Many have attributed the success of hydraulic fracturing in unconventionals to the development of increased reservoir contact area through complex fracture networks with significant shear-enhancement of flow on preexisting natural fractures. Core samples recovered in recent field experiments have revealed large numbers of subparallel hydraulic fractures (swarms) that provide an alternative explanation for the enhanced stimulated surface area. We argue that understanding the mechanisms behind the development of this stimulated surface area is key to optimizing the stimulation of unconventional resources. We investigate potential phenomena that could lead to fracture swarming. First, we consider whether "swarming" is indeed a correct term. We show that the null hypothesis that the hydraulic fractures are actually randomly placed cannot be rejected with significant confidence. We next investigate several mechanical explanations for the genesis of closely-spaced fractures (branching, poromechanics, variations in situ stress orientation, and near wellbore effects associated with perforations). Our analysis eliminated all mechanisms other than near wellbore effects to be the primary cause for the observed closely spaced hydraulic fractures.

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