Modeling the Size, Shape and Connectivity of Stratal Bodies in River, Wave and Tide-Dominated Deltas
Modeling the size, shape, and connectivity of deltaic stratal bodies has important implications for hydrocarbon reservoir assessment and prediction. Here, we present initial results from 12 depth-averaged simulations of river, tide, and wave-dominated deltaic systems in Delft3D. All runs were computed on a 24 × 16 km grid with 100 × 100 m cell size. For the offshore boundary conditions, we specified time-varying wave conditions, and/or constant semidiurnal tidal forcing. For the upstream boundary condition we specified a steady river discharge of 11,000 m3/s, carrying an initial sand to cohesive mud ratio of approximately 2:1. Analyses of runs consisted of extracting sand and mud body dimensions. Results show that river delta sand bodies are predominantly composed of elongate levees and mouth bars, which together create digitate deposits with long axes parallel to local flow. Fluvial sand bodies have limited connectivity due to intervening interdistributary muds. Tide-dominated delta sand bodies are composed of two types: near fluvial channels there are elongate levee and crescentic mouth bar sands, while in interdistributary bays tidal flats develop with shore-perpendicular sandy tidal shoals. As a result, tide-dominated deltas tend to have high sand connectivity along the delta perimeter. On the contrary, in the delta interior where river influence is low, connectivity is low due to local accumulation of mud that caps and separates sandy tidal shoals. Early in the development of wave dominated deltas, sand bodies are similar to those of river-dominated deltas. However, as deltas prograde into deeper water where wave influence is higher, levees and mouth bars are replaced by spits and sand ridges. In areas downdrift of alongshore transport, sand body connectivity is low due to the presence of sandy spits with intervening mud deposits. Initial analysis suggests that small-scale gyres trap mud leading to enhanced deposition in these regions. At the delta front, sediment distribution by longshore transport leads to the formation of laterally continuous sand ridges forming sheet-like sand bodies. Additional experiments show that basin bathymetry is a major control on stratal body dimensions. For example, for the same tidal amplitude, tidal strength is amplified in shallower basins compared to deeper basins. Conversely, for the same wave height, wave strength is amplified in deeper basins relative to shallower basins.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015