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Origin and Significance of Thin-Bedded Packets Interleaved in Sandy Deep-Sea Fan Successions


Sandy deep-water reservoirs commonly contain subordinate or even minor low-net sand, thin-bedded packets that can have a significant impact on production. These thin-bed packets can occur in a wide range of settings (lobe fringes, splays, overbanks, channel margins, local or fan-wide abandonments) and correctly diagnosing their origin is important for reconstructing hierarchical stacking patterns, their use in reservoir zonation and incorporating these elements correctly in reservoir models.

Recent outcrop and core-based studies from the Pennsylvanian Ross Sandstone Formation, western Ireland, have highlighted significant thin-bedded and fine-grained intervals outwith the condensed sections that subdivide the succession into stacked fan complexes. The thin-bed packets are typically m-scale, but also occur in thicker siltstone-dominated packets up to 10 m thick, some of which can be correlated laterally for several kilometres down dip. A common motif in the latter is that they display fining upwards vertical patterns and are exclusively composed of well-structured thin (<5 cm) to very thin (<1 cm) beds of current-rippled sandstone isolated in siltstone. These are locally cut and partly interleaved with mid- and upper Ross channel fills. Some of these thin-bedded packets have similar thicknesses to laterally adjacent channel fills which incise into them. Lateral faces transitions and palaeoflow data in the thin bed packets suggest a genetic relationship with the channels. In other cases, the thin-bed packets are perched above the level of the adjacent channel floor but can be followed laterally into channel margin facies without a break. These relationships suggest at least some of the Ross channels had low-relief levees and locally thin-bed frontal splays into which they were incised. Away from channels, metre- and sub-metre thick bundles of thin beds commonly separate thicker sandy lobe elements. These show strong bed type partitioning in that some are exclusively composed of stacked very-thin well-structured turbidites, whereas others are made up of thin hybrid event beds, commonly displaying sandying-upward cycles beneath cleaner sandstones. The latter tend to be associated with and fill erosional relief and are related to lobe reoccupation of scoured and abandoned mid- and inner-fan sectors (repair facies of Elliott, 2000). The erosional relief is key to their preservation. The latter are interpreted as overspill sheets from channels delivering sand to lobes further down-dip.