Unraveling the Timing of Fluid Migration and Trap Formation in the Brooks Range Foothills: A Key to Discovering Hydrocarbons

Naturally occurring fractures can play a key role in the evolution and producibility of a hydrocarbon accumulation. Understanding the evolution of fractures in the Brooks Range/Colville basin system of northern Alaska is critical to developing a better working model of the hydrocarbon potential of the region. This study addressed this problem by collecting detailed and regional data on fracture distribution and character, structural geometry, temperature, the timing of deformation along the Brooks Range rangefront and adjacent parts of the Colville basin, and the in situ stress distribution within the Colville basin. This new and existing data then were used to develop a model of how fractures evolved in northern Alaska, both spatially and temporally. The results of the study indicate that fractures formed episodically throughout the evolution of northern Alaska, due to a variety of mechanisms. Four distinct fracture sets were observed. The earliest fractures formed in deep parts of the Colville basin and in the underlying Ellesmerian sequence rocks as these rocks experienced compression associated with the growing Brooks Range fold-and-thrust belt. The orientation of these deep basin fractures was controlled by the maximum in situ horizontal stress in the basin at the time of their formation, which was perpendicular to the active Brooks Range thrust front. This orientation stayed consistently NS-striking for most of the early history of the Brooks Range and Colville basin, but changed to NW-striking with the development of the northeastern Brooks Range during the early Tertiary. Subsequent incorporation of these rocks into the fold-and-thrust belt resulted in overprinting of these deep basin fractures by fractures caused by thrusting and related folding. The youngest fractures developed as rocks were uplifted and exposed. While this general order of fracturing remains consistent across the Brooks Range and adjacent Colville basin, the absolute age at any one location varies. Fracturing started in the southwest deep in the stratigraphic section during the Late Jurassic and Early Cretaceous, moving northeastward and upsection as the Colville basin filled from the west. Active fracturing is occurring today in the northeastern parts of the Colville basin, north of the northeastern Brooks thrust front. Across northern Alaska, the early deep basin fractures were probably synchronous with hydrocarbon generation. Initially, these early fractures would have been good migration pathways, but would have been destroyed where subsequently overridden by the advancing Brooks Range fold-and-thrust belt. However, at these locations younger fracture sets related to folding and thrusting could have enhanced reservoir permeability and/or served as vertical migration pathways to overlying structural traps.