Deformation and uplift in a Late Cainozoic fold belt, eastern Otway Basin: implications for fault reactivation and petroleum potential
thesisposted on 2017-03-02, 23:21 authored by Matthews, Jack
The aim of this project was to obtain insight into the Neogene (late Cainozoic) deformation and uplift of the Basal Tertiary Unconformity in the eastern Otway Basin. Late Cainozoic compression, resulting from an intraplate boundary stress field has produced a fold belt ~ 70km wide from the Colac Trough, across the Otway Ranges and into the Torquay Sub-basin. Structure maps of the Basal Tertiary Unconformity were constructed in order to highlight the nature of faults and folds in the region, using depth converted 2D seismic reflection data, as well as petroleum and borehole data. Seismic interpretation and mapping of the Top of Palaeozoic Basement and Intra-Oligocene Unconformity was also undertaken. 2D seismic cross sections were used to show the structural features of the fold belt, specifically faults that have reactivated with reverse movement. Structural restoration of a depth converted seismic profile was used to determine the amount of shortening that has occurred due to Neogene compression. The petroleum potential of the region was also examined, given the lack of exploration success in the eastern Otway Basin. ENE trending early Cretaceous normal faults that developed during the rifting stage of basin development have reactivated with reverse movement due to compression in the late Cainozoic, which has caused deformation and uplift. The maximum vertical variation in the position of the Basal Tertiary Unconformity is approximately 2000m, which suggests there has been up to 2000m of uplift in the fold belt, consistent with previous thermal studies, and with the most uplift and erosion occurring in the central Otway Ranges, where the Cainozoic sediments have mostly been eroded and the Early Cretaceous Eumeralla Formation is exposed. Restoration of a structural cross section has found approximately 0.7% crustal shortening of the Intra-Oligocene Unconformity, and approximately 5% crustal shortening of the Base Tertiary Unconformity, values consistent with previous structural studies. Hydrocarbon exploration in the eastern Otway Basin has been largely unsuccessful, and relates to the deformation and uplift observed in this study. Burial histories and thermal data reveal hydrocarbons were generated in Early Cretaceous source rocks, however, uplift and cooling of source rocks effectively ceased hydrocarbon generation, and subsequent burial coupled with a lower geothermal gradient has not provided enough heat to generate a second phase of migration. Also, many of the petroleum traps targeted formed in the Neogene, and post-dated the main phase of hydrocarbon generation in the Early Cretaceous. Preferential reactivation of steeply dipping >60º Early Cretaceous faults has occurred around the Otway Ranges and central region of the fold belt. Fault displacement and deformation of the Basal Tertiary Unconformity is greatest in the middle, then diminishes from either side to the NW and SE. Fault reactivation is influenced by various factors, and it is clear that fault reactivation has undoubtedly played an important role in the formation of the fold belt.