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The implications of tectonic compaction on pore pressure, porosity and stresses

Most of the methods routinely used in the industry to predict pore pressure have an underlying assumption of 1D compaction driven by the overburden stress. However in nature the sediments are exposed to evolving 3D stress states resulting from the combined effects of the overburden weight, tectonic deformation, erosion, etc. Nonetheless the effects of tectonics in sediment properties and overpressure and still poorly understood.

In the paper entitled “Assessing the implications of tectonic compaction on pore pressure using a coupled geomechanical approach” we use ParaGeo to gain insights on the effect of tectonic strain on effective stresses, porosity, overpressure and stress paths. To that end we build a set of 2D plane strain column models that simulate different histories with different values for:

  1. Sedimentation rate prior to the tectonic deformation

  2. Amount of tectonic deformation

  3. Tectonic deformation rate

  4. Sedimentation rate after tectonic deformation

Column model stages

We then have used the porosities predicted by the models in order to apply the equivalent depth method to predict overpressure and quantify the error in each case.

The results shown the importance of accounting for the evolution of the full 3D stress tensor. The stress paths have shown that tectonic compaction leads to a large increase in deviatoric stress which results in compaction at the same time that overpressure increases. Because this is contrary to the disequilibrium compaction principle (i.e. overpressure leads to porosity preservation and hence under compaction) this invalidates usage of 1D methods for overpressure prediction in tectonically active basins.

In this figure the results for three different cases accounting for different sedimentation rates during the deposition stage are shown. Note that results O1, O2 and O3 are results after sedimentation stage (before tectonic deformation is applied)
Results for cases with different sedimentation rates


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