4D Fracture Distribution as a Signature of Structural Evolution in the Otway Basin
Hugo Bonython BURGIN
Bachelor of Science (Petroleum Geology and Geophysics) 2015
Australian School of Petroleum
The University of Adelaide
An understanding of the distribution of natural fractures throughout time and space within sedimentary basins can enable the optimisation of exploration and production methods. Additionally insights into the structural and tectonic evolution of a region can also be obtained.
Natural fractures within the Otway Basin, Australia, have been identified via wellbore image log data (from the Penola Trough) and outcrop analysis (along the Great Ocean Road), and have been catalogued with regard to their direction of dip, degree of dip and inferred chronological regime of paleo-stress.
Evidence for up to four paleo-tectonic stress regimes within the study area have been identified, with 8 individual fracture sets detected in well data and 6 sets detected from outcrop analysis at twelve localities within the study area. Once detected, each individual fracture set was assigned a conjugate pair based upon an inferred paleo-stress regime. Following the integration of well and field data sets, relative chronology between conjugate fracture pairs was determined via: the presence or absence of fracture sets within consecutive stratigraphic units, cross cutting relationships observed in the field and bedding / fracture orientation relationships.
The result of this study was a more complete understanding of the history of regional tectonics within the Otway Basin. In situ stress was found to be approximately NW-SE supporting earlier work by Hillis and Reynolds (2002) and Bailey et al. (2014). Initial extension within the Otway Basin was in all likelihood orientated NE-SW, occurring over a prolonged period (Berriasian-Albian) during the Early Cretaceous. This is supported by other studies including Teasdale et al. (2003) and Perincek and Cockshell (1995). Additionally, basin inversion within the Otway is proposed to have occurred during the Mid-Cretaceous (Cenomanian-Turonian), orientated NE-SW followed by renewed extension in a NW-SE orientation. Lastly a change to a NW-SE compressional regime from the Eocene-Recent is also proposed and supported by earlier works including Holford et al. 2014.