Evidence and Implications for Significant Late and Post-Fold Fracturing on Detachment Folds in the Lisburne Group of the Northeastern Brooks Range

Abstract

Summary In exposed detachment-folded Lisburne Group carbonates, field evidence and statistical analysis suggest that a significant population of fractures postdate folding. Both prefold fractures and penetrative strain associated with peak folding are overprinted by late-folding and post-folding fractures. Late-folding fractures strike east/west, parallel to the fold axes. These, and the earlier structures, are consistently overprinted by pervasive late north/south extension fractures. Both east/west and north/south fracture sets have similar average and median spacings. Statistical analysis of fold angle and fracture spacing indicates that, as the folds tighten, both the east/west and north/south fracture spacings increase by a factor of two or three and become slightly more variable. This behavior is opposite from that expected if the fractures were closely related to folding. It suggests that the two sets are similar to each other and are only weakly affected by the folding. This weak genetic relationship between folding and formation of the most obvious fractures serves as an important example with major consequences for reservoir modeling. Complex genetic and timing relationships between fractures and folds may result in several fracture sets, each having different characteristics (e.g., size, amount of fill, and termination type). Unless recognized, this could result in inappropriate wellbore placement or inaccurate productivity and recovery estimates. Modeling of this fractured system, for example, showed permeability changes greater than 80%, depending on fracture fill, timing, and flow direction.