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

摘要

In exposed detachment-folded Lisburne Group carbonates,field evidence and statistical analysis suggest that a significantpopulation of fractures post-date folding.Both prefold fractures and penetrative strain associatedwith peak folding are overprinted by late-folding and postfoldingfractures. The fractures in the first set strike EWparallel to the fold axis, are perpendicular to bedding, areoften only partially filled with cement, and generally terminateat bedding planes. These and the earlier structures areconsistently overprinted by pervasive late NS extensionfractures. The later fractures strike perpendicular to the foldaxes, are vertically extensive, evenly spaced and unfilled. Bothhave similar average and median spacings.Statistical analysis of fold angle and fracture spacingindicates that, as the folds tighten, both the EW and NSfracture spacing increases by a factor of two or three andbecomes slightly more variable. This behavior is oppositefrom that expected if the fractures were closely related tofolding. It suggests that the two sets are similar to each otherand are only weakly affected by the folding.This weak genetic relationship between folding andformation of the most obvious and open fractures serves as animportant example with major consequences for reservoirmodelling. Complex genetic and timing relationships betweenfractures and folds may result in several fracture sets, eachhaving different characteristics (e.g., size, amount of fill, andtermination type). Unless recognized, genetically disparatefractures may be combined into one or a few sets to produce areservoir model with fracture properties which do not apply toany of the sets. This could result in inappropriate wellboreplacement or inaccurate productivity and recovery estimates.