Insights from Geomechanical Modelling of 2D Cross-Sections in Fold-Thrust Belt Structures

Abstract

A holistic approach to structural model validation in fold-thrust belts (FTB) by integrating geomechanical modelling demonstrates the value of including this component into the balancing/restoration workflow. The approach enables plausible structural models to be tested with consideration of the mechanical stratigraphy and the fault framework. The workflow improves interpretation confidence in areas of poor quality seismic. Understanding the complex subsurface geometries of FTB typically involves geometrical restoration techniques to validate 2D seismic interpretation. Although these methods involve well-established geometric rules based on field and subsurface studies, they do not include any consideration of the elastic moduli and physical properties of the rocks. Geomechanical tools, such as Dynel 2D, enable investigation of rock behaviour under applied stress, the strain distribution and the resulting geometries. Dynel 2D uses Finite Element Modelling (FEM) code to include physical rock properties to assess: (a) The strain distribution throughout the rock mass as it deforms from lithostatic pressure and applied lateral compressive stress, (b) The differing response and interaction between contrasting mechanical stratigraphy, and; (c) How this behaviour influences faulting and folding styles which govern the overall structural geometries. A natural example of a fault-propagation fold typically found in a FTB was restored to test the fault framework interpreted. The structure presents small-scale internal deformation produced during its evolution. Structural geologists commonly use natural examples of various scales as analogues for explaining features or processes at much larger scales. A geometrical model approach describes the overall shape of the structure used in this study. This technique may overlook subtle features that when up-scaled, have a larger impact on key horizon geometry and fault complexity. Mapping the strain magnitude and distribution throughout the major structure can provide insights into the position, geometry and extent of the internal deformation. Similar to traditional sandbox models, forward modelling performed by applying a lateral compressive stress to the horizontal stratigraphy, simulates the changes in the strain distribution during each deformation stage. Several models featuring a typical flat-ramp fault geometry were trialled. Each simulation tests a combination of varied elastic properties. This technique reveals how rock types of different mechanical contrasts interact within the fault framework during applied stress. In summary, combining well-established geometrical methods and geomechanical simulation allowed testing of structural evolution scenarios and identifying implausible models.

AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020