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The Potential Impacts of Substrate Erosion and Facies Heterogeneity of Blocky MTDs on Reservoir Volumes and Seal Quality: Insights From the Ventimiglia Flysch Formation (Eocene, NW Italy)


Mass Transport Deposits (MTDs) play an important role in deep-water hydrocarbon systems as they may represent vertical and lateral seals as well as erode significant portions of underlying reservoir units. Insights from outcrop into sub-seismic features of MTDs may thus facilitate better assessment of exploration targets associated with such deposits. Here we report on multiple MTDs from the Ventimiglia sub-basin of the Alpine foreland basin, NW Italy. This tectonically active, N-S elongate depocentre was predominantly filled by southerly-sourced turbidites of the Eocene Ventimiglia Flysch (VF), unconformably deposited onto hemipelagic marlstones of the Olivetta San Michele Fm (OSM). Correlation of multiple sections through the best exposed of the studied MTDs allowed recognition of: i) an inner/proximal region with high positive relief and deep (though localized) basal erosion consisting of a clast-supported OSM megabreccia (thickness in the range 30 to 75 m) capped by a OSM conglomerate of variable thickness, ii) a dual layer medial region (thickness in range of 5 to 30 m) lying on a distally-shallowing basal erosion and consisting of a bottom layer of intensely deformed turbidites and of an upper layer with an association of sparse OSM megaclasts and a mixed-composition conglomerate; iii) an outer/distal region where the conglomerate sits conformably onto the substrate with no discernible erosion and is enriched in turbidite clasts. Such a depositional architecture can be interpreted as being generated by an initial debris slide of OSM (possibly with a focused mass-flow axis) undergoing a significant compositional and rheological modification as a result of the entrainment of unconsolidated sediments eroded from the substrate. It is suggested that the development of pore water overpressure in the entrained turbidite material may have resulted in an increased flow mobility and capacity to transport megaclasts. Finally, the capping conglomerate is interpreted as the deposit of a trailing debris flow that was cogenetic to the initial slide and capable of outrunning the main deposit of the MTD with only minor compositional hybridization. This study provides constraints on shape and internal heterogeneity of the so-called ‘detached’ MTDs (sensu Moscardelli and Wood, 2008) which, though common in the subsurface, often cannot be well imaged seismically. The outcrop constraints enable better prediction of both preserved reservoir volumes and seal quality.