Otway Basin, structural permeability, Eumeralla Formation, fracture mechanics, fault

Abstract

Over 1900 naturally occurring fractures were recorded in the field, from outcrop of the Eumeralla Formation in the Otway Basin, Victoria, Australia. Two distinctive fracture sets were identified with strike orientations of N-S and NE-SW. Natural fractures were further characterised as open or closed. A total of 623 open fractures in two dominant sets were observed at surface, with mean strikes of N-S and WNW-ESE. A total of 892 closed fractures (generally cemented) in two dominant sets were also observed at surface, also with mean strikes of N-S and WNW-ESE. Further investigation showed that despite a majority of fractures being optimally aligned with the present-day stress field, they remained closed.

Six discrete fracturing events were interpreted in total; Two Cretaceous bedding perpendicular Mode 1 sets; a late Cenozoic vertical Mode 1 set; a late Cenozoic bedding parallel set; a Miocene continuous crack-seal related set; and finally an Early Cretaceous Fault related set, reactivated during the Miocene.

Thin sections revealed multiple generations of fracture cement inferring a crack-seal history or that mineral cement renders the fractures stress insensitive. Failure of fractures to reactivate and remain open to fluid flow in the favourable stress conditions of the Otway Basin has potential to adversely affect and limit the secondary permeability of the system. This also validates the hypothesis that the in-situ stress regime is not always the dominant factor in the propensity for a fracture to be open to fluid flow.