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The Diagenesis Model in ParaGeo

During basin history, from initial deposition to present day state at depth, basin sediments experience both mechanical and non-mechanical processes that have an impact on their properties. Porosity decrease due to mechanical compaction is a result of the increasing stresses with burial and tectonic deformation. On the other hand non-mechanical processes such as diagenesis may also lead to porosity and permeability decrease as well as an increase in strength.

In our paper “A Diagenesis Model for Geomechanical Simulations: Formulation and Implications for Pore Pressure and Development of Geological Structures” we present the diagenesis model available in ParaGeo and we show the potential implications of diagenesis in over pressure generation and in the predicted structural style due to the ductile – brittle transition lead by the diagenetic overprint on geomechanical properties. The model is formulated in a modular manner enabling flexibility in terms to whether or not to incorporate certain effects of diagenesis in sediment mechanical properties. The different model components are described as follows:

  1. The kinetic reaction component defines the rate of diagenetic porosity decrease which is formulated via either an exponential or power law Arrhenius type equation that is a function of temperature

  2. The compaction model defines the increase in sediment stiffness and strength due to the diagenetic compaction via three factors:

    1. Increase in the pre-consolidation pressure leading to a yield surface size increase

    2. Decrease in Lambda, reducing further sediment mechanical compressibility

    3. Decrease in kappa, leading to an increase in sediment Bulk and Young’s modulus (increased stiffness)

  3. The cementation model defines an increase in pt (tensile strength) due to the diagenesis overprint.

In such model each diagenetic reaction is characterized by a threshold temperature above which the diagenetic porosity loss starts to occur. Each reaction has also a maximum allowable porosity change and a corresponding maximum reduction in lambda and kappa. Furthermore more than one reaction with different threshold temperatures may be assigned to any material (e.g. we may model shallow early carbonate cementation and deeper smectite to illite transformation in the same formation).

In the paper we demonstrate the calibration of the model using published experimental data for kimmeridge clay. The geomechanical experiments from Nygard et al. (2004a, 2004b, 2006) performed on two sets of samples of collected at two locations with different burial histories and different maximum burial depths before being uplifted enabled to derive properties for the assumed non-diagenetically modified sediments (Kimmeridge Westbury Clay) and the diagenetically modified sediment (Kimmeridge Bay Clay). Then we calibrate the diagenesis model in ParaGeo to capture the transition between both. We also calibrate a diagenesis reaction for Berea sandstone so that after simulating its burial history we capture the transition from a sand to the present day consolidated and cemented properties.

A diagenesis reaction was calibrated for Kimmeridge Clay so that initially the material has properties consistent with Kimmeridge Westbury Clay (KWC) data and after diagenesis a behaviour consistent with Kimmeridge Bay Clay (KBC) data is captured
Simulation of Kimmeridge Clay history

Published data for Bera Sandstone (Wong et al 1997) is used to calibrate the SR4 model parameters and define the yield surface in the p`-q space
Calibration of yield surface for Berea Sandstone

We then show the implications of diagenesis in overpressure generation and the predicted structural style in ParaGeo. The simulation of a North Sea scenario previously published by Gutierrez and Wangen (2005) accounting for mechanical compaction shows that the addition of diagenesis increases overpressure due to two different factors:

  1. The overpressure generated by the diagenetic volumetric strain

  2. The additional overpressure generated by disequilibrium compaction due to the lower permeability of the diagenetically modified sediments.

We also simulated the deposition followed by tectonic deformation in a thrust using the previously characterized materials. We considered two cases, one considering mechanical compaction only and another including the effects of diagenesis. Such example illustrates how the diagenesis may induce a transition from ductile to brittle behaviors due to the diagenesis-induced over consolidation.

The simulation consisted on a deposition stage followed by a stage of tectonic shortening. Material characterisations previously calibrated for Kimmeridge Clay and Berea sandstone are used. Two cases were considered; only aconting for diagenesis and the other acounting solely for mechanical compaction.
Predicted structure in a compressional model for cases with and without diagenesis


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