An introduction to the Special Issue of Marine and Petroleum Geology: Fluid-rock-tectonics interactions in basins and orogens Introduction

1. Introduction Fluid flow is a first-order feature of the geodynamic evolution of basins and orogens. Fluids interact with rocks from the earliest stages of sedimentation until these rocks are deformed and/or metamorphosed and then possibly exhumed. Fluids are major contributors to mineralization and ore deposition, hence to resources. The interactions between fluids and rocks lead to the evolution of rock physical properties hence affect hydrocarbon potential (e.g.,in porous media/reservoirs). Fractures and faults are preferred pathways for fluids, and in turn physical and chemical interactions between fluid flow and tectonic structures, such as fault zones, strongly influence the mechanical behaviour of the crust at different space and time scales. Where in the past attentionwas paid to specific diagenetic products and processes, such as for example hydrothermal dolomitisation or dickite formation, today diagenesis is more often placed in the geodynamic evolution of the area under investigation. Thus a change from a more static towards a more dynamic approach took place as exemplified by the concept of “structural diagenesis” (Laubach et al., 2010). In this context one of the challenges remains to reconstruct the evolution of the Pressure (P), Volume (V), Temperature (T), chemical composition (X) through time (t) during deformational stages such as the development of a foreland foldand- thrust belt (Roure et al., 2011). New analytical as well as numerical tools were developed that helped unravel the fluide rocketectonics relationships. Furthermore over the last years research results became more quantitative and precise, e.g. by the use of Lidar imaging techniques (with or without hyperspectral analysis; Kurz et al., 2011; Buckley et al., 2013) to acquire large datasets for example on fracture patterns or distribution of dolomite bodies, or by the application of computed tomography as a 3D quantitative petrographical technique. Also in the field of statistical analysis of data the development of multiple point geostatistics (Jung et al., 2013) set new avenues to explore for the future. With respect to analytical development of crush e leach analysis (Gleeson and Turner, 2007) and the use of laser ablation fluid inclusion analysis, stable isotope analysis of fluid inclusions, microprobe analysis of individual cement phases as well as micro-milling of individual diagenetic products for isotope studies are new developments that allowed to acquire unique datasets that help unravel the fluid-rock interactions. Another emerging field deals with the use of clumped isotopes to infer the temperatures at which specific diagenetic reactions took place (Huntington et al., 2011). Finally the use of noble gases as geochemical tracers to understand the multiphase crustal fluid system useful to identify hydrocarbons and fluid migration from mantle into the crust and across the crust (Burnard,2013). In basins and orogens, fluid-rock interactions occur in relation to tectonic structures such as folds, diffuse fracture sets and fault zones. A complex interplay exists in folds between fluids, (mechanical)stratigraphy, stages of fold development and fold style (Evans and Fischer, 2012), and many studies have focused on the changes in fluid PeTeX conditions during folding (e.g. Bradbury and Woodwell, 1987; Evans, 1995; Beaudoin et al., 2011). Despite some (but few) attempts at upscaling local fluid flow reconstructions to the scale of the basin or the fold-and-thrust belt (e.g.,Trave et al., 2007; Beaudoin et al., 2014), the variability of the fluid system at the scale of the individual fold makes it to date still difficult to extrapolate local reconstructions and to build reliable basin/fold-and-thrust belt scale paleohydrogeological models. Fluids exert an effect on crustal rock strength. They for instance permit pressure solution and chemical reactions to occur, thus stiffening the rock by depositing cements and/or weakening it through development of low-strength hydrated mineral phases such as phyllosilicates, and they reduce rock strength by overpressure.Conversely, the structural and stratigraphic permeability architecture influences fluid migration pathways. In recent years our understanding of fluid flow through faults has been improved through the combination of laboratory measurements of fluid flow properties of natural and synthetic fault rocks and observations in nature of migrating seismicity at depth, shallow reservoir-induced seismicity and in situ behaviour of geothermal systems. The permeability of a fault zone, both along-plane and across-plane, is controlled by the permeability of the individual fault rocks/fractures and by their 3D geometric architecture (Lunn et al., 2008). Open fractures and slip surfaces have a permeability governed by their aperture distribution (which is in turn influenced by their orientation with respect to the (local) stress field) and by their healing capacity in relation to the fluid PeTeX properties, but their influence on the permeability of the fault zone strongly depends on their connectivity and their ability to cut across lower permeability units at the time of fault activity.status: publishe