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Westward-Ho: Why Do Most Turbidite Reservoirs Offshore Ghana Stack to the West, and What Impact Does This Have on Reservoir Heterogeneity, Modeling, Connected Volume, Well Placement and on Exploration?

Abstract

Upper Cretaceous deep-water oil and gas reservoirs offshore Ghana are found in slope and intraslope channel complex systems and ponded fans, testament to highly irregular seafloor topography during sedimentation. Shelfal feeder systems and width were variable, with some deep-water slope systems fed by deltas directly into canyon heads, and some through canyons a long distance from fluvio-deltaic systems by long-shore drift migration of shelfal sands. Two of the larger ponded slope fans, where the Jubilee and Ntomme oilfields are located, comprise reservoir intervals made of offset-stacked frontal splay complexes, with layered and amalgamated turbidite sandstone beds arranged into stacked lobe elements, and larger-scale offset-stacked lobes.

In MH4 and MH1 intervals in Jubilee, and all of Ntomme, the larger-scale offset-stacked lobes vary internally in heterogeneity but all stack progressively towards the west, each separated by bounding and laterally persistent heterolithic packages. The Enyenra slope channel complex is very different, though equivalent in age to the Jubilee MH1 reservoir, comprising three or more phases of channel complex system occupancy depending on the hierarchy of channel architecture being examined, whether for chronological or connectivity. The channel phases correspond to the reservoir intervals (EO1U, EO1L, EO2) and are bounded by abandonment heterolithics. Each interval has multiple offset-stacked channel elements, each comprising layered and amalgamated sandstone beds with interleaved margin heterolithics. The Enyenra slope channel complex system is levee-confined with prominent meandering and straight planform reaches, and stacking of channel elements within each interval is prominently to the west, stacking at almost 80° on western, and vertical to 10° on eastern meanders. Shallower non-producing intervals in the Santonian and Campanian also show prominent westwards-stacking.

The lobe and channel element pattern of westwards-stacking has a huge impact on reservoir connectivity. Lobes are separated by shales that control fluid flow tortuosity and are the main uncertainty in reservoir connectivity. Channel stacking impacts on well-planning in Enyenra, as trajectories, and the seafloor facility location, are largely predicated by this pattern. Future wells are now designed to enter top reservoir at a very high angle from the west, to maximise reservoir penetration for production or injection. However, most of the subsea facilities are currently located to the east. Westward-stacking of lobe and channels is explained, showing how all fields and prospects along this trend will have the same character. This impacts on current field development and management planning, and on nearfield exploration and perhaps new ventures, where the pattern has clearly been sustained in the Gulf of Guinea from post transform breakup to the current day.