Timing Constraints on the Structural History of the Western Otway Basin and Implications for Hydrocarbon Prospectivity Around the Morum High, South Australia

Abstract

Reconstructed thermal and structural histories derived from new AFTA Apatite Fission Track Analysis, vitrinite reflectance and (U-Th)/He apatite dating results from the Morum–1 well, Otway Basin, reveal that the Morum High is a mid-Tertiary inversion structure. Uplift and erosion commencing in the Late Paleocene to mid-Eocene (57–40 Ma) removed around 1,500 m of sedimentary section. The eroded section is attributed to the Paleocene- Eocene Wangerrip Group which is considered to have been deposited in a major depocentre in the vicinity of the present Morum High. This depocentre is interpreted to have been one of a number of transtensional basins developed at the margin of the Morum Sub-basin and adjacent to the Tartwaup Hinge Zone and Mussel Fault during the Early Tertiary. The Portland Trough in Victoria represents a similar depocentre in which over 1,500 m of Wangerrip Group section, mostly represented by deltaic sediments of the Early Eocene Dilwyn Formation, is still preserved.Quantification of the maximum paleotemperature profile in Morum–1 immediately prior to Late Paleocene to mid-Eocene inversion shows that the paleo-geothemal gradient at the time was between 21 and 31°C/km, similar to the present-day level of 29°C/km, demonstrating that there has been little change in basal heat flow since the Early Tertiary.Reconstruction of the thermal history at the Trumpet–1 location reveals no evidence for any periods of significant uplift and erosion, demonstrating the relative stability of this part of the Crayfish Platform since the Late Cretaceous.The thermal and burial histories at Morum–1 and Trumpet–1 have been used to calibrate a Temis2D hydrocarbon generation and migration model along seismic line 85-13, encompassing the Crayfish Platform, Morum High and Morum Sub-basin. The model shows the cessation of active hydrocarbon generation from Eumeralla Formation source rocks around the Morum High due to cooling at 45 Ma (within the range 57–40 Ma) resulting from uplift and erosion of a Wangerrip Group basin. There has been almost no hydrocarbon generation from the Eumeralla Formation beneath the Crayfish Platform.Migration of hydrocarbons generated from the Eumeralla Formation began in the Late Cretaceous in the Morum Sub-basin and is predicted to continue to the present day, with the potential for accumulations in suitably placed reservoirs within the Late Cretaceous package both within the Morum Sub-basin and at the southern margin of the Crayfish Platform.

Impacts of Neogene-Recent compressional deformation and uplift on hydrocarbon prospectivity of the passive southern Australian margin

“…This episode of uplift, erosion and elevated heat flow is recorded by AFTA and VR data from the Anglesea-1 well (Green et al, 2004) and is inferred to have occurred at Nerita-1. Although the top-Otway Group unconformity associated with the ~95 Ma event is found throughout the Otway Basin, it is clear that uplift and deformation was far less severe in the Shipwreck Trough and regions further to the west (Duddy, 2003). Whereas the Torquay sub-basin and Otway Ranges likely remained as structural highs, with late Cretaceous sediments thin or absent, in the Shipwreck Trough rifting and sedimentation resumed by the Turonian and continued throughout the late Cretaceous (Duddy, 2003;Schneider et al, 2004).…”

“…The Otway Group sequences are sufficiently organic rich to be considered the primary hydrocarbon source rocks for reservoir targets in the Shipwreck Trough (Sharp and Wood, 2004). The Otway Group is separated by a regional midCretaceous unconformity from the overlying Upper Cretaceous Sherbrook Group (Duddy, 2003). The Sherbrook Group contains the main reservoir (Waarre Formation) and sealing (Flaxman Formation and Belfast Mudstone) horizons in the Shipwreck Trough (Duddy and Erout, 2001;Sharp and Wood, 2004).…”

“…The sedimentary successions of the southern margin become progressively more marine-influenced and calcareous throughout the Cenozoic, reflecting the progressive establishment of open marine circulation (Blevin and Cathro, 2008). Uplift and deformation during the mid-Cretaceous, intra-Maastrichtian and mid-Eocene times are reflected by a number of unconformities (Duddy et al, 2003;Krassay et al, 2004;Totterdell et al, 2004). Only the most recent phase of uplift and deformation which began during the late Miocene and continues to the present-day is addressed in the current study.…”

“…During the early Cretaceous, the Shipwreck Trough and Torquay-sub basin were both part of a major continental rift system that encompassed the Otway, Bass and Gippsland basins (Duddy, 2003). At the end of the early Cretaceous (~95 Ma), some parts of this rift system, including the Torquay sub-basin and the adjacent Otway Ranges experienced major uplift and erosion related to compressional deformation that reactivated earlier extensional faults (Hill et al, 1995;Duddy, 2003). AFTA and VR data from wells and outcrops in the Otway Basin suggest that heat flow at ~95 Ma was approximately double present-day levels, and then declined rapidly, reaching present-day levels at ~80 Ma (Duddy, 2003).…”

“…At the end of the early Cretaceous (~95 Ma), some parts of this rift system, including the Torquay sub-basin and the adjacent Otway Ranges experienced major uplift and erosion related to compressional deformation that reactivated earlier extensional faults (Hill et al, 1995;Duddy, 2003). AFTA and VR data from wells and outcrops in the Otway Basin suggest that heat flow at ~95 Ma was approximately double present-day levels, and then declined rapidly, reaching present-day levels at ~80 Ma (Duddy, 2003). This episode of uplift, erosion and elevated heat flow is recorded by AFTA and VR data from the Anglesea-1 well (Green et al, 2004) and is inferred to have occurred at Nerita-1.…”