--> Controls on Modern Delta Development With Implications for Predicting Reservoir Distribution in Deltaic Systems

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Controls on Modern Delta Development With Implications for Predicting Reservoir Distribution in Deltaic Systems

Abstract

A significant proportion of the world's remaining hydrocarbon reserves occur within deltaic deposits, thus understanding the key controls on reservoir development in deltaic systems has important economic implications. We are undertaking a study of modern day deltas to help understand reservoir development and compiled a global database of all (n=97) the large (>30 km in length from apex to toe) modern deltas using remotely sensed imagery. Deltas were classified according to dominant process – wave, fluvial, tidal or mixed, and modifiers applied depending on the influence of subordinate processes e.g. wave (tidal). Although there is overlap between different delta types, some general observations can be made. Wave dominated deltas account for 60% of the dataset, mixed process deltas are uncommon (10%) and tidal influenced and fluvial influenced deltas both account for 15% each. Most large deltas occur on passive margins and delta type is strongly influenced by latitude, climate and shoreline physiography. All deltas comprise a radial, distributive fluvial system (DFS) centred on the delta apex. Large, non-fluvial deltas (>75 km) show a disconnect between the DFS and shoreline although this distance varies depending on scale of fluvial system. Smaller non-fluvial deltas (30–75 km) generally show a good connection between the DFS and shoreline, again dependent on fluvial system scale. The significance of this observation is that small non-fluvial deltas supply sand directly to the shoreface whereas larger non-fluvial deltas do not. Fluvial-dominated deltas supply sand directly to the marine realm. Locating the apex of a delta – the point at which channels either bifurcate from or avulse from – is important as the majority of the transported sand is dropped immediately downstream of the apex. The backwater effect (the distance upstream of the delta mouth to where the base of the channel intersects sea-level) has been highlighted as a key controls apex location in modern deltas. Analysis of our dataset suggests that the backwater effect has no control on the apex location of large rivers, instead, the primary control on apex location of virtually all of the large deltas studied is relict topography. The dataset is definitive in terms of including all the large marine deltas on Earth, it is therefore the most comprehensive dataset available for comparison with ancient deltaic successions and should at least in part be representative of the rock record.