Polygonal faults in the Ormen Lange Field, Møre Basin, offshore Mid Norway

Abstract

3D seismic and well data from the Ormen Lange Field, Mid Norway have been used to analyse the development of a system of polygonal faults affecting the Late Cretaceous-early Paleocene reservoir. These faults have the typical properties of polygonal fault systems recognized elsewhere in mainly fine-grained successions. They grew by upward propagation from the thick, shale-prone interval of the Late Cretaceous in the MOre Basin and were reactivated during the deposition of the Balder Formation. They have throws ranging from a few metres to 80 m, are typically 1-3 km in length and have highly irregular throw distributions along strike, mainly as a result of complex fault intersection geometries. The Ormen Lange Field is the first described example of polygonal faults that completely transect a major sandstone reservoir interval. The presence of these faults has important implications for the likely production behaviour of the field. Fault seal analysis shows that they are unlikely to form juxtaposition seals, except locally, but that they may have a significant risk for clay smear seals, particularly in the lower reservoir unit.

Displacement field in contraction‐driven faults

“…In typical blind normal faults and polygonal faults, the displacement patterns are commonly symmetric with depth. Notable exceptions are when a weak layer is present near the base or where a shallow propagating fault approaches the free surface (sediment-water interface); in these cases, a significant increase in local displacement gradients is generally observed [e.g., Watterson et al, 2000;Nicol et al, 2003;Stuevold et al, 2003].…”

“…When observed close to their original position i.e., with their upper tips close to the modern seabed, they are generally planar (e.g., Figure 1a), and exhibit a range of fault plane dips from 50 to 80° . Once buried and inactive, they are passively flattened by vertical compaction, and have much shallower dips, often in the range of 30-50° [Lonergan et al, 1998;Stuevold et al, 2003]. In planform, they can be linear or highly curved, reflecting variations in lithology or mechanical interactions between neighboring faults during propagation [Lonergan et al, 1998;Goulty, 2008].…”

“…The second model applies to nearsurface faults and the stiffness at depth z is assumed to be linearly dependent on the mean stress at that depth (linearity with effective stress applies to large strain processes such as those modeled here [Terzaghi and Peck, 1967]). This second case is intended to simulate the case where polygonal faults initiate within a few tens of meters of burial, and propagate to the free surface, where they subsequently behave as small syn-sedimentary faults, displacing the sediment-water interface (see examples in the work by Stuevold et al [2003]). Results for both end-members are presented below together with a modification to incorporate addition of new load due to continued sedimentation during fault propagation.…”

“…This diagenetically "coupled" model for physical property evolution and stress development then offers an explanation for the fundamental and defining characteristic of polygonal fault systems, namely their polygonal planform geometry. To develop a truly polygonal system with the intersection relationships that are universally observed , it is clearly necessary that some faults nucleate and propagate earlier than others, and indeed the displacement transfer that occurs at intersections testifies to this simple principle [Stuevold et al, 2003]. It follows from this that as one fault is reaching almost full propagation, and accumulating strain accordingly, others nearby are only just initiating.…”

“…Polygonal faults also represent a major challenge for soil mechanics: they are an expression of shear failure, hence the stress path cannot follow the classical 1D k 0 -stress condition that would prevail under zero-lateral strain conditions. Polygonal faults occur widely in passive continental margin basins where hydrate resources are contained [e.g., Berndt et al, 2003], and occur pervasively in sediments that constitute the major sealing sequences for petroleum accumulations [e.g., Watterson et al, 2000;Stuevold et al, 2003;Cartwright et al, 2007]. Therefore, the understanding of polygonal fault systems can be of crucial importance.…”