--> New Insights Into the Contribution of Sub-Seismic Faults to Deformation in the Earth's Crust

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New Insights Into the Contribution of Sub-Seismic Faults to Deformation in the Earth's Crust

Abstract

Fault imaging techniques, such as seismic reflection surveys, are limited in their spatial extent and resolution. This results in small faults and fractures (sub-seismic) being missed in low resolution datasets, which is particularly important when using conventional seismic reflection surveys as these typically only detect faults down to 10-20m displacement. A consequence is that the majority of fault intensity is not observed and studies will underestimate extension within basins and reservoirs. Furthermore, the abundance of these sub-seismic faults and fractures control important rock properties, such as porosity and permeability, so this can affect predictions of fracture controlled properties within reservoirs. The amount of extension accommodated by sub-seismic faults and fractures is widely debated and difficult to accurately quantify. In this contribution we use a compilation of different fault datasets (normal and strike-slip faults; low and high strain regions) that provide direct measurement of extension on structures across a wide range of resolutions (0.1-100m displacement). These provide a complete record of extension on faults across the resolution cut-off of many seismic reflection datasets (i.e. ∼10m displacement). Thus we focus on comparing the extension on sub-seismic faults (<10m displacement) with seismically resolvable large faults (10m displacement). This radical approach vastly improves on previous studies that rely heavily on the application of general scaling laws to limited fault size data in order to extrapolate extension down scale. Results highlight fundamental relationships that indicate the proportion of total extension on sub-seismic faults changes as strain increases. In low strain regions sub-seismic faults are of greater importance, whereas in high strain regions the majority of extension is accommodated by large seismically resolvable faults. This can be explained by strain localization onto larger structures as a fault population grows resulting in sub-seismic faults generating a background strain. We demonstrate the application of a potential ‘universal’ relationship that provides a new method for calculating total extension from observable extension on seismically resolvable faults. This will improve predictions of fracture controlled properties within reservoirs as well as reconcile estimates of extension within basins and reservoirs.