Three-Dimensional Modeling of Facies Architecture and Connectivity Variations of Meandering River Successions in Evolving Rift Basins

Abstract

The spatial organization of meandering river deposits varies greatly within rift-basin fills, depending on how differential rates of subsidence and sediment supply interplay to drive changes in 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 active settings remain limited.

A numerical forward stratigraphic model – the Point-Bar Sedimentary Architecture Numerical Deduction (PB-SAND) – is used to explore the relationships between differential rates of subsidence and resultant fluvial channel-belt migration, reach avulsion and 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. Point-bar connectivity and stacking patterns are predicted in response to variations in rates of fault-driven subsidence, resulting accommodation generation, rates of river migration and avulsion frequency. Results show how the connectivity of point-bar sandbodies can be quantified as a function of subsidence rate, which itself decreases both along and away from the locus of fault displacement. Model outputs are analyzed quantitatively in terms of horizontal and vertical changes in static-connectivity metrics at multiple scales, to document the modeled connectivity of channel belts, point bars within them, and intra-bar sand-prone packages. Spatial variations in the connectivity of point-bar sandbodies are controlled by the relative significance assigned to input parameters that mimic allogenic and autogenic processes.

PB-SAND facilitates understanding of facies heterogeneity and connectivity variations of fluvial successions in rift basins, allowing examination of the influence of geologic boundary conditions on sedimentary architecture at different scales. Model outputs incorporate sedimentary architecture and stratigraphic heterogeneities of fluvial system elements realistically, and in a format that can be integrated into conventional reservoir-modeling practice, particularly to aid in the assessment of sandbody connectivity.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018