Mechanical restoration of gravity-driven deformations using Limit Analysis theory: the Baram delta in NW Borneo.

Josselin Berthelon and Antoine Bouziat and Xiaoping Yuan and Bertrand Maillot and Pauline Souloumiac and M-C. Cacas and T. Cornu. ( 2016 )
in: 2016 RING Meeting, ASGA

Abstract

The Baram Delta System, Brunei, north-west Borneo, is a Tertiary delta system located on an dead continental margin. The margin failure is also accommodated by proximal extension and distal shortening corresponding to gravity spreading above a Miocene shale decollement. Standard petroleum system models in such contexts use geometrical structural restorations as input data to provide the sequential steps of basin configuration. The accuracy of these geological scenarios is strongly dependent on the quantity and quality of the available data. For example, in the offshore Brunei, seismic resolution and well- correlations are lacking to describe the calendar of the late Pliocene-Pleistocene gliding deformations. Because folds and thrusts emplacements are strongly linked to the pore pressure, stress and fluid content, it is critical to reduce structural and kinematic uncertainties. As such, it has long been suggested that kinematic structural restoration must respect some mechanical criteria in addition to standard geometric ones. In this work, we aim to restore the geological evolution of the Plio-Pleistocene Baram Delta, respecting mechanical equilibrium, volume conservation and the maximum rock strength. This gravity- driven deformation is constrained using the kinematic approach of limit analysis (SLAMTec), considering geological materials as Coulomb frictional materials. SLAMTec is a forward method that predicts at each increment of shortening and/or sedimentation an optimum thrust-fold emplacement and updates the basin geometry using a set of geometrical rules. Various geological parameters are available to reproduce the diversity of tectonic structures, such as sedimentation, material softening and fluid pressure, while the efficiency of the algorithm eases testing large varieties of initial geometries and parameters. Boundary conditions and mechanical properties of the Brunei margin are determined from available literature and two interpreted seismic profiles. Several hundred of simulations are run to explore the influence of the pore pressure and mechanical parameters on the geometry and kinematic of the Plio-Pleistocene gliding deformations. It gives insights into the mechanics of the Baram Delta System, as several relationships can be discussed: importance of fluids overpressure to create the conditions for gravity gliding, shelf break progradation vs position of collapsing normal faults, evolutionary mechanical parameters to explain the present-day geometry of the margin, etc. Finally, eight quantitative criteria are defined to grade the simulated models compared to the seismic profiles; these are used to point the models that match the best the interpreted seismic sections, in a proto-inversion attempt. The lessons learned from this work are used to conceive a “mechanically balanced” structural restoration which integrates first-order information on the fluids pressure and mechanical parameters that controlled its evolution.

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BibTeX Reference

@INPROCEEDINGS{,
    author = { Berthelon, Josselin and Bouziat, Antoine and Yuan, Xiaoping and Maillot, Bertrand and Souloumiac, Pauline and Cacas, M-C. and Cornu, T. },
     title = { Mechanical restoration of gravity-driven deformations using Limit Analysis theory: the Baram delta in NW Borneo. },
 booktitle = { 2016 RING Meeting },
      year = { 2016 },
 publisher = { ASGA },
  abstract = { The Baram Delta System, Brunei, north-west Borneo, is a Tertiary delta system located on an
dead continental margin. The margin failure is also accommodated by proximal extension
and distal shortening corresponding to gravity spreading above a Miocene shale
decollement. Standard petroleum system models in such contexts use geometrical structural
restorations as input data to provide the sequential steps of basin configuration. The
accuracy of these geological scenarios is strongly dependent on the quantity and quality of
the available data. For example, in the offshore Brunei, seismic resolution and well-
correlations are lacking to describe the calendar of the late Pliocene-Pleistocene gliding
deformations. Because folds and thrusts emplacements are strongly linked to the pore
pressure, stress and fluid content, it is critical to reduce structural and kinematic
uncertainties. As such, it has long been suggested that kinematic structural restoration must
respect some mechanical criteria in addition to standard geometric ones. In this work, we
aim to restore the geological evolution of the Plio-Pleistocene Baram Delta, respecting
mechanical equilibrium, volume conservation and the maximum rock strength. This gravity-
driven deformation is constrained using the kinematic approach of limit analysis (SLAMTec),
considering geological materials as Coulomb frictional materials. SLAMTec is a forward
method that predicts at each increment of shortening and/or sedimentation an optimum
thrust-fold emplacement and updates the basin geometry using a set of geometrical rules.
Various geological parameters are available to reproduce the diversity of tectonic structures,
such as sedimentation, material softening and fluid pressure, while the efficiency of the
algorithm eases testing large varieties of initial geometries and parameters. Boundary
conditions and mechanical properties of the Brunei margin are determined from available
literature and two interpreted seismic profiles. Several hundred of simulations are run to
explore the influence of the pore pressure and mechanical parameters on the geometry and
kinematic of the Plio-Pleistocene gliding deformations. It gives insights into the mechanics of
the Baram Delta System, as several relationships can be discussed: importance of fluids
overpressure to create the conditions for gravity gliding, shelf break progradation vs position
of collapsing normal faults, evolutionary mechanical parameters to explain the present-day
geometry of the margin, etc. Finally, eight quantitative criteria are defined to grade the
simulated models compared to the seismic profiles; these are used to point the models that
match the best the interpreted seismic sections, in a proto-inversion attempt. The lessons
learned from this work are used to conceive a “mechanically balanced” structural
restoration which integrates first-order information on the fluids pressure and mechanical
parameters that controlled its evolution. }
}