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The Forelimb Problem: Testing Models for Fold-Thrust Structures and Their Implication for Trap Geometry and Reservoir Damage


Structural deformation can be critical in determining the success or failure of exploration and development in foreland thrust belts. Establishing structural geometry, especially of the steep fold forelimbs that are difficult to image seismically, is critical for reducing uncertainty and risk on a variety of scales – from defining trap integrity, quantifying trap volumes, assessing juxtaposition of associated faults to forecasting bed-scale damage within reservoirs and seals which will impact hydrocarbon column height, reserve estimation and production rates. Steep fold forelimbs also provide significant challenges when drilling wells including accurate definition of sub-surface targets, directional control of the well-bore and well-bore stability problems; all of these issues can add significantly to the cost of drilling and the risk of having to side-track the well and having a well-bore that enables a successful completion and testing program for the well. Existing approaches apply a narrow range of fold-thrust relationships that make arbitrary choices for strain and fault localization in the forelimbs. Yet our previous research shows that, when faced with interpretation decisions of real structures on excellent seismic data, experts do not use these models but adopt a greater range of structural styles. This suggests that the routine application of the standard spectrum of styles engenders an overly optimistic appreciation of structural risk. The challenge is to decide which structural models are appropriate to given real-world examples, and where they break down – using a greater range of well-exposed outcrop analogues for less-well understood subsurface applications. We present descriptions of forelimb geometries and relaying deformation styles from the Mesozoic carbonate-shale multilayer of the French Subalpine chains and from fold-thrust complexes developed in inter-bedded sandstone-shale (turbidite) multilayers in the French Alps. We show the rapidity of structural variation and illustrate the degree of coherence between the finite results of forward modelling using standard kinematic models and outcrop geometry. These comparisons provide explicit illustrations of the risks associated with the application of current models for predicting trap geometry, possible juxtaposition across forelimb thrusts and broader damage distributions.