R&D Studies - coordinated by Fadi Nader
Dionisos - 3D Stratigraphic Modelling of Sedimentary Basins
Didier Granjeon
Introduction
As oil becomes less and less accessible, one of the biggest challenges is to find new opportunities in frontier areas such as in pre-salt carbonates or in deep sea clastic layers, which both are complex heterogeneous sedimentary systems. Sequence stratigraphic conceptual models (e.g. Catuneanu et al, 2009) are very helpful to analyze a sedimentary system, but they are usually limited to a few parameters such as accommodation and sediment supply, cannot provide quantitative evaluation of strata architecture and address uncertainties. Stratigraphic forward numerical models have been developed since the early 1960s. They represent the dynamics of sedimentary systems and simulate the filling of sedimentary basins. This project is based on Dionisos, a stratigraphic forward model developed at IFP since 1992 (Granjeon, 1996, Granjeon and Joseph, 1999, Granjeon, 2009).
Methodology
Dionisos is a process-based modelling tool that accounts for accommodation, supply and transport. Accommodation is controlled by sea level variations, plus basin and strata deformation induced by compaction, vertical uplift and subsidence, thrusts and growth faults, salt and shale diapirs (Figure 1). Supply of sediment may correspond either to boundary condition (inflow / outflow of water and sediment into / out of the simulated area), basement erosion, carbonate production or evaporite precipitation. Short-term transport processes are lumped in two large-scale processes: a slow hillslope creeping and a fast water-driven overland sheet flow. The creeping of sediment is assumed to be proportional to the local slope of the landscape, while the sheet flow is defined using a non-linear slope- and water dischargedependent law. Sedimentation and erosion at each point of the basin is defined from the mass conservation equation and these transport equations. Sedimentation occurs at a point of the basin if the transport capacity decreases. On the contrary, erosion occurs if the transport capacity increases. In such a case, the erosion rate predicted by the transport equation cannot exceed a maximum mechanical incision rate.
Applications
To illustrate these principles, two nested Dionisos simulations of the Rhone system are presented (Figure 2): a first modeling was performed at a regional scale (300 x 450 km²), to capture the long-term and large-scale behavior of this sedimentary system, then a second modeling was performed at a more local scale (60 x 90 km²), to better study the impact of shelf dynamics on deep-water turbiditic systems. To keep this project simple, accommodation was kept constant. Water and sediment were supplied through a narrow source located on the head of the Petit Rhone deep sea canyon. Fluxes of water and sediment were assumed to vary through time as in-phase sinusoidal functions. Four climatic cycles were simulated with a period of 120 ky (the simulated time span is 400 ky). During times of high water and sediment inflow, water velocity and sediment concentration at the head of the canyon were high enough to generate strong hyperpycnal flows, the strength of which increased progressively inside the canyon due to water incorporation. These fluxes generated detached lobes far away from the canyon mouth. On the opposite, during times of low water and sediment inflow, sediment concentration was not very high and weak hyperpycnal flows fed the turbiditic system, inducing a relative starvation of the system. Despite the simplicity of the parameters, this simulation provides insights into the distribution and morphology of the Petit-Rhone deep-sea fan, which can be applied to conceptual and quantitative models of large-scale turbiditic system architecture.
Conclusion
Stratigraphic forward models such as Dionisos are powerful tools to simulate in three dimensions the interaction between climatic, tectonic and sedimentary processes. The evolution of sedimentary basins can now be investigated from source to sink, and a series of questions on complex sedimentary systems can be approached such as: how evaporite interfingers with carbonate to form seals and traps? Which is the role of shelf-edge deltas as sand staging areas between fluvial sources and deep-water sinks (e.g. Burgess and Steel, 2008, Somme et al, 2009)? Such a modeling allows us to test different sequence stratigraphic conceptual models, and thus to reduce uncertainties in the prediction of stratigraphic architecture and sedimentary facies distribution of frontier areas (e.g. Burgess et al, 2006, Alzaga-Ruiz et al, 2009).
References
Alzaga-Ruiz, H., Granjeon, D., Lopez, M., Seranne, M., Roure, F., 2009. Gravitational collapse and Neogene sediment transfert across the western margin of the Gulf of Mexico: insights from numerical models. Tectonophysics 470, n. 1-2, 21-41
Burgess, P.M., Lammers, H., Van Oosterhout, C., Granjeon, D, 2006. Multivariate sequence stratigraphy: tackling complexity and uncertainty with stratigraphic forward modeling, multiple scenarios and conditional frequency maps. AAPG Bulletin 90, 1883-1901.
Burgess, P.M., and Steel, R., 2008, Stratigraphic forward modeling of basin margin clinoform systems: Implications for controls on topset and shelf width and timing of formation of shelf-edge deltas: SEPM Sp. Pub., v. 90, p. 35–45.
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Granjeon, D., 1996. Modelisation stratigraphique deterministe—conception et applications d’un modele diffusif 3D multilithologique. Memoires Geosciences Rennes, PhD Dissertation. Geosciences Rennes, Rennes, France, 189 pp.
Granjeon, D., Joseph, P., 1999. Concepts and applications of a 3-D multiple lithology, diffusive model in stratigraphic modeling. Numerical experiments in stratigraphy: recent advances in stratigraphic and sedimentologic computer simulations. SEPM Spec. Publ. 62, 197– 210.
Granjeon, D., 2009. 3D stratigraphic modeling of sedimentary basins. AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009.
Somme, T.O., Helland-Hansen, W., Granjeon, D., 2009. Impact of eustatic amplitude variations on shelf morphology, sediment dispersal, and sequence stratigraphic interpretation: Icehouse versus greenhouse systems. Geology 37, 587-590.