PASSIVE MARGIN STRATIGRAPHY FOR NUMERICAL MODELS CALIBRATION: COEFFICIENT OF DIFFUSION MEASUREMENT IN THE OGOOUÉ DELTA (GABON)
Abstract
One major and undepreciated point in diffusion process-based stratigraphic modelling is the large range
of coefficients of diffusion used to reproduce natural examples without considering their meaning and their
validity in term of transport and deposition processes. Most of the time, present-day stratigraphic models are
not used as real forward models but more as a tool to make semi-inversion based on a “best-fit” approach to
reproduce well-constrained sedimentary systems.
The aim of this work is to consolidate inputs of stratigraphic numerical modelling by calibrations based
a natural passive margin deltaic system: the relative small and structurally well delimited Cenozoic post-rift
Ogooué Delta in Gabon. From seismic and wells analyses, this system evolves from Paleogene aggrading ramp
to a significant deltaic progradation resulting from a major Lower Miocene uplift.
Calculations from high resolution seismic stratigraphy and wells analysis are performed in three steps:
(1) measurement of the sand/shale ratio evolution (calibrated on wells) along the depositional profile; (2)
restoration of the slope at time of deposition (including correction of differential compaction); (3) calculation
of depositional fluxes using a basin-scale age model at the highest time resolution possible and quantification
of uncompacted volumes for each time slice and stratigraphic context considered (ratio between
accommodation and sedimentation).
Diffusion coefficient values range over one order of magnitude (0.1 to 1 km2/Ka), that is to say, show
less variability than values classically used in published diffusion processed-based stratigraphic modelling
(x0.0001 to x10 km2/Ka). These results also suggest (1) low influence of stratigraphic context on coefficient
of diffusion values, (2) higher values of coefficients for the most proximal (shelf) and distal (basin floor) parts
of the depositional profile and (2) an increase of coefficient values for clayey dominated facies.