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Deep-Water Reservoir Distribution in a Syn-Depositionally Active Salt-Confined Mini Basin-Fill

Abstract

Deep-marine successions onlapping salt diapirs form excellent reservoirs with combinations of structural-stratigraphic trapping. Typically, they are poorly-imaged by seismic in the subsurface due to salt overgrowth, steep dips and variable lithological distributions. This leads to increased uncertainty in hydrocarbon column heights and reservoir quality distributions. Outcrop studies provide a valuable analog to constrain sedimentary facies and geometrical configurations of these onlaps, reducing subsurface uncertainty. The Triassic (Keuper) Bakio Diapir and Guernica salt bodies in the Basque-Cantabrian Basin, Spain, are rare exhumed structures with associated halokinetically-influenced deep-marine successions. The structures grew reactively, then passively during the Aptian-Albian and are flanked by deep-marine carbonates (Aptian-earliest Albian) and clastics (middle Albian-Cenomanian). Continuous exposure enables a rare opportunity to study salt-sediment interactions where palaeoflow is at a low angle to structural strike. The clastics show an overall upward coarsening, thickening of beds, and reduction of fines. The mudstone-rich succession is dominated by thin-bedded low-density turbidites at its base and is overlain by thin hybrid beds, then medium-high-density turbidites interspersed with slump deposits. The upper part comprises stacked, amalgamated, scoured, high-density turbidites, pebbly mega-debrites and slumps with thick hybrid beds. Axial turbidites and hybrid beds, and lateral slumps and debrites complicate the onlap geometries. Paleoenvironments pass upwards from distal-fringe to lobe axis-off axis to channel lobe transition zone, indicating a progradational system influenced by halokinetic and autocyclic controls. Soft-sediment deformation, slumping and hybrid beds, together with bed pinch-out and flow deflection evidence from paleocurrents indicate active topographic adjustment during deposition. A tapered halokinetic sequence bound by progressive unconformities showing bed shallowing and rapid facies changes over 500 m laterally, on the western flank of the Bakio Diapir signify simultaneous halokinesis and deposition. Subsurface data from the eastern Central Graben of the North Sea has been integrated to create a multi-scalar model predicting reservoir facies distribution in halokinetically influenced deep-marine successions. This model is directly applicable to petroleum exploration and development in many salt basins globally.