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Three-Dimensional Forward Stratigraphic Modeling of the Sedimentary Architecture of Meandering-River Successions in Half-Grabens

Abstract

The architecture of meandering-river deposits varies greatly within the infills of rift-basins, depending on how differential rates of fault propagation and subsidence interplay with autogenic processes to drive changes in fluvial channel-belt position and rate of migration, avulsion frequency, and mechanisms of meander-bend cut off. This process fundamentally influences stacking patterns of the accumulated successions. Quantitative predictions of the spatio-temporal evolution and internal architecture of meandering fluvial deposits in such tectonically active settings remain limited.

A numerical forward stratigraphic model, PB-SAND, is used to explore the relationships between differential rates of subsidence and resultant fluvial channel-belt migration, reach avulsion and channel-deposit stacking in active, fault-bounded half-grabens. The model is used to reconstruct and predict the complex morphodynamics of fluvial meanders, their generated bar forms, and the associated lithofacies distributions that accumulate as heterogeneous fluvial successions in rift settings, constrained by limited data from seismic images and outcrop successions. The 3D modeling outputs can be used to explore heterogeneity of both intra- and inter-bar deposits at various temporal scales.

Results show how the connectivity of sand-prone geobodies can be quantified as a function of subsidence rate, which decreases both along and away from the basin-bounding fault. In particular, results highlight the spatial variability in the size and connectedness of sand-prone geobodies that is seen in directions perpendicular and parallel to the basin axis, and that arises as a function of the interaction between spatial and temporal variations in rates of accommodation generation and tectonically driven changes in river morphodynamics. Optimal locations or ‘sweet spots’ for hydrocarbon exploration in half-grabens is primarily determined by the interplay between the frequency of fault slip, the rate of subsidence, the style of basin propagation, the rates of migration of channel belts, the frequency of avulsion, and the proportion and spatial distribution of variably sand-prone channel and bar deposits. The 3D model provides linkage between local outcrop measurements and large-scale evolutionary behavior, and allows quantitative assessments of possible scenarios depicted in traditional qualitative facies models. Outputs from this approach can be used to better inform reservoir models.