Abstract

The Carboniferous Lisburne Group of northern Alaska has been deformed into a variety of map-scale structures in both compressional and extensional structural settings, thus providing a series of natural experiments for observing the formation, distribution, and behavior of fractures in this thick carbonate unit. Two fracture sets dominate the Lisburne Group carbonates of the North Slope subsurface and the nearby northeastern Brooks Range fold and thrust belt. North-northwest-striking regional extension fractures probably formed in front of the northeastern Brooks Range fold and thrust belt. In the North Slope subsurface, this fracture set overprints east-northeast-striking fractures related to earlier extensional deformation; in contrast, in the fold and thrust belt, the north-northwest-striking fracture set is overprinted by younger east-northeast-striking fractures related to subsequent contractional deformation. Lithology is the primary control on the fracture density of both sets. In mildly deformed Lisburne Group carbonates, grainstones are the least fractured and dolomitic mudstones the most fractured. However, structural position plays a greater role in the overall fracture character and density in folded and thrust-faulted Lisburne carbonates. Explorationists in the Lisburne Group and other potentially fracture-controlled or -enhanced carbonate reservoirs need to consider both carbonate lithology and structural position when developing predictive models for fracture development. Fracture character and density can change abruptly across a given area, and even across an individual structure. Rock types less prone to fracturing in the less deformed sections may experience the greatest relative increase in fracture intensity as deformation increases.

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