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Inversion structures are now recognized in most orogenic belts and in many other tectonic settings, however, this has not always been the case. For many years the thin-skinned paradigm dominated the interpretation of thrust belts which resulted in many inversion structures being erroneously interpreted as thin-skinned thrusts. In rift basins, intra-cratonic basins and on passive margins inversion structures were often interpreted as having alternative origins, for example, strike-slip deformation. The classic paper of Bally described the geometric characteristics of inversion structures, in which he identified the extensional fault geometry and its subsequent compressional reactivation as essential requirements of an inversion structure. The increased recognition of inversion structures in a wide variety of tectonic settings is illustrated by four hydrocarbon exploration case studies. The geometric characteristics of inversion structures are remarkably consistent irrespective of the tectonic setting and by applying a few simple criteria they can easily be identified. The impact of inversion on hydrocarbon prospectivity is a function of the control on the petroleum system elements from the initial extensional tectonic history and of the later compressional tectonic history which commonly creates new, or modifies old trap configurations.

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Where primary porosity and permeability of a rock are unfavourable for hydrocarbon production, fractures can improve reservoir potential by enhancing permeability. Higher fracture intensity may create a better-connected fracture network, improving fractured-reservoir quality. Investigations into the controls on fracture intensity commonly conclude that either structural or lithological factors have the greatest influence on fracture abundance. We use the Swift Reservoir Anticline in northwestern Montana to investigate how fracture intensity varies throughout the structure and determine that although structural factors do influence fracture intensity, lithology is the main control at outcrop.The Swift Reservoir Anticline exposes bedding surfaces of the Mississippian Castle Reef Formation dolomite. Field data indicates that fracture intensity is highest in the fold forelimb, decreasing into the backlimb except in outcrops of coarse dolomite where fracture intensity is low, regardless of structural position. Field fracture intensity correlates with whole-rock quartz, kaolinite and porosity percentages. We suggest porosity and composition influence bulk-rock mechanical properties, which, in turn, control the fracture intensity at outcrop. Fracture intensity has a stronger relationship with lithological than structural factors, therefore we suggest that the key to predicting fracture intensity in the subsurface here is understanding how lithology varies spatially.

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