Abstract
In exposed detachment-folded Lisburne Group carbonates, field evidence and statistical analysis suggest that a significant population of fractures post-date folding.
Both prefold fractures and penetrative strain associated with peak folding are overprinted by late-folding and post-folding fractures. The fractures in the first set strike EW parallel to the fold axis, are perpendicular to bedding, are often only partially filled with cement, and generally terminate at bedding planes. These and the earlier structures are consistently overprinted by pervasive late NS extension fractures. The later fractures strike perpendicular to the fold axes, are vertically extensive, evenly spaced and unfilled. Both have similar average and median spacings.
Statistical analysis of fold angle and fracture spacing indicates that, as the folds tighten, both the EW and NS fracture spacing increases by a factor of two or three and becomes 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 and open fractures serves as an important example with major consequences for reservoir modelling. 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, genetically disparate fractures may be combined into one or a few sets to produce a reservoir model with fracture properties which do not apply to any of the sets. This could result in inappropriate wellbore placement or inaccurate productivity and recovery estimates.
