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Prediction of reservoir quality of turbidite systems in steep passive margins: a challenge for deepwater exploration


Prediction of presence of turbidite reservoirs is highly facilitated by detailed imaging of depositional deepwater systems by high quality 3D seismic surveys. In addition, outcrop studies of deepwater sediments provide a better understanding of deepwater depositional facies and related processes and their relation with reservoir architecture. Nevertheless, prediction of reservoir quality remains a key risk in deepwater exploration. In a few recently drilled exploration wells, reservoir quality issues were experienced and interpreted to result from immature turbidite sediments and consequently from drastic diagenetic processes downgrading petrophysical properties of the reservoirs. Slope morphology and drainage area were considered to be the main controlling factors on sediment immaturity and related poor reservoir quality.

Deep-water hydrocarbon exploration results across the slope and the basin are strongly influenced by the morphology of the shelf to basin relief and more importantly by the dip of the slope. Brittle substrate and ductile substrate are the two mechanical end-members which govern the basin typology across the shelf break down to the base of slope. Presence of thick mobile shale or salt section has a direct implication on the confinement of the basin. Continental margins where mobile section is intensely deformed by younger prograding shelfal sediments favor the development of ponded basins.

Steep slopes which are locally fault-controlled seem to favor the incorporation of rip-up clasts and clay flakes in the gravity flow, dominated by immature sediments rather than mature quart-rich sediments. Combination of rip-up clasts (constitutional clay) and clay flakes in the pore network (disperse clay) affect drastically the permeability of turbidite reservoirs.

Small drainage areas with a steep gradient coupled with a short distance from the hinterland to the deepwater basin seem to be the geomorphic elements having a strong negative impact on deepwater reservoir characteristics.