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Lateral Facies Heterogenity in Hybrid Event Beds and its Applied Significance: Examples From the Miocene Cilento Flysh and the Cretaceous-Paleocene Gottero Sandstone (Italy)


Hybrid event beds (HEBs) found in association with conventional turbidite deposits characterise two well-exposed sections in the Cilento flysh and the Gottero Sandstone (Apennines, Italy). HEBs are a type of deep-water sediment gravity flow deposit that generally comprise a basal clean sandstone overlain by a variety of muddier and less-permeable sandy facies thought to be emplaced by combinations of debris flow and turbiditic processes as part of the same event. HEBs are commonly found in the outer and marginal parts of deep-water systems where they replace beds composed dominantly of clean sand up-dip over scales of kms to 10s km. Here, we report that in addition to these broad patterns, important yet poorly understood short-length facies changes (over metres to 100s m) can occur, modifying the overall texture and reservoir characteristics at or beneath the typical spacing of production wells. Detailed facies analysis combined with bed for bed correlations shows that marked lateral variations in the internal bed make-up of most of the HEBs occur along depositional strike. This variability typically involves lateral changes in the proportions of the bed divisions and in the texture of the debritic interval and the scale of substrate blocks it may contain. Generally, these changes occur without significant variation in the overall event bed thickness. The statistical evaluation of such facies transitions is used to develop a revised model of sandstone to mud abundance in hybrid flow generated lobes, and to estimate the character of the critical transition from up-dip turbidite-dominated intervals to the down-dip, HEB-dominated fringe, incorporating small-scale lateral variability. The nature of this transition is inferred to be related to the pattern and mechanism of up-dip erosion and entrainment of mud clasts into the flow, inducing turbulence damping. Once formed, linked debris flows can locally remove some or all of the just-deposited basal sand, especially where large entrained rafts of substrate plough into it. The variable thickness and continuity of the basal clean sandstone and the rugosity on the contact with the overlying debrite have important implications for reservoir characterisation. Significant variability in bed character at the inter-well scale can be anticipated; the intra-bed rugosity may impact drainage and sweep efficiency during hydrocarbon production, particularly in cases where the lower sandstone is locally completely removed.