systematic Gross Depositional Environment mapping, mapping of syn-depositionally active versus inactive faults, and prediction of lateral variation in reservoir properties (thickness and stacking/connectivity of depositional elements, and porosity/permeability distributions). Only then can sensible risking of reservoir presence and quality and of seal potential be achieved in these depositionally and structurally complex settings.