AAPG Annual Convention and Exhibition

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How Can We Use Tectonic Reactivation to Control Stress Distribution at Reservoir and Basin Scale? Learning from Paleopiezometry and Analogues


Reactivation is a key tectonic process from several reasons. For operations, several applications based on the concept of critically stressed fractures have been developed in the last decade concerning reservoir management and seal integrity analysis. More fundamentally, the Byerlee law is still a reference that is used to control the mechanical behavior of the upper crust. It is generally used assuming that properly oriented planes of weakness are available in the rock mass and that therefore reactivation governs crustal strength. In addition, reactivation is studied at various scales from stress tensor inversion at small-scale to trap formation by basin scale fault reactivation. The objective of this presentation is to share some key learnings about reactivation and their consequences on our understanding of stress distribution. At small scale, reactivation can be directly observed as superimposed slickensides on faults or calcite twinning for example. These observations have been used to propose various methods to define paleo-stress. This point is discussed using recent results of paleo-stress history quantification from calcite twins with application to the evolution of the Appalachian foreland basin. At reservoir scale, we will demonstrate how the reactivation of fractures by folding can control the properties of fracture network. The chosen outcrop analogue shows striking similarities with what is expected for the stimulation of a pre-existing fracture network by hydraulic fracturing. This stresses the importance of reactivation at that scale. These successes at small scale have led to overestimate the role of reactivation. This will be demonstrated using analogue model of graben inversion by transpression. The proposed experiments reveal that the fault characteristics are not the key factor to explain the structural evolution of these models. In practice, at basin scale, the occurrence and geometry of decoupling layers (such as salt and overpressure shale) matters much more than reactivation. These examples reveal that during the past decade the study of reactivation has progressively shifted from large-scale tectonics to reservoir characterization. We advocate that it is time to revisit the large scale with the recent advances concerning paleopiezometry, taking into account properly the role of decoupling layers. This should lead to innovative achievements concerning the modeling of stress distribution.