Correlation and Development of Fluvial Systems: Implications for three-dimensional Facies Models

Abstract

The variety of controls on the deposition of fluvial systems, whether autocyclic or allocyclic including tectonic, climatic or eustatic influences, produce complex spatial architectural geometries and facies variations, which are challenging to correlate and predict across sedimentary basins. In extensional basins, tectonic subsidence is the main influence upon generation of accommodation space and the spatial distribution of drainage networks. This is further complicated during marine transgressions when a direct competition exists between tectonism, climate, and sea-level rise producing a complex series of interactions. This is manifested in a change in depositional style such as bed thickness, geometry or grain size changes of a fluvial sequence. Establishing predictive basin-scale models and frameworks linking controls on sedimentation to preserved architecture is vital, especially as fluvial systems are prolific reservoirs for the extraction or injection of fluids and targets for mineral resources globally. The multi-disciplinary and multi-scale approach of this research using sedimentology, chemostratigraphy and statistical-based facies modelling aims to investigate the Triassic syn-rift fluvial architecture in the Central Iberian Basin, north-eastern Spain, to build a correlated, basin-wide depositional model constrained by the main controls on fluvial architectural development. Observations from outcrops are incorporated in fluvial facies models to investigate the three-dimensional facies and fluid flow relationships.
Firstly, on the basin scale, the use of chemostratigraphy identified key horizons independent of fluvial architecture and, in combination with sedimentology, allowed a high-resolution spatial and temporal correlation of Triassic syn-rift fluvial sandstones, identifying a major fluvial system along ~80 km of the rifted basin margin for ~10 Ma. The advance of the Middle Triassic Tethyan marine transgression (0.04–0.02 m/year) is recorded by a transition from a fluvial succession to marginal marine clastic sediments and sabkha evaporites and subsequently widespread thick carbonate deposition infilling the basin and recording the final stage of syn-rift deposition. The nonmarine to marine transition is characterised by significant changes in the fluvial architecture from predominantly braided to meandering and a transition from a tectonically to a climatically driven fluvial system.
Secondly, outcrops of the syn-rift Triassic Upper Buntsandstein fluvial succession revealed three phases of development for the fluvial system. Stage one is dominated by an early ephemeral, sheetflood system comprised of isolated, single-storey laminated sand sheets. Stage two is identified by a distinct basin-wide correlative boundary characterised by changing geochemical sediment composition and sharp transition to multi-storey, braided fluvial systems. Stage three comprises highly amalgamated, multi-storey channels with an increased occurrence of floodplain and paleosols facies. This transition is directly attributed to the rapidly approaching Tethyan marine transgression with the Central Iberian Basin.
Finally, statistically-based facies modelling has been utilized to quantitatively determine relationships between fluvial channel shapes, channel widths, net-to-gross and fluid flow characteristics of the Triassic fluvial succession in the Central Iberian Basin. Using field data sets from outcrops allowed detailed conditioning of statistical models with multi-point statistical simulations producing the most realistic and truthful representation of the fluvial architecture compared to the outcrops. The complexity of the facies models as represented by different channel shapes is in relation to the overall recovery factor, where models built from simple, well-defined channel shapes result in the highest recovery factors. In high net-to-gross systems, as recognised from the models, internal heterogeneities are the main drivers and controls for fluid flow and are best represented utilising process-based models. In contrast, in decreasing net-to-gross fluvial systems, data conditioning becomes more important as sand body external configurations increase their impact on fluid flow behaviour and are best represented using multi-point statistical simulation models.
This research contributes to the understanding of sedimentation in rift basins, the spatial and architectural distribution of fluvial facies and their representations in models as many fluvial successions are targets for carbon capture and hydrogen storage and prime gas reservoirs in north-western Europe.