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Coupled simulation of a single compressional thrust structure

In the paper “Hydro-mechanical modelling of stress, pore pressure and porosity evolution in fold-and-thrust belt systems” ParaGeo was used to build a thrust model constrained with data for NW Borneo. The model consisted of an initial over consolidated shale wedge over which 5 additional shale layers are deposited during the application of the shortening.

The model predicted a thrust fault with several conjugate backward thrust developed as the thrust hanging wall climbed the fault. A contrast in the deformational regime between the over consolidated initial wedge (brittle behavior) and the under consolidated deposited synthectonic layers (ductile deformation) was observed as only mechanical compaction was considered (over consolidation in the synthectonic layers would be required to predict brittle behaviour).

In the paper we discuss the role of the high pore pressure in the predicted structural style by comparing the solution with a hydrostatic case: the hydrostatic case predicted a ductile anticline with no clear strain localization, as high pore pressures were required to drive the stress path towards the shear side of the yield surface.

In addition to the main model a hydrostatic case was simulated to illustrate the role of overpressure in the predicted structural style. Stress pathts at point p1 are plotted.
Comparison of the predicted structure with a hydrostatic case

In the figure y1 (dotted line) corresponds to the initial yield surface whereas y2 (continuous line) corresponds to the final yield surface
Comparison of stress paths at point p1

Three notional well locations within the synthectonically deposited sediments were selected in order to:

  1. Plot stress paths to illustrate the deformational and stress histories

  2. Compare pore pressure and porosities to their corresponding 1D model

  3. Assess the overpressure underestimation by the Equivalent Depth Method (EDM) when the predicted porosities from the geomechanical model are used as an input

We show that the error in the EDM prediction is positively correlated with the effective stress ratio values for the two wells that are located in compressional regions at present day.

Well 1 and well 3 are in locations which are compressional at present day. Well 2 is in a location that was initially compressional but is extensional at pressent day (anticline location)
Normalized EDM error as a function of the effective stress ratio


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