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Fluvio-deltaic stratigraphy develops under continuous morphodynamic interactions of allogenic and autogenic processes, but the role and relative contribution of these processes to the stratigraphic record are poorly understood. We analysed synthetic fluvio-deltaic deposits of several accommodation-to-supply cycles (sequences) with the aim to relate stratigraphic variability to autogenic and allogenic controls. The synthetic stratigraphy was produced in a series of long time-scale (10 5 years) numerical experiments with an aggregated process-based model using a typical passive-margin topography with constant rates of liquid and solid river discharge subjected to sinusoidal sea-level fluctuation. Post-processing of synthetic stratigraphy allowed us to quantify stratigraphic variability by means of local and regional net sediment accumulation over equally spaced time intervals (1-10 kyr). The regional signal was subjected to different methods of time-series analysis. In addition, major avulsion sites (>5 km from the coastline) were extracted from the synthetic stratigraphy to confirm the interpretations of our analyses. Regional stratigraphic variability as defined in this study is modulated by a long-term allogenic signal, which reflects the rate of sea-level fluctuation, and it preserves two autogenic frequency bands: the intermediate and high-frequency components. The intermediate autogenic component corresponds to major avulsions with a median inter-avulsion period of ca. 3 kyr. This component peaks during time intervals in which aggradation occurs on the delta plain, because super-elevation of channel belts is a prerequisite for large-scale avulsions. Major avulsions occur occasionally during early stages of relative sea-level fall, but they are fully absent once the coast line reaches the shelf edge and incision takes place. These results are consistent with a number of field studies of falling-stage deposition in fluvial systems. The high-frequency autogenic component (decadal to centennial time scales) represents mouthbar-induced bifurcations occurring at the terminal parts of the system, and to a lesser extent, partial or small-scale avulsions (<5 km from the coastline). Bifurcation intensity correlates strongly with the rate of progradation, and thus reaches its maximum during forced regression. However, its contribution to overall stratigraphic variability is much less than that of the large-scale avulsions, which affect the entire area downstream of avulsion nodes. The results of this study provide guidelines for predicting fluvio-deltaic stratigraphy in the context of co-existing autogenic and allogenic processes and underscore the fact that the relative importance and the type of autogenic processes occurring in fluvio-deltaic systems are governed by allogenic forcing.

Multiscale simulation of fluvio‐deltaic stratigraphy was used to quantify the elements of the geometry and architectural arrangement of sub‐seismic‐scale fluvial‐to‐shelf sedimentary segments. We conducted numerical experiments of fluvio‐deltaic system evolution by simulating the accommodation‐to‐sediment‐supply (A/S) cycles of varying wavelength and amplitude with the objective to produce synthetic 3‐D stratigraphic records. Post‐processing routines were developed in order to investigate delta lobe architecture in relation to channel‐network evolution throughout A/S cycles, estimate net sediment accumulation rates in 3‐D space, and extract chronostratigraphically constrained lithosomes (or chronosomes) to quantify large‐scale connectivity, that is, the spatial distribution of high net‐to‐gross lithologies. Chronosomes formed under the conditions of channel‐belt aggradation are separated by laterally continuous abandonment surfaces associated with major avulsions and delta‐lobe switches. Chronosomes corresponding to periods in which sea level drops below the inherited shelf break, that is, the youngest portions of the late falling stage systems tract (FSST), form in the virtual absence of major avulsions, owing to the incision in their upstream parts, and thus display purely degradational architecture. Detailed investigation of chronosomes within the late FSST showed that their spatial continuity may be disrupted by higher‐frequency A/S cycles to produce “stranded” sand‐rich bodies encased in shales. Chronosomes formed during early and late falling stage (FSST) demonstrate the highest large‐scale connectivity in their proximal and distal areas, respectively. Lower‐amplitude base level changes, representative of greenhouse periods during which the shelf break is not exposed, increase the magnitude of delta‐lobe switching and favour the development of system‐wide abandonment surfaces, whose expression in real‐world stratigraphy is likely to reflect the intertwined effects of high‐frequency allogenic forcing and differential subsidence.

We propose a new workflow for building static reservoir models of siliciclastic fluvio-deltaic systems. The proposed strategy requires a process-based stratigraphic simulation model which incorporates a reservoir-scale alluvial architecture module nested within a low-resolution basin-scale (sequence-stratigraphic) model. The basin-scale model is run with the intent to approximate large-scale basin-fill properties (based on geological/geophysical background information about palaeotopography, sea level, sediment supply, subsidence, and so forth). Subsequently, the model may be stochastically optimised by dedicated post-processing software to mimic sub-grid (reservoir-scale) properties of selected parts of the basin fill. This approach allows us to narrow down the range of possible scenarios (realisations) from the outset, which results in more reliable uncertainty estimates associated with reservoir models. Pilot studies suggest that the improvement of geological credibility of stochastically simulated fluvial reservoir models may go hand-in-hand with a significant reduction of the computational effort of inverting basin-scale (process-based) stratigraphic forward models. The implementation of geological constraints on object-based models is expected to improve estimation of sand-body connectivity and dynamic reservoir behaviour, and will therefore contribute to reduction of the non-uniqueness in current static reservoir models. Furthermore, the uncertainty associated with each basin-scale parameter can be propagated all the way through to reserve estimation. The partitioning of the overall uncertainty into contributions at the basin and reservoir scales may be quantitatively assessed, and the information content of all available data may be quantified.

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