Abstract:
[Objective] Shallow-water deltas are common sedimentary systems widely recorded in petroliferous basins and host abundant hydrocarbon resources. Based on differences in sediment types and contents supplied by rivers, they can be classified into three categories: sand-rich, sandy-muddy, and mud-rich deltas, which exhibit significant variations in geomorphology, sedimentary characteristics, sedimentary architecture, and sediment heterogeneity. [Methods] To clarify the sedimentary dynamics, sedimentary architecture patterns, and heterogeneity characteristics of these three delta types, this study employed sedimentary numerical simulations to reproduce their depositional evolution processes. Geomorphic and grain-size data were extracted to reconstruct 3D digital models of delta architecture and sediment grain-size distribution. Through detailed analysis of depositional evolution, sedimentary architecture patterns of shallow-water deltas were established, and heterogeneity characteristics within architectural frameworks were elucidated. [Conclusions and Discussion] The results indicate: (1) Variations in sediment types and contents supplied by rivers determine deltaic sediment composition, influence levee construction capacity and anti-scouring strength of distributary channels, and thereby control channel formation, evolution, filling characteristics, sedimentary architecture, and heterogeneity. (2) As the sand-to-mud ratio decreases, distributary channels exhibit reduced quantity, hierarchical complexity, and areal proportion, alongside increased sinuosity, enhanced stability, diminished lateral migration capacity, and a transition from lateral migration-dominated filling to abandonment-dominated filling. (3) Sand-rich deltas typically display lobate or triangular morphologies with extensively developed multi-stage radial distributary channels. Lateral migration dominates, forming broadly connected high-quality reservoirs in delta plains-to-fronts through complex stacking and incision, containing frequent fine-grained interbeds. Sandy-muddy deltas exhibit multi-finger branching patterns with fewer channels dominated by trunk distributaries that bifurcate terminally, forming laterally amalgamated finger-shaped bar complexes. These complexes create irregular broad-banded connected sand bodies with frequent muddy interbeds. Mud-rich deltas feature sparse, sinuous ribbon-like trunk channels lacking lateral accretion. Channels are predominantly filled with muddy sediments after abandonment, while bar complexes form bead-like discontinuous high-quality reservoirs along channel margins, laterally isolated by mud-filled channels. (4) Sand-rich deltas show near-continuous sand distribution with downstream fining. Coarsest grains occur in trunk channels, followed by mouth bars, yielding weak planar heterogeneity but strong vertical heterogeneity due to multi-stage lateral/progradational fine-grained interbeds. Sandy-muddy deltas exhibit broad-banded sand bodies with coarse-grained reservoirs in trunk channels and mouth bars, weak planar heterogeneity, and strong vertical heterogeneity from muddy interbeds. Mud-rich deltas display strong planar heterogeneity as reservoirs are restricted to mouth bar cores, laterally isolated by muddy channels, with weak vertical heterogeneity from multi-stage muddy interbeds.
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