Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractOverpressure prediction in tectonic environments is a challenging topic. The available pore pressure prediction methods are designed to work in environments where compaction is mostly one dimensional and driven by the vertical effective stress applied by the overburden. Furthermore, the impact of tectonic deformation on stresses, porosity and overpressure is still poorly understood. We use a novel methodology to capture the true compaction phenomena occurring in an evolving 3D stress regime by integrating a fully-coupled geomechanical approach with a critical state constitutive model. To this end, numerical models consisting of 2D plane strain clay columns are developed to account for compaction and overpressure generation during sedimentation and tectonic activity. We demonstrate that a high deviatoric stress is generated in compressional tectonic basins, resulting in a substantial decrease in porosity with continuing overpressure increase.The overpressure predictions from our numerical models are then compared to those estimated by the equivalent depth method (EDM) in order to quantify the error induced when using classical approaches, based on vertical effective stress, in tectonic environments. The stress paths presented here reveal that a deviation from the uniaxial burial trend can substantially reduce the accuracy of the EDM overpressure predictions.
. (2016) 'Stress and pore pressure histories in complex tectonic settings predicted with coupled geomechanical-uid ow models.', Marine and petroleum geology., 76 . pp. 464-477. Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractMost of the methods currently used for pore pressure prediction in sedimentary basins assume one-dimensional compaction based on relationships between vertical effective stress and porosity. These methods may be inaccurate in complex tectonic regimes where stress tensors are variable. Modelling approaches for compaction adopted within the geotechnical field account for both the full three-dimensional stress tensor and the stress history. In this paper a coupled geomechanical-fluid flow model is used, along with an advanced version of the Cam-Clay constitutive model, to investigate stress, pore pressure and porosity in a Gulf of Mexico style mini-basin bounded by salt subjected to lateral deformation. The modelled structure consists of two depocentres separated by a salt diapir. 20% of horizontal shortening synchronous to basin sedimentation is im-posed. An additional model accounting solely for the overpressure generated due to 1D disequilibrium compaction is also defined. The predicted deformation regime in the two depocentres of the mini-basin is one of tectonic lateral compression, in which the horizontal effective stress is higher than the vertical effective stress. In contrast, sediments above the central salt diapir show lateral extension and tectonic vertical compaction due to the rise of the diapir. Compared to the 1D model, the horizontal shortening in the mini-basin increases the predicted present-day overpressure by 50%, from 20 MPa to 30 MPa. The porosities predicted by the mini-basin models are used to perform 1D, porosity-based pore pressure predictions. The 1D method underestimated overpressure by up to 6 MPa at 3400 m depth (26% of the total overpressure) in the well located at the basin depocentre and up to 3 MPa at 1900 m depth (34% of the total overpressure) in the well located above the salt diapir. The results show how 2D/3D methods are required to accurately predict overpressure in regions in which tectonic stresses are important.
We present coupled, critical state, geomechanical‐fluid flow simulations of the evolution of a fold‐and‐thrust belt in NW Borneo. Our modeling is the first to include the effects of both syntectonic sedimentation and transient pore pressure on the development of a fold‐and‐thrust belt. The present‐day structure predicted by the model contains the key first‐order structural features observed in the field in terms of thrust fault and anticline architectures. Stress predictions in the sediments show two compressive zones aligned with the shortening direction located at the thrust front and back limb. Between the compressive zones, near to the axial plane of the anticline, the modeled stress field represents an extensional regime. The predicted overpressure distribution is strongly influenced by tectonic compaction, with the maximum values located in the two laterally compressive regions. We compared the results at three notional well locations with their corresponding uniaxial strain models: the 2‐D thrust model predicted porosities which are as much as 7.5% lower at 2.5 km depth and overpressures which are up to 6.4 MPa higher at 3 km depth. These results show that one‐dimensional methods are not sufficient to model the evolution of pore pressure and porosity in contractional settings. Finally, we performed a drained simulation during which pore pressures were kept hydrostatic. The predicted geological structures differ substantially from those of the coupled simulation, with no thrust faulting. These results demonstrate that pore pressure is a key control on structural development.
Forward basin modeling is routinely used in many geological applications, with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented. Here, a new, temperature-dependent, kinetic diagenesis model is formulated and implemented within a hydromechanical framework. The model simulates the macroscopic effects of diagenesis on (1) porosity loss, (2) sediment strength, (3) sediment stiffness and compressibility, (4) change in elastic properties, (5) increase in tensile strength due to cementation, and (6) overpressure generation. A brief overview of the main diagenetic reactions relevant to basin modeling is presented and the model calibration procedure is demonstrated using published data for the Kimmeridge Clay Formation. The calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation. The incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 MPa due to both stress-independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability. Finally, a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behavior to be captured due to the increase in strength that results in an overconsolidated stress state. This is illustrated by comparison of the present-day structures predicted by a geomechanical-only model, where a ductile fold forms, and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted.
Many sedimentary basins host important reserves of exploitable energy resources. Understanding of the present-day state of stresses, porosity, overpressure and geometric configuration is essential in order to minimize production costs and enhance safety in operations. The data that can be measured from the field is, however, limited and at a non-optimal resolution. Structural restoration (inverse modelling of past deformation) is often used to validate structural interpretations from seismic data. In addition, it provides the undeformed state of the basin, which is a pre-requisite to understanding fluid migration or to perform forward simulations. Here, we present a workflow that integrates geomechanical-based structural restoration and forward geomechanical modelling in a finite element framework. The geometry and the boundary kinematics derived from restoration are used to automatically create a forward geomechanical model. Iterative correction may then be performed by either modifying the assumptions of the restoration or modifying the restoration-derived boundary conditions in the forward model. The methodology is applied to two problems; firstly, a sand-box scale benchmark model consisting of sand sediments sliding on silicon leading to the formation of a graben structure; secondly, a field-scale thrust-related anticline from Niger Delta. Two strategies to provide further constraint on fault development in the restoration-derived forward simulation are also presented. It is shown that the workflow reproduces the first order structural features observed in the target geometry. Furthermore, it is demonstrated that the iterative approach provides improved understanding of the evolution and additional information of current-day stress and material state for the Niger Delta Case.
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