3D GEO 3D Victoria Project Department of Primary Industry

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A framework sequence stratigraphic study of the onshore and offshore eastern Otway basin has been conducted as part of the 3D Victoria Initiative, in an attempt to resolve the complex stratigraphy and contradictions and confusions present in the published literature. The results are promising at the present pilot scale, but the full details of the chronostratigraphy have not yet been finalised.

A number of seismic and well database issues have been addressed and partially resolved, but further database clean-up and improvements in general usability are required.

A series of seismic and well facies maps have been produced for the major depositional sequences and a number of interesting sub-sequences identified for further work. These maps provide a useful basis for regional-scale exploration and for the beginnings of prospectivity and play fairway analysis.

In the deepwater, gravity sliding is prominent as post-breakup differential subsidence tilted the margin. Some changes of structural style are also seen in the deepwater but these cannot be resolved at the present scale of study.

The results are presented in terms of petroleum systems and the implications for non-petroleum minerals and resources such as geothermal energy.

Further work is recommended in several areas:-
1) Definitively resolving the lithostratigraphic nomenclature and sequence stratigraphy from the eastern Otway to the Torquay embayment
2) Continuing this work with a further study at finer spatial and temporal scale
3) Additional database consolidation
4) Ties to other nearby basins along the Australian margin
5) Potential deepwater studies
6) Communication and distribution of the results of this present study
7) Consolidation of prior learnings from key individuals

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3D GEO 3D Victoria Project Department of Primary Industry

  1. 1. Otway Basin: stratigraphic and tectonic framework GeoScience Victoria 3D Victoria Report 2. Department of Primary Industries. Prof. Mike Hall & Dr Jeff Keatley 3D Geo Pty Ltd
  2. 2. Bibliographic Reference: HALL, M. & KEATLEY, J. 2009. Otway Basin: stratigraphic and tectonic framework. GeoScience Victoria 3D Victoria Report 2. Department of Primary Industries. © The State of Victoria, Department of Primary Industries, 2009 ISSN 1324-0307 ISBN 978-1-74217-511-9 Keywords: 3D modelling, Otway Basin, cross sections, fence diagram, seismic, regional, sequence stratigraphy, Geological Survey of Victoria. This report may be obtained from: For further information contact: Information Centre 3D Modelling Manager Department of Primary Industries GeoScience Victoria 1 Spring Street Department of Primary Industries Melbourne, VIC 3000 Australia GPO Box 4440 Melbourne, VIC 3001 Australia Telephone: (61 3) 9658 4440 Email: Tim.Rawling@dpi.vic.gov.au Facsimile: (61 3) 9658 4760 Email: information.centre@dpi.vic.gov.au Website: www.dpi.vic.gov.au/minpet/store Authorship and acknowledgements: This report is the result of a collaboration between all of the members of the 3D Geo team and a number of GeoScience Victoria staff. The authors wish to thank all who contributed and in particular Kevin Asquith, Nick Hoffman, Rob Kirk, Alan Tait and Lorenzo D’Auria from 3D Geo and Geoff O’Brien, Peter Tingate, Louise Goldie- Divko, Kusum Gunatillake, Bob Harms, Eddie Frankel, Tim Rawling and Terry Smith from GeoScience Victoria. Disclaimer: This report was prepared for GeoScience Victoria and the 3D Victoria project by Professor Mike Hall and Dr Jeff Keatley of 3D-Geo Pty Ltd on a contract basis. This publication may be of assistance to you, but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequences which may arise from you relying on any information in this publication. All photographs, images, maps, charts, tables and written information in this publication are copyright under the Copyright Act and may not be reproduced by any process whatsoever without the written permission of the Department of Primary Industries. i
  3. 3. EXECUTIVE SUMMARY A framework sequence stratigraphic study of the onshore and offshore eastern Otway basin has been conducted as part of the 3D Victoria Initiative, in an attempt to resolve the complex stratigraphy and contradictions and confusions present in the published literature. The results are promising at the present pilot scale, but the full details of the chronostratigraphy have not yet been finalised. A number of seismic and well database issues have been addressed and partially resolved, but further database clean-up and improvements in general usability are required. A series of seismic and well facies maps have been produced for the major depositional sequences and a number of interesting sub-sequences identified for further work. These maps provide a useful basis for regional-scale exploration and for the beginnings of prospectivity and play fairway analysis. In the deepwater, gravity sliding is prominent as post-breakup differential subsidence tilted the margin. Some changes of structural style are also seen in the deepwater but these cannot be resolved at the present scale of study. The results are presented in terms of petroleum systems and the implications for non-petroleum minerals and resources such as geothermal energy. Further work is recommended in several areas:- 1) Definitively resolving the lithostratigraphic nomenclature and sequence stratigraphy from the eastern Otway to the Torquay embayment 2) Continuing this work with a further study at finer spatial and temporal scale 3) Additional database consolidation 4) Ties to other nearby basins along the Australian margin 5) Potential deepwater studies 6) Communication and distribution of the results of this present study 7) Consolidation of prior learnings from key individuals ii
  4. 4. Contents Executive Summary ii List of Contents iv List of Figures v 1 Introduction and project Overview 1 2 Basin Overview 3 3 Geological Summary 7 4 Late Jurassic-Early Cretaceous 9 5 Late Cretaceous 14 6 Tertiary 24 7 Petroleum Systems 29 8 Non-Petroleum Implications 32 9 Conclusions and 34 10 Recommendations 36 11 References and Further Reading 38 Appendix A Regional Sequence Stratigraphy Study 41 Appendix B Regional Seismic Cross Sections 53 Appendix C Regional Well Defined Facies Maps 59 Appendix D Seismic Sequence Facies Maps 63 Appendix E Seismic Sequence Well Log Picks 67 Appendix F Regional Structural Restoration Sections 68 iii
  5. 5. List of Figures - Main Text Figure 1: Well and Seismic database Figure 2: Plate Tectonics setting Figure 3: Regional structural setting and previous studies Figure 4: Otway Basin structural setting Figure 5. Stratigraphic section showing horizons used to interpret seismic Figure 6: Late Jurassic-Early Cretaceous depocentres Figure 7: Magnetic image showing basement structural trends Figure 8: Gravity image showing basement trends Figure 9: Early Cretaceous half graben Figure 10: Woolsthorpe-1 seismic tie showing Early Cretaceous stratigraphy Figure 11: Early Cretaceous super sequences Figure 12: Late Cretaceous structural elements Figure 13: Late Cretaceous continental margin Figure 14: Formation tops used for regional correlation Figure 15: Well correlation sections Figure 16: Well correlations showing formations and regional variations in log character Figure 17: Well correlation section 1, Prawn-1A to Bridgewater Bay-1 Figure 18: Well correlation section 2, Torquay Sub-Basin, Snail-1 to Warracburunah-2 Figure 19: Well correlation section 3, Armit-1 to Normanby-1 Figure 20: Well correlation section 4, Prawn-1A to Iona-1 Figure 21: Facies map, lower part of Waarre Formation Figure 22: Facies map, Waarre Formation Figure 23: Facies map, Flaxman Formation Figure 24: Facies map, Nullawarre Formation Figure 25: Facies map, Paaratte Formation Figure 26: Facies map, Timboon Formation Figure 27: Tertiary structural - stratigraphic elements Figure 28: Tertiary shelf edge Figure 29: Shelf edge and shallow water canyons Figure 30: Tertiary deep water canyons Figure 31: Late Tertiary fold belt, offshore extension of Otway Ranges Figure 32: Western margin of Late Tertiary fold belt, west of Otway Ranges Figure 33: Eastern margin of Late Tertiary fold belt, Torquay Sub-basin Figure 34: Isoreflector contours showing effect of Late Tertiary uplift Figure 35: Potential DHI downdip of Amrit-1 iv
  6. 6. List of Figures - Appendix A Figure A1: Hardcopy and digital Otway Basin grids Figure A2: Megasequence chronostratigraphy Figure A3: Otway megasequences-facies map, isochron and selected seismic Figure A4: Onshore seismic through Otway megasequences Figure A5: Basal Otway half graben facies Figure A6: Middle Otway half graben facies Figure A7: Upper Otway basinal gravity slide mounds Figure A8: Cretaceous facies maps, isochrons and selected seismic Figure A9: Turonian shallow marine shelf sequences Figure A10: Turonian gravity slide complex Figure A11: Turonian grabens Figure A12: Deepwater Belfast seismic facies Figure A13: Deepwater Belfast facies near Thylacine Figure A14: Belfast submarine fan candidate, downdip from Whelk-1 Figure A15: Sherbrook shallow marine sequences Figure A16: Sherbrook high amplitude quot;Amritquot; facies Figure A17: Tertiary facies maps, isochrons and selected seismic Figure A18: Wangerrip shallow marine shelf facies Figure A19: Wangerrip lowstand deltaics Figure A20: Nirranda sequence List of Figures - Appendix B Figure B1: Regional Seismic Line A Figure B2: Regional Seismic Line B Figure B3: Regional Seismic Line C Figure B4: Regional Seismic Line D Figure B5: Regional Seismic Line E Figure B6: Regional Seismic Line F List of Figures - Appendix C Figure C1: Facies map, lower part of Waarre Formation Figure C2: Facies map, Waarre Formation Figure C3: Facies map, Flaxman Formation Figure C4: Facies map, Nullawarre Formation Figure C5: Facies map, Paaratte Formation Figure C6: Facies map, Timboon Formation List of Figures - Appendix D Figure D1: Seismic Facies Map – Basal Otway Group Figure D2: Seismic Facies Map – Mid Otway Group Figure D3: Seismic Facies Map – Upper Otway Group v
  7. 7. Figure D4: Seismic Facies Map – Turonian – Waarre-Flaxman Fm. Figure D5: Seismic Facies Map – Belfast Fm./Shipwreck Sequence Figure D6: Seismic Facies Map – Sherbrook Sequence Figure D7: Seismic Facies Map – Wangerrip Sequence Figure D8: Seismic Facies Map – Nirranda Sequence List of Figures - Appendix F Figure F1: Regional Structural Restoration Section D Figure F2: Regional Structural Restoration Section E Figure F3: Regional Structural Restoration Section F vi
  8. 8. 1 Introduction and Project Overview 3D-GEO Pty Ltd was commissioned by the Minerals and Petroleum section of the Victorian Department of Primary Industry (DPI) to compile this review of the Victorian part of the Otway Basin This review is based on data from the Petroleum Atlas of Victoria, well completion reports and additional palynological reports commissioned by the DPI, as well as seismic lines provided by DPI (Figure 1). A B Figure 1. A. Seismic grid; B. Wells used in study 1
  9. 9. The project was designed as a reconnaissance-level review of the structural evolution, stratigraphic fill, and petroleum resource potential of the basin, intended to amalgamate a variety of earlier piecemeal projects into a coherent whole with a consistent framework. Areas of further interest will be flagged for additional future. Over the past 5-10 years, major progress has been made in Petroleum Exploration of the offshore Otway basin, with significant commercial gas finds in the Shipwreck trough (Thylacine & Geographe, Casino, La Bella, Minerva). Smaller finds onshore and the presence of some liquids suggest that future potential exists for further discoveries. One crucial knowledge gap is in the onshore-offshore transition, where datasets disconnect and Petroleum resource management and licence administration shifts from State to federal authorities. Over the past decade, a number of Industry studies and academic research projects have addressed some of the continuity issues but to date, no major open-file framework study has been produced to properly fill the data gap – hence the commissioning of the present study. Data products of earlier DPI-supported academic research projects have been located, restored, and transcribed to a common standard. These background grids and maps are available for future workers, as are the products of the current study. It was originally envisaged that a fully phase-matched and balanced seismic dataset would be available, with all relevant well checkshot and velocity data. This was demonstrably not the case and part of the project time was devoted to balancing and tying the selected regional lines. The remainder of the seismic dataset was NOT fully matched and balanced so future studies should be wary of data matching issues. Well documentation was quite comprehensive, but catalogued by a proprietary indexing system which assigns numbers and not names to all documents. Whilst economical, this is not very helpful to the casual browser outside of the proprietary indexing system. Thought should be given to updating document file names to include information about their content, or to supplying user-friendly guides to file identities. Many scanned TIFF images are either unreasonably large, or formatted with unusual compression algorithms and do not open with “normal” image browsers. The value of these items is therefore substantially degraded and again, thought should be given to improving the usability of the image data. Further work in database consolidation clearly required to be done, and the next logical step is to infill the present study with a finer grid of seismic data and to resolve the key issues that are flagged at appropriate points in this report and summarised in the concluding section. 2
  10. 10. 2 Basin Overview The Otway Basin, along with the Torquay Sub-basin, Bass Basin and the Gippsland Basin, formed as part of a rift between Australia and Antarctica. Rifting started in the mid to late Jurassic, while India was breaking away from Australia and Antarctica, and continued until the end of the Cretaceous (Figure 2). Two separate episodes of rifting, in the early and late Cretaceous, were followed by thermal subsidence, although the precise timing of these events varies along the basin. Significant fold-related uplift in the late Tertiary formed the Otway Ranges and led to the formation of a number of petroleum traps (Figures 3,4). A B C Figure 2. Plate Tectonics setting A. Latest Cretaceous; B. Latest Eocene; C. Present Day 3
  11. 11. Figure 3. Regional structural setting and previous study areas A large number of previous detailed studies have been conducted by industry, government, or academic groups (Figure 3). These have often been limited in scope to a sub-basin or specific formation and there is a general lack of integration and a very obvious inconsistency of nomenclature. The need is clear for an overall well and seismic-based sequence architecture to provide a chronostratigraphic context for the lithostratigraphic nomenclature. 4
  12. 12. Figure 4. Otway Basin structural setting The published stratigraphy of the Otway Basin is a confusing topic. Five or six alternative and inconsistent versions have been published, and few of these have been tied into a proper sequence stratigraphic context. As a consequence, the chronostratigraphy of the Otway basin is a significant weakness and a plethora of local formation names is used to describe local details of the stratigraphy with little knowledge of their true depositional, tectonic, and chronostratigraphic context. For this work, we have adopted the generic stratigraphy of Geary and Reid, 1998 (Figure 5), and have begun the process of interpreting the various formation names in a true sequence stratigraphic framework. The work of Kraasay et al., 2004 comes close to the desired chronostratigraphic goal but still requires some modification. Appendix A details the seismic sequence interpretation, and the basinwide framework of regional lines and seismic sequences that has resulted. In figure 5, we merely note the inconsistencies of well-based formation tops, published lithostratigraphy, and our attempts to generate a first-pass sequence stratigraphy. Clearly, more work is required in the Sherbrook Group – especially the “grab-bag” lithostratigraphic name “Belfast Mudstone” which merely represents anything that cannot be distinguished. Appendix A shows that the Sherbrook/Belfast can be subdivided into at least two clearly distinguishable seismic sequences, and more local sub- sequences are suggested. There is scope for a stand-alone project to investigate just this issue, but the value of further work would be enhanced by avoiding further piecemeal studies and instead working to definitively revise the entire chronostratigraphy. 5
  13. 13. Seismic Formation Tops Horizons From Wells: Timboon Paaratte Nullawarre Flaxman Waarre Lower Waarre. Figure 5. Stratigraphic section showing horizons used to interpret seismic 6
  14. 14. 3 Geological Summary The Otway Basin originated as part of a developing rift between Australia and Antarctica, as Gondwana progressively broke up during the Late Jurassic and early Cretaceous. Signatures of both local rift events and the rifting of nearby fragments to west and east are recorded in the basin. Final break-up and continental separation did not occur until End Cretaceous times and due to the oblique Sorrell margin of Tasmania, only a limited ocean developed until Eocene times. The Victorian section of the rift is dominated by half grabens controlled by mainly north-dipping faults and is continuous with similar structures in South Australia and Gippsland. Deposition began in a series of lakes that formed individual depocentres and eventually became linked into a through-going, fluvial-lacustrine system. The oldest sediments in the Otway Basin, the Casterton Formation and Crayfish Sub-group (lower part of the Otway Group), comprise locally derived sands, and muds from a more distant source. Basaltic volcanism occurred in the west during the earliest stage of rifting. Minor uplift and erosion in the late Barremian were followed by deposition of extensive coal measures (Killara Coal Measures) and the Aptian-Albian Eumeralla Formation (upper part of the Otway Group), dominated by fluvial sand, much of it derived from volcanic sources to the east. These sediments become finer to the west, with a marine influence present in the South Australian part of the basin, indicating that the rift was opening from the west and forming a long, marine inlet. Extensional faulting died out eastwards, so that in western Victoria deposition of only the lowest Eumeralla Formation is fault-controlled, while in the Torquay region the major faults remained active until late Albian. Minor folding, related to structural inversion, occurred in the region of the Otway Ranges and Torquay Sub-basin in the late Albian-Cenomanian. During a major tectonic change in the middle of the Cretaceous, rifting began along the east coast of Australia, eventually forming the Tasman Sea. In the Otway region, renewed extension led to the development of NW striking normal faults that dip dominantly southwest. The northern portion of the Early Cretaceous rift was uplifted and parts of the Otway Group were eroded to build the shallow marine shelf of the Mussel Platform. At the eastern end of the Late Cretaceous basin, the major faults change to a more NNW orientation and extend southwards along the west coast of Tasmania, while the Torquay Sub-basin became structurally separated from the main Otway Basin and linked to the Bass Basin. The focus of this extension was in the (present day) offshore area and eventually led to sea floor spreading and thermal subsidence of the Otway continental margin at the end of the Cretaceous. Upper Cretaceous sediments can be divided into six sequences (Lower Waarre, Waarre, Flaxman, Nullawarre, Paratte and Timboon). Each sequence represents progradation into the basin, mostly from the northwest and east. The northwest sediment source was part of an extensive delta, fed from an ancestral Murray River that derived an enormous volume of sediment from the large catchment area along the dip slope of the eastern Australian uplift. Sediment also entered the eastern end of the Otway Basin from the Tasmanian area, as deltas built out in lower Waarre, Waarre, Flaxman and Timboon times, spreading across the Prawn Platform and prograding westwards into 7
  15. 15. the rift and eventually into deeper water south of the Mussel Platform. In the north the Port Campbell embayment appears to have been supplied by local sources or longshore drift. Separation of Australia and Antarctica at the end of the Cretaceous, and the resulting thermal subsidence, allowed marine transgression over much of the northern, onshore portion of the Otway Basin. The Tertiary sediments are dominated by coastal and shallow marine, progradational sequences interrupted by marine transgressions. The oldest, Wangerrip Group, comprises clastic sediments while the younger Nirranda and Heytesbury Groups are largely carbonates. These units built out the prominent present day shelf, but much of the basin is now at water depths of greater than 100 metres and was partly starved of sediments during the Tertiary. Canyons on many of the Tertiary slopes are not necessarily related to sea level fluctuations but probably developed by upslope retreat of base of slope seepage and slumping. In the Torquay Sub-basin, fluvial sedimentation continued into the early Tertiary, but marine conditions became established by the late Eocene. Folds, related to inversion of some Early Cretaceous faults in the mid Miocene–early Pliocene, were caused by regional, NW-SE directed crustal shortening of up 12% and led to uplift of the Otway Ranges and smaller, local areas in the far west and central west. The King Island-Mornington High also developed at this time, forming a barrier between the Torquay Sub-basin and Bass Basin, while erosion of uplifted sediments lead to a major change from carbonate to clastic deposition in the offshore areas. 8
  16. 16. 4 Late Jurassic-Early Cretaceous The Otway Basin initially developed as a late Jurassic-early Cretaceous intra-continental rift system, extending E-W between (present day) southern Australia and Antarctica and cutting across the N-S grain of the Paleozoic basement. In the Otway region the rift system comprises a series of half grabens controlled by large, dominantly north-dipping normal faults that strike NW-SE in far western Victoria, east-west in the central west and NE-SW in the Otway Ranges and Torquay Sub-basin, and cut across the grain of the underlying Paleozoic basement (Figures 6,7,8). Figure 6. Late Jurassic-Early Cretaceous depocentres 9
  17. 17. Figure 7. Magnetic image showing basement structural trends and trend of Early Cretaceous faults in onshore Otway Basin (dashed red line). 10
  18. 18. Figure 8. Gravity image showing basement trends and trend of Early Cretaceous faults. The half grabens initially developed as individual depocentres, including locally deep lakes (Figure 9), but eventually linked to form a through-going rift. The oldest sediments in the Otway Basin belong to the Casterton Formation and Crayfish Sub-group (lower Otway Group) and consist of fluvial sandstones with floodplain and lacustrine claystones and some coals (Figure 10). Basaltic volcanism occurred during the beginning of rifting in the central west. S N Figure 9. Early Cretaceous half graben 11
  19. 19. Figure 10. Woolsthorpe-1 seismic tie showing Early Cretaceous stratigraphy Uplift and erosion in the far west and central west in the late Barremian were followed by the deposition of extensive coal measures in local depocentres (Killara Coal Measures) and later, the Aptian-Albian Eumeralla Formation (upper Otway Group) (Figure 11). This is dominated by fluvial sand, much of it derived from volcaniclastic sources somewhere to the east and northeast, probably volcanoes associated with the future opening of the Tasman Sea. The sediments become finer to the west and marine influences are present in the South Australian part of the Otway Basin, indicating that the rift was opening from the west, forming a very long marine inlet. Extensional faulting died out eastwards, so that in the far west and central west deposition of only the lowest Eumeralla Formation is fault- controlled, while in the Otway Ranges and Torquay Sub-basin the major faults remained active until late Albian. Minor folding, related to structural inversion, occurred in the region of the Otway Ranges and Torquay Sub-basin in the late Albian-Cenomanian. 12
  20. 20. Figure 11. Early Cretaceous super sequences, central onshore Otway Basin 13
  21. 21. 5 Late Cretaceous During a major tectonic change at the end of the Early Cretaceous, rifting began along the east coast of Australia, eventually forming the Tasman Sea. In the Otway region NE-SW directed extension in the Turonian led to the development of dominantly south-dipping normal faults that strike NW-SE in the far west and central west. Close to Cape Otway, at the eastern end of the Late Cretaceous basin, the major faults change to a more NNW-SSE/N-S orientation and extend southwards along the west coast of Tasmania (Figure 12). Figure 12. Late Cretaceous structural elements The focus of this extension was in the offshore area and eventually led to sea floor spreading and thermal subsidence of the Otway continental margin at the end of the Cretaceous (Figure 13). 14
  22. 22. Figure 13. Late Cretaceous continental margin; A. Time; B. Depth During the Late Cretaceous the Sherbrook Group was deposited as the Otway Basin continued to widen. The eastern end of the basin was now controlled by movement along the “Sorell Fault Zone”, underlying the linear zone of subsidence named the Shipwreck Trough. At the same time the northern side of the original Early Cretaceous rift was uplifted and parts of the Otway Group were eroded to form the shallow marine shelf of the Mussel Platform. Also at this time the Torquay Sub-basin was separated from the main Otway Basin and became linked to the Bass Basin. The Sherbrook Group consists of a number of depositional sequences but is here split into six; lower Waarre, Waarre, Flaxman, Nullawarre, Paaratte and Timboon, from oldest to youngest (Figure 14-20). 15
  23. 23. Figure 14. Formation tops used for regional correlation. Figure 15. Well correlation sections 16
  24. 24. GR DT GR DT Timboon Paaratte Nullawarre Flaxman Waarre Lower Waarre Eumeralla Bridgewater Bay 1 Prawn 1A Figure 16. Well correlations showing formations and regional variations in log character. 17
  25. 25. Figure 17. Well correlation section 1, Prawn-1A to Bridgewater Bay-1. Figure 18. Well correlation section 2, Torquay Sub-Basin, Snail-1 to Warracburunah-2 18
  26. 26. Figure 19. Well correlation section 3, Armit-1 to Normanby-1 Figure 20. Well correlation section 4, Prawn-1A to Iona-1 Each sequence represents progradation into the basin, mostly from the northwest and east (Figures 21-25). The northwest sediment source was part of an extensive South Australian delta that appears to have been fed from an ancestral Murray River. The enormous volume of sediment was derived from a large catchment area along the dip slope of the eastern Australian uplift caused by Tasman Sea rifting. The delta was blocked by the Tartwaup and Mussel Fault systems from extending farther into the Victorian part of the Otway Basin. It appears to have started to build into the basin at Waarre times and reached its maximum extent in Paaratte or Timboon times. 19
  27. 27. Sediment entered the eastern end of the Otway Basin through the Torquay Sub-basin and also from the Tasmanian area. Sediment from these two sources spread across the Prawn Platform and prograded westwards into the rift across the Shipwreck Trough onto the Mussel Platform or into deeper water south of the Mussel Platform. Deltas built across the southern end of the Shipwreck Trough during lower Waarre, Waarre and Timboon times, but less coarse sediment entered the northern end of the Shipwreck Trough, which contains mostly claystone, except at Timboon times. The Port Campbell embayment did not have a major sediment source but seems to have been supplied by local sources or by longshore drift. Figure 21. Facies map, lower part of Waarre Formation                         Legend       Sediment Transport    Facies Boundary                          ‐‐‐‐‐‐‐‐‐‐‐‐  DT  Delta Top  SM  Shallow Marine  SF  Shore Face  OS  Outer Shelf  20
  28. 28. Figure 22. Facies map, Waarre Formation Figure 23. Facies map, Flaxman Formation 21
  29. 29. Figure 24. Facies map, Nullawarre Formation Figure 25. Facies map, Paaratte Formation 22
  30. 30. Turbidites have not yet been identified in the Shipwreck Trough. The massflow sandstones in Geographe-1 and Thylacine-1 and -2 appear to be remobilised shelf or shoreline sands and are overlain by shallow marine sediments. They may be the proximal ends of channels feeding turbidite sands to the deep water area west of the Prawn Platform and south of the Mussel Platform. The sediment source in the northwest may also have supplied sand to the deep water area to the south. In the Torquay Sub-basin accumulation of sediments recommenced in Timboon times with fluvial sandstones and floodplain claystones and some coals, as subsidence in the Bass Basin extended northwest (Figure 26). Figure 26. Facies map, Timboon Formation 23
  31. 31. 6 Tertiary Separation of Australia and Antarctica occurred around the end of the Cretaceous, and the resulting thermal subsidence allowed marine transgression over much of the northern, present onshore portion of the Otway Basin (Figure 27). Figure 27. Tertiary structural – stratigraphic elements The Tertiary sediments consist of coastal and shallow marine, progradational sequences interrupted by marine transgressions. The oldest, Wangerrip Group, comprises clastic sediments while the younger Nirranda and Heytesbury Groups are largely carbonates. These units built out the prominent present day shelf, but much of the basin is now under water depths greater than 100 metres and was partly starved of sediments during the Tertiary (Figure 28). Canyons on many of the Tertiary slopes are not necessarily related to sea level fluctuations but probably developed by upslope retreat of base of slope seepage and slumping (Figures 29-30). 24
  32. 32. Figure 28. Tertiary shelf edge Figure 29. Shelf edge and shallow water canyons 25
  33. 33. Figure 30. Tertiary deep water canyons In the Torquay Sub-basin, fluvial sedimentation continued into the early Tertiary, but by the late Eocene marine conditions became established. Folds, related to inversion of many Early Cretaceous faults in the mid Miocene–early Pliocene, were caused by regional, NW-SE directed shortening of up 12% and lead to uplift of the Otway Ranges and smaller, local areas in the far west and central west (Figures 31-34). The King Island-Mornington High also developed at this time, forming a barrier between the Torquay Sub-basin and the Bass Basin. Erosion of uplifted sediments led to a major change from carbonate to clastic deposition in the offshore areas. 26
  34. 34. Figure 31. Late Tertiary fold belt, offshore extension of Otway Ranges Figure 32. Western margin of Late Tertiary fold belt, west of Otway Ranges 27
  35. 35. Figure 33. Eastern margin of Late Tertiary fold belt, Torquay Sub-basin, east of Otway Ranges Figure 34. Isoreflector contours showing effect of Late Tertiary uplift on previously deeply buried Early Cretaceous sediments in the Otway Ranges 28
  36. 36. 7 Petroleum Systems A variety of petroleum systems have been described or hypothesised for the onshore and offshore Otway Basin. Geoscience Australia report 3 petroleum systems which they denote Austral 1, 2, & 3. Austral 1 sources hydrocarbons to the western Otway Basin/Penola Trough, from latest Jurassic to earliest Cretaceous Crayfish fluvio-lacustrine shales. Austral 2 Sources the major gas finds in the Shipwreck Trough, and the smaller gas discoveries and minor oil in the eastern onshore Otway. Eumeralla Aptian-Albian fluvial and coaly facies are the active sources Austral 3 is not a proven commercial petroleum system. It invokes Late Cretaceous to early Tertiary fluvio-deltaic facies to source hydrocarbons in the deeper water parts of the offshore Otway basin. According to GA:- “There are strong stratigraphic and geographic controls on oil families within the Otway Basin. Oils in the west and onshore belong to Austral 1 families and were sourced from Late Jurassic - Early Cretaceous, syn-rift, dominantly fluvio-lacustrine organic facies. Oils in the east belong to an Austral 2 family derived from Early Cretaceous, post-rift coaly organic facies. Oils in the central part of the basin have a mixed source affinity, but are predominantly from Eumeralla Supersequence sources (Boreham et al., 2004).” “Natural gases in the Otway Basin show clear geochemical differentiation between those from the western and eastern parts of the basin. The western gases (e.g., Jacaranda Ridge 1, Katnook 2, Ladbroke Grove 2, Redman 1 and Troas 1) belong to the Late Jurassic - Early Cretaceous, Crayfish Supersequence-sourced Austral 1 Petroleum System (Boreham et al., 2004). The eastern gases (e.g., Thylacine 1, Geographe 1, La Bella 1, Minerva 3, Casino 1, Casino 2) belong to the Aptian-Albian, Eumeralla Supersequence-sourced Austral 2 Petroleum System (Boreham et al., 2004). Gases from the central Otway Basin (e.g., Port Fairy 1, Caroline 1) are the products of mixing from both sources within local depocentres.” “Multiple charge histories in the natural gas reservoirs are evident from the widespread influx of overmature, dry gas to an initially in-place wet gas, particularly in the western Otway Basin. Both gas charges have the potential to displace and/or alter the composition of any reservoired oil. In the east, however, most natural gases (e.g., Geographe 1, Thylacine 1, La Bella 1, Lavers 1) are interpreted as the result of a single gas charge (Boreham et al., 2004). “ “Otway Basin natural gases show a strong geochemical association with their respective oils, suggesting that both are generated from the same source (Figures 8 and 9). Also, the gases and oils and their effective source rocks have a strong stratigraphic and geographic relationship, indicating mainly short- to medium-range migration distances from source to trap (Boreham et al., 2004).“ Boult (2006) suggests that Upper Albian (uppermost Eumeralla equivalent) marine source rocks may be active in the Morum sub-basin offshore South Australia. This would introduce a new source facies into play in the offshore Otway, where the proven fluvio-deltaic and coaly sources of the Austral 1 and 2 systems would be suspect. These sources may not extend into the eastern offshore Otway – the Nelson sub-basin. 29
  37. 37. Boult states:- “Geochemical analysis undertaken on an oil show within Upper Albian–Cenomanian(?) rocks from the Crayfish-A1 well, which is close to the edge of this depocenter, suggest its origin is an anoxic marine source rock, and aromatic hydrocarbon analysis strongly suggests that it is migrated oil. The oil show is also consistent with the modeled development of a significant oil-prone source pod in the Morum Sub-basin north of the Discovery Bay High, which possibly correlates with the prolific world-wide Albian ocean anoxic event (OAE). “ On the other hand, anomalous amplitudes with phase opposite to seabed (i.e. a soft kick) are noted downdip of the Amrit-1 well in the nelson Sub-basin. This suggests that at least one petroleum system is still active here, but the failure of Amrit-1 suggests that the system is not prolific, or that sealing capacity of the shallow section is problematic. Figure 31: Potential DHI downdip of Amrit-1 In conclusion, the demonstrably-effective source rock of the eastern (Victorian) Otway is Geoscience Australia’s Austral 2 (Eumeralla) source system, based on coals and carbonaceous shales. In the vicinity of the Shipwreck Trough, two critical moments have been identified by burial history modelling for generation and expulsion from the Eumeralla Formation-sourced, Austral 2 petroleum system. Firstly, the end of the Late Cretaceous (approximately 68 Ma) when deposition of the thick clastic Sherbrook Group provided the necessary overburden to mature source intervals near the base of the Eumeralla Formation (Ryan et al, 2005). Secondly, the Late Miocene inversion event. Prior to this inversion, the deposition of thick Oligocene-Miocene carbonates in the vicinity of the Shipwreck Trough resulted in continued generation and expulsion of hydrocarbons throughout the Paleogene. (Ryan et al, 2005), but only limited trapping opportunities existed 30
  38. 38. Detailed facies mapping and paleogeographies of the Eumeralla-Waarre sequences are required to identify the most prospective source kitchens. In some cases, specific units such as the Killara coals can be mapped in very fine detail, showing localisation of coaly facies to particular half-grabens. 31
  39. 39. 8 Non-Petroleum Implications This framework study is important not just for Petroleum exploration. Many other geological and resource endeavours require a good knowledge of the present-day geometry, facies distribution, and structural evolution of the Otway basin. Geothermal Energy The deeper parts of the onshore Otway basin have porous sandstones in the Crayfish formation (Pretty Hill sst) which are potentially deep enough and hot enough for a geothermal power project, extracting hot formation waters and using the enthalpy for steam generation and power turbines. In order to exploit this resource, a good knowledge of burial depth and temperature is required, as well as detailed maps of porosity, original depositional facies, and diagenetic modification through, e.g. deep burial and uplift. The present framework is ideal to place further detailed studies and is probably suitable for general geothermal prospectivity delineation at a regional scale, in context but is not yet sufficient for detailed local calculations of resource availability. Further work along the lines of that described in Appendix 1 – seismic sequence stratigraphy, will enable a detailed 3D model of facies, porosity, thermal conductivity, heat flow, and temperature to be produced. Carbon Sequestration and Storage In the modern era, disposal of CO2 into the subsurface is an emerging issue. A detailed framework of the basin is required to identify long-term stable reservoir units into which CO2 can be sequestered. The Otway basin is particularly useful for these studies because it has a geological history of trapping CO2 and the formation waters are demonstrably stable in the presence of CO2. Again, the framework is only useful for regional studies, and a detailed CCS study would require more detailed follow-on work. 32
  40. 40. Gold and other Minerals The deep lead alluvial gold system probably extends south from the prolific Moyston-Ballarat area and is now buried beneath the Newer Volcanics. A good basin framework will enable prospecting and potential bore-drilling to target alluvial gold in locations where the balance of subsidence and uplift/erosion has encouraged stream channels to focus alluvial systems, but has retained them at economically-mineable depths. Other economic minerals can also be mapped through the integration of bedrock terranes and cover, within this framework. Coal exploration in the Otway basin has not shown promise to date. The coals at outcrop are thin and the coal measures relatively sand-prone. Some high rank coals outcrop in the Otway Ranges. Detailed facies mapping of coal sequences may, however, reveal areas where better-developed coal sequences occur at economic depths. For example the Killara coals are well-developed in the Killara-1 and Taralea-1 wells, and can be easily mapped on seismic data – distributed within a local graben. It should be relatively easy to show whether these coals retain their thickness and quality at shallower depths within this local graben, or in another nearby area. 33
  41. 41. 9 Conclusions A brief but comprehensive review of the Otway basin well and seismic sequence stratigraphy has been conducted to generate a robust framework for the basin to support regional-scale exploration activities and to tie together disparate and scattered mapping exercises in various parts of the onshore and offshore Otway basin (Figure 36). The study has been limited in scope by the modest scale at which it was applied. Clearly, a more-detailed and slightly refined product could be made using more of the available seismic data and looking more closely at detailed facies patterns in wells and seismic, but that goal is best reserved for a follow-on study to take this framework forward and to elaborate upon it. The major limitation of this study is that, in the time available, it has not been possible to definitively resolve the myriad alternative lithostratigraphies and chronostratigraphies that have been published in piecemeal studies of parts of the Otway Basin. The existence of distinct depocentres and sediment sources in the eastern and western Otway Basin leads to major lateral facies variations. The demonstrable evidence of diachronous tectonism between east and west mean that any lithostratigraphic correlation is fraught with difficulty and that sequence stratigraphy must be carefully applied with detailed seismic grids to resolve the merging and splitting of unconformities and other sequence boundaries. Although we have attempted a comprehensive regional-scale sequence stratigraphy here, it is far from complete and many important details are yet to be resolved. In particular, the Belfast Mudstone is a “grab-bag” lithostratigraphic formation name which describes anything which is not distinguishable, within a wide age range that encompasses many sedimentary cycles and a major tectonic episode. As a result of this inherent complexity, and the database limitations, the links to other basins and sub-basins are less clear. In particular, we have limited documented correlation into key wells in the western Otway (Morum) sub- basin, and the present work is limited in its application to the eastern Otway (Nelson) sub-basin 34
  42. 42. Several issues are unique to the deeper water area. Whist this is more the purview of Geoscience Australia, it is still part of the Otway Basin history and merits consideration in future work:_ a) Several key sequences in the deepwater are unrepresented or have a poorly-understood correlation with the better known shelfal areas. b) Candidate gravity slides have been identified which must be explained in a basin context as tectonic events and subsidence presumably cause basin tilting and shelf-edge instability. c) Tectonics in the deepwater is poorly-constrained due to the very limited number of deepwater seismic lines. In particular, fault orientation and correlation is poorly understood. Certainly, there is a change in apparent dip of the faults in the deepwater section, but is this a change of orientation or a change of structural style? In part, GA swath bathymetry can assist locally with fault correlation where seabed effects are seen, but swath bathymetry is only locally available. It is possible that other hydrographic data could be used to assist with this study. d) The palaeobathymetric evolution of the present-day deepwater and the advance of successive shelf edges needs to be properly understood in a 4D context. We have conducted preliminary one-line bathymetric restorations. These need to be extended into a consistent map-based history of palaeobathymetry Given these limitations, however, the general geological evolution of the eastern Otway basin is reasonably well- understood and the essentials of the petroleum system that are responsible for large offshore gas reserves and more modest onshore accumulations can be documented and used as a basis for prospecting and for risk assessment. The most important petroleum system centres on the Waarre formation of the Shipwreck Trough where extensive shallow marine reservoirs and coaly source rocks combine to produce an effective petroleum system, with the right burial history. Other non-petroleum resources can also usefully be assisted by this framework study, with their own controlling factors being fitted-in to the stratigraphic framework. A number of database issues arose during the course of this study. Not all have been fully-resolved and we will recommend that further work be done on the various databases to make them more open and user-friendly, and to debug various navigation and description errors. 35
  43. 43. 10 Recommendations 1) The published lithostratigraphy of different parts of the Otway Basin is a confusing melange of local lithostratigraphic terms with little consistency. This study has begun to resolve the issues in terms of an objective sequence stratigraphy and chronology but falls short of being a definitive solution. Further work is clearly needed to define a coherent chronostratigraphy based on sequence analysis for the Otway basin and adjacent areas. 2) A follow-on study should be considered which will infill in greater detail the present framework, using a much larger proportion of the available seismic data and revisiting some of the key uncertainties in well dating and correlation. The study will offer refined spatial and temporal resolution of the structural and stratigraphic evolution of the Otway basin, detailed fault mapping and correlation, and mapping of the distribution of sedimentary facies within the basin. This follow-on study should aim to support all types of mineral and energy activity, generating a detailed understanding of the distribution of facies through time, and their subsequent modification through burial, fluid movement, and later uplift. 3) Additional database consolidation work is required. For instance, the onshore well database held by DPI is catalogued using a proprietary system which, whilst efficient, limits access to the data without the appropriate indexing tools. Many of the digital images are scanned with unconventional compression algorithms or are wrongly described and will not open. Some seismic data has poorly-described or inaccurate navigation and time-reference data so the location of the lines in space and time requires adjustment to tie the rest of the dataset. These issues are relatively minor for the framework study but will be crucial when a more-detailed follow-up study is contemplated. These issues should therefore be addressed with a specific database clean-up exercise. 4) A further study should be considered to review more closely the linkage between the tectonics and stratigraphy of the Otway basin, and other nearby basins on the Australian margin such as the Torquay Embayment, the Bass and Gippsland Basins, Sorrel basin in Tasmania, and potentially extending west to the Great Australian Bight. Whilst much of this is outside the remit of the Victorian DPI, there is some value in at least generating a consistent modern chronology and stratigraphic correlation of the various Victorian basins and examining the interplay of tectonics and sedimentation from one basin to another. 5) A specific deepwater tectonic/palaeobathymetric study may be considered – probably jointly with Geoscience Australia, which will address the issues of fault correlation and orientation, gravity slides, and the evolution of palaeobathymetry of Australia’s southern margin. 6) The results of this present study and any follow-on should be made available to all mineral and energy explorers and the geological community at large. Appropriate methods of dissemination and communication of the information should be developed as part of the strategy of informing and supporting economic activity in Victoria. 7) At a time of generational change in DPI and Victorian Universities, several key workers with unique knowledge of the Otway Basin are approaching the end of their professional lives. It is important to capture 36
  44. 44. and download their knowledge base into a consolidated documentation of the Otway Basin and its links with other nearby depocentres. These key individuals should be a vital part of near-term future studies to resolve the issues described above. 37
  45. 45. 11 References and Further Reading Alexander, E.A. & Morton, J.G.G., 2001. Northern Otway Basin exploration opportunities - Blocks OT2001-A and B. Primary Industries and Resources South Australia, Petroleum Exploration Data Package 10. Arditto, P.A., 1995—The eastern Otway Basin Wangerrip Group revisited using an integrated sequence stratigraphic methodology. The APEA Journal 35(1), 372-384. Bernecker, T. And Moore D.H., 2003—Linking basement and basin fill: implications for hydrocarbon prospectivity in the Otway Basin region. The APEA Journal, 43(1), 39-58. Birch, W.D. (Editor), 2003 Geology of Victoria. Geol Soc. Australia Special Publication 32 Boreham, C.J., Hope, J.M., Jackson, P., Davenport, R., Earl, K.L., Edwards, D.S., Logan, G.A. and Krassay, A.A., 2004. Gas-oil-source correlations in the Otway Basin, southern Australia. In: Boult, P.J., Johns, D.R. and Lang, S.C. (Eds), Eastern Australasian Basins Symposium II, Petroleum Exploration Society of Australia, Special Publication, 603-627. Boult, P.J. & Hibburt, J., 2002. Petroleum Geology of South Australia Volume 1: Otway Basin, Second Edition. (CD- ROM). Boult, P.J., et.al., 2006, The Morum Sub-basin Petroleum System, Otway Basin, South Australia Search and Discovery Article #10095 online at :- http://www.searchanddiscovery.net/documents/2006/06004boult/index.htm on 27-01-2009 Cliff, D.C.B., Tye, S.C. And Taylor, R., 2004—The Thylacine and Geographe gas discoveries, offshore eastern Otway Basin. The APPEA Journal 44(1), 441-462. Constantine A., 2001—Otway Basin. In: Woollands, M.A. and Wong, D. (eds), Petroleum Atlas of Victoria, Australia. Department of Natural Resources and Environment. Cooper, G.T. And Hill, K.C., 1997—Cross-section balancing and thermochronological analysis of the Mesozoic development of the eastern Otway Basin. The APPEA Journal 37(1), 390-414. Dickinson, J.A., Wallace, M.W., Holdgate, G.R., Daniels, J., Gallagher, S.J. And Thomas, L., 2001—Neogene tectonics in south eastern Australia: implications for petroleum systems. The APPEA Journal 41(1), 37-52. Duddy, I.R., 1994—The Otway Basin: thermal, structural and tectonic and hydrocarbon generation histories. Compiled by Finlayson, D.M., NGMA/Petroleum Exploration Society of Australia Otway Basin Symposium, Melbourne, April 20, 1994: extended abstract, Australian Geological Survey Organisation Record, 1994/14, 35 42. Duddy, I.R., 1997—Focussing exploration in the Otway Basin: understanding timing of source rock maturation. The APPEA Journal, 37(1), 178-191. 38
  46. 46. Edwards, D.S., Struckmeyer, H.I.M., Bradshaw, M.T. & Skinner, J.E., 1999. Geochemical characteristics of Australia's Southern Margin petroleum systems. The Australian Petroleum Production and Exploration Association (APPEA) Journal 39(1), 297-321. Etheridge, M. A., Branson, J.C. & Stuart, S.P.G., 1985. Extensional basin-forming structures in Bass Strait and their importance for hydrocarbon exploration. The APEA Journal, 25(1), 344-361. Finlayson, D.M., et. al., 1993 The Western Otway Basin - a tectonic framework from new seismic, gravity and aeromagnetic data. ASEG Exploration Geophysics 24(4) 493-500 Geary, G.C. And Reid, I.S.A., 1998—Geology and prospectivity of the offshore eastern Otway Basin, Victoria - for the 1998 Acreage Release. Victorian Initiative for Minerals and Petroleum Report 55, Department of Natural Resources and Environment. Geological Survey of Victoria, 1995. The stratigraphy, structure and geophysics and hydrocarbon potential of the Eastern Otway Basin, Geological Survey of Victoria Report 103, 241p. Krassay, A.A., Cathro, D.L. and Ryan, D.J., 2004. A regional tectonostratigraphic framework for the Otway Basin. In: Boult, P.J., Johns, D.R. and Lang, S.C. (Eds), Eastern Australasian Basins Symposium II, Petroleum Exploration Society of Australia, Special Publication, 97-116. Lisk, M., 2004 Constraints on the oil prospectivity of the Penola Trough, onshore Otway Basin. In Boult P.J., Johns, D.R. and Lang, S.C. (editors) In: Eastern Australasian Basins Symposium II, Conference Proceedings. Petroleum Exploration Society of Australia Special Publication. 2, 628-641. Lyon, P.J., Boult, P.J., Hillis, R.R. and Bierbrauer, K., 2007. Basement controls on fault development in the Penola Trough, Otway Basin, and implications for fault-bounded hydrocarbon traps. Australian Journal of Earth Sciences, 54 (5), 675-689. Monteil, E., Kelman, A. And Krassay, A.A., 2004—New and revised palynological data for the Otway Basin. Geoscience Australia, Record, 2004/22. Moore, D.H., 2002—Basement-basin relationships in the Otway Basin, Victoria, Australia Victorian Initiative for Minerals and Petroleum Report 78, Department of Natural Resources and Environment. Moore, A.M.G., Stagg, H.M.J., & Norvick, M.S., 2000. Deep-water Otway Basin: A New Assessment of the Tectonics and Hydrocarbon Prospectivity. The APPEA Journal 40(1), 66-85. Morton, J.G.G., 1995. Otway Basin, Mesozoic (Chapter 9). IN: J.F. Drexel and W.V. Preiss, (Editors), 1995 The Geology of South Australia, Volume 2, The Phanerozoic. South Australia. Geological Survey, Bulletin, 54, 142- 147. Norvick, M.S., & Smith, M.A., 2001. Mapping the plate tectonic reconstruction of southern and southeastern Australia and implications for petroleum systems. The APPEA Journal, 41(1), 15-36. 39
  47. 47. O'Brien, G.W., Reeves, C.V., Milligan, P.R., Morse, M.P., Alexander, E.M., Willcox, J.B., Yunxuan, Z., Finlayson, D.M. & Brodie, R.C., 1994. New ideas on the rifting history and structural architecture of the Western Otway Basin: evidence from the integration of aeromagnetic, gravity and seismic data. APEA Journal, 34(1), 529-554. Palmowski, D. B., 2004 Ph.D. Thesis, University of Melbourne Palmowski, D., Hill, K.C. and Hoffman, N., 2004. Structural-stratigraphic styles and evolution of the offshore Otway Basin - a structural seismic analysis. In: Boult, P.J., Johns, D.R. and Lang, S.C. (Eds), Eastern Australasian Basins Symposium II, Petroleum Exploration Society of Australia, Special Publication, 75-96. Perincek, D. & Cockshell, C.D., 1995, The Otway Basin: Early Cretaceous rifting to Neogene inversion: the APEA Journal, 35(1), 451-466. Perincek, D., Simons, B., Pettifer, G.R., 1994. The tectonic framework and associated play types of the Western Otway Basin, Victoria, Australia. The APEA Journal, 34(1), 460-478. Ryan, D.J., Boreham, C.J., Deighton, I., Krassay, A.A. And Cathro, D.L., 2005—Petroleum systems of the Otway Basin, Southern Australia: oil and gas in a complex multi-phase rift basin. 25th Annual GCSSEPM Foundation Bob F. Perkins Research Conference: Petroleum Systems of Divergent Continental Margin Basins, Dec 4–7, 2005, Houston, Texas. Schneider, C.L., 2005 Ph.D. Thesis, University of Melbourne Smith, M.A., Cathro, D.L., Earl, K.L., Boreham, C.J. and Krassay, A.A., 2003. An audit of selected offshore petroleum exploration wells in the Otway Basin, southeastern Australia. Geoscience Australia Record, 2003/21, 158p. Woollands, M.A. & Wong, D., 2001. Petroleum Atlas of Victoria, Australia. Department of Natural Resources and Environment. 40
  48. 48. APPENDIX A Regional Sequence Stratigraphy Study. Summary. This study involved the initial interpretation of 10 paper seismic lines from on to offshore Otway Basin with 8 wells with geology available (composite logs). The results of this work were transferred onto a Kingdom project and a larger dataset of seismic was interpreted. Figure A1. Hardcopy and digital Otway Basin grids. These data, however, are not a regular grid, especially out in the deep water Otway Basin and so most of the outboard picking is relatively unconstrained and, added to the lack of any deepwater well control, there is uncertainty in the offshore picking. Palaeogeography maps were made for each of the megasequences but a caveat is that there is little good geology calibrating the maps due to lack of wells and, with those wells available, lack of data such as composite logs. The data were subdivided into nine 2nd order cycles (“megasequences”) And these are (with approximate ages and seismic horizon names): Lower Otway Group (Berriasian to Barremian-RK_basement to RK_Otway30) Middle Otway Group (Aptian-RK_Otway30 to RK_Eumerella30) Upper Otway Group (Albian-RK_Eumerella to RK_Otway_MegaS) Turonian (Turonian to Cenomanian-RK_Otway_MegaS to RK_Turonian_MegaS) Belfast (Santonian to Coniacian-RK_Turonian_MegaS to RK_Belfast_MegaS) Sherbrook (Campanian to Maastrichtian-RK_Belfast_MegaS to RK_Sherbrook_MegaS) 41
  49. 49. Wangerrip (Maast to mid Eocene-RK_Sherbrook_MegaS to RK_Wangerrip_MegaS) Nirranda (mid Eocene to Oligocene-RK_Wangerrip_MegaS to RK_Nirranda_MegaS) Heytesbury (Oligocene to Miocene-RK_Nirranda_MegaS to RK_Heytesbury_MegaS) Figure A2. Megasequence chronostratigraphy. Otway Group. In general, the seismic data quality for the Otway is not good but there are “windows” of better data quality. A considerable thickness of Otway Group has been interpreted outboard (namely, in deep water, away from the last “shallow” well control) but this disagrees with the interpretations of Norvick and Palmowski who have no outboard Otway. This interpretation difference is concerned with a deep reflector that this study contends is basement, but which other authors refer, enigmatically, to as the “regional detachment surface”. The little good well data available for this unit show considerable sand deposition and published data discuss fluvial systems with some intermixed volcaniclastics and lacustrine shales, with some thin coal development within the Eumeralla Formation. Lower Otway Megasequence. The Lower Otway Group (“Casterton/Lower Crayfish” lithostratigraphic names) is mapped inboard as a well developed E- W half graben with lake and “delta” facies belts with broader, less asymmetric and more N-S basins to the east. This half graben fill can be subdivided into 3 sequences, each of which has a lacustrine shale basin centre and an infilling lacustrine delta facies. The lakes are relatively shallow and this may be a negative factor for good lacustrine source rock development. 42
  50. 50. Figure A3. Otway megasequences-facies map, isochron and selected seismic. Figure A4. Onshore seismic through Otway megasequences. 43
  51. 51. Figure A5. Basal Otway half graben facies. Middle Otway Megasequence. The Middle Otway unit (“Upper Crayfish”) has poor data quality but appears to be present in broader and possibly more symmetric half grabens (when compared to the basal Otway unit). There may be better communication between grabens that could have an impact on lacustrine source rock development. Windows of better data quality show a well bedded section, often with higher amplitude basal sheet facies whose geology is unknown (could be anything from basal transgressive sand (as seen in the Ridge Basin, near Los Angeles) to lacustrine turbidites (ditto). Inboard this unit underwent uplift and erosion as there are well developed truncational erosion geometries beneath the Upper Otway. 44
  52. 52. Figure A6. Middle Otway half graben facies. Upper Otway Megasequence. The Upper Otway unit (“Eumeralla Formation”) is a thin, often well bedded sheet facies deposited on the erosional Middle Otway unconformity and infilling relict half graben accommodation space. Outboard this unit is mounded and, in isolation, could be interpreted as a volcano or a reef, but given the age and basinal setting the mounds are interpreted to be due to gravity sliding as the basin underwent its first regional tilting to the southwest. (Note that Palmowski in his PhD thesis interprets this unit to be Turonian in age). The sliding implies a high shale content and the bedding character may be related to coals in a flood plain setting. Figure A7. Upper Otway basinal gravity slide mounds Turonian Megasequence. The Turonian Megasequence was deposited (in the Shipwreck Trough area) as two shallow marine sequences (deltas and basal transgressive sands-based mostly on log character) infilling many small, faulted depocentres, some of which may be in relatively deep water. 45
  53. 53. Figure A8. Cretaceous facies maps, isochrons and selected seismic The deltas have a relatively high amplitude and continuity seismic character. Asymmetry in some grabens suggests that isolated lacustrine shales may be present (e.g. downdip from Whelk-1). Figure A9. Turonian shallow marine shelf sequences. Outboard there is a large structural high that is difficult to interpret. This study considers it to be a series of stacked thrusts of distal Turonian sediments related to gravity sliding into the tilting basin. 46
  54. 54. Figure A10. Turonian gravity slide complex. Updip from here we interpret the associated extensional faults that sole out along the shaly slide plane (and up which Palmowski interpreted igneous intrusions). The Shipwreck Trough is interpreted to be the main focus for clastic input to the basin, possibly due to nearby uplift to the north or northeast. There may be a landward fluvial facies belt that grades basinward into a wave dominated deltaic facies belt. An additional, isolated, dominantly fluvial basin has been interpreted in the Nerita-1 area. There is a high clastic input along the axis of wells from Fergusons Hill-1 to Whelk-1 and a good proximal to distal facies change in these belts from Iona-1 to La Bella-1. Figure A11. Turonian grabens. Belfast Megasequence. The Belfast Megasequence was deposited as two inboard shallow marine sequences that grade to deepwater outboard. The proximal deltaic facies in Loch Ard-1 and Fergusons Hill-1 grade rapidly to distal shales in Prawn-1A and Minerva-1. Possible shelf progradation geometries are evident downdip from Whelk-1. Most clastics (shallow and deep water) entered from the eastern (or northern) margin of the Shipwreck Trough. Several potential large submarine fans have been interpreted downdip from the Thylacine-1 area. The latter well has a thin fan 47
  55. 55. sand which is on production at the Woodside facility (but its connectivity to Thylacine-2 may be more complex than published by WOPD). Figure A12. Deepwater Belfast seismic facies. These fans are in areas of potential hydraulic jumps (e.g. damming up against adjacent topography) and are higher amplitude and continuity with associated channels. Picking downdip from Thylacine-1 into the basin is not easy due to a series of fault terraces but a model has been used for this unit whereby all fan candidates are made coeval to the Thylacine fan. Most of these potential fans may have updip pinchout traps and there could be several associated with the downdip Thylacine-1 fault terraces. Some of these may be within reach of “shallow” wells, similar to Thylacine-1 and it is suggested that these features, especially if allied to migration work, may be of interest to industry. Figure A13. Deepwater Belfast facies near Thylacine. 48
  56. 56. Figure A14. Belfast submarine fan candidate, downdip from Whelk-1. Sherbrook Megasequence. The Sherbrook Megasequence was deposited as up to three inboard shallow marine sequences, whose development in the Shipwreck Trough is similar to that of the previous unit. Possible prograding geometries are associated with the sands in Bridgewater Bay-1 and also in the adjacent onshore. The deltas (braided stream facies?) in Iona-1 have a well developed higher amplitude-continuity seismic facies compared to the underlying marine shales. Figure A15. Sherbrook shallow marine sequences. 49
  57. 57. There is an unusual high amplitude and continuity facies in the basin downdip from Amrit-1, which tagged the “distal” edge of this unit (with two thin “deltaic” sands and a lot of shale). This facies thins into the basin and was interpreted as a possible fan but more Amrit-1 data is needed to sort this out. On the next fault block downdip from Amrit-1 there appears to be a small gas accumulation within the high amplitude facies. This extensive basinal facies has, in places, faults of a younger unit soling out onto its top. There are erosional channels and mounds associated with a sequence above this basal facies and this study recommends more work be done in this as it may be of interest to industry. Figure A16. Sherbrook high amplitude “Amrit” facies. Wangerrip Megasequence. The Wangerrip Megasequence was deposited as (at least) two inboard shallow marine sequences with little deepwater clastic sedimentation occurring. The wave dominated delta belts are readily mapped and show N and NE sediment supply, not the NW component seen for the Sherbrook. 50
  58. 58. Figure A17. Tertiary facies maps, isochrons and selected seismic. The input from the Nerita-1 area may be significant, with a large inboard fluvial facies belt grading to a sandy, wave dominated delta belt from Whelk-1 to Discovery Bay-1 and beyond. Seismic facies are excellent within this and the younger megasequences and sophisticated facies mapping is possible. Large canyons separate the sequences in Discovery Bay-1 but these may be due to local shelf edge collapse rather than to regional base level fall (or both). When lines of wells such as Minerva-1 to La Bella-1 and Loch Ard-1 to Prawn-1A are considered, the prograding slope facies suggest a continually falling base level between successive sequences (up to 4 or 5 of them?). This means low accommodation, high sediment supply and hence high N/G (net to gross sand percentage) for the highstand deltaics - eg in Prawn-1A. 51
  59. 59. Figure A18. Wangerrip shallow marine shelf facies. Figure A19. Wangerrip lowstand deltaics. Nirranda Megasequence. The Nirranda Megasequence has not had facies maps made but may consist of a single shallow marine sequence and is similar to the Wangerrip in most respects. One difference is the very thick potential basal transgressive sand- volcaniclastic package of the Boonah Formation, as seen in Nerita-1. Northeast of Prawn-1A a series of discontinuous mounds are interpreted to be Nirranda-aged volcanoes and more igneous facies may be present in the Wild Dog-1 area. 52
  60. 60. Figure A20. Nirranda sequence. Heytesbury Megasequence. The Heytesbury Megasequence has not had facies maps made and is similar to the Wangerrip/Nirranda units except that most of the shallow marine highstand units are now platform carbonates rather than clastic deltas. There may be very distal lowstand clastics out in the basin and possible igneous intrusives in the Voluta-1 area. Issues. There are several “issues” associated with this study: • An inadequate seismic grid means interpretations are more subjective than usual when the grid is tighter. It may be necessary to run with alternative interpretations at the same time. Deepwater picking will always be equivocal due to the lack of deep water wells. • Inadequate well calibration. Composite log and other “hard” well geology, together with formation picking and sequence stratigraphy, needs to be fully integrated with the seismic stratigraphic study in order to construct palaeogeography maps. Further integration is still required between these disparate datasets and the structural framework study. • The seismic facies mapping was done in a short time (entire study some 13 days). • There is a different interpretation for the basinal extent of the Otway between this study and Norvick- Palmowski. It was interesting to note that in the DPI workshop (2 Sept, 2008) a key line was worked by all participants and most agreed with this study’s conclusion that the Otway Group may extend well beyond the shelf edge. 53
  61. 61. APPENDIX B Regional Seismic Cross Sections Five regional sections were interpreted and depth converted, with three key sections being restored. Where possible they link the offshore geology with that onshore, and illustrate the structural and stratigraphic development of the basin. Regional Line A A c.290 km long line in the west of the basin, with a short, N-S onshore section and a long, NNE-SSW offshore portion tying Breaksea Reef-1. Onshore imaging is poor but the structure appears to be dominated by moderately spaced, south dipping, Late Cretaceous faults. The faulting is more closely spaced immediately offshore, beneath the shelf, but becomes more widely spaced further offshore. The faulted section is unconformably overlain by thin Tertiary sediments that can be traced back to the progrades that build the continental shelf. The Early Cretaceous section interpreted above basement thins towards the southern end of the line and is overlain by thinner, largely unfaulted Late Cretaceous and Tertiary basin fill. Regional Line B Another onshore to offshore dip line in the west of the basin, extending NNE-SSW for c.350 km, and tying Bridgewater Bay-1. The line images an Early Cretaceous half graben in the north, the Portland Trough, shelf edge and deep water portion of the basin. Top basement is imaged clearly only in the north. Late Cretaceous faulting is conspicuous south of the shelf edge, as is thinning of the upper crust in the deep water portion of the line. The base Tertiary unconformity and prograding units building the shelf are also clearly imaged. Regional Line C An onshore to offshore dip line in the central part of the basin, extending NNE-SSW for c.160 km. The line ties Caramut-1 and Woolsthorpe-1 in the onshore portion, across an Early Cretaceous half graben controlled by mainly north dipping faults. Offshore, south dipping Late Cretaceous faults are also well imaged, as is the base Tertiary unconformity and shelf build up. Regional Line D A NNE-SSW line extending for c.395 km in the east of the basin, with only the northernmost 35 km onshore. The line ties Iona-1, Minerva-1 and -2A and La Bella-1 on the shelf and continues into the offshore, deep water part of the basin. In the north small scale faulting is imaged within the Late Cretaceous and Early Tertiary section, as are the small Late Tertiary folds. Late Cretaceous faults are imaged beneath the base Tertiary unconformity on the shelf, as well as the Early Tertiary progrades that build the shelf and the Late Tertiary Minerva Anticline. Offshore the Late Cretaceous section is much less faulted but slightly folded beneath the base Tertiary unconformity. Further offshore the Late Cretaceous onlaps a prominent high of possible Otway Group, south of which is a thin, largely unfaulted, deep water basin. 54
  62. 62. Regional Line E This is a c.190 km long, onshore-offshore line across the Torquay Sub-basin, in the east of the Otway Basin. It extends south from near Warracburunah-2, north of the Otway Ranges, across the ranges to the coast, then NW-SE across the offshore part of the sub-basin to the NW corner of the Bass Basin, tying Nerita-1 and Snail-1. Early Cretaceous faults are (poorly) imaged in the north, as are the base Tertiary unconformity and the edges of the Otway Ranges anticline. Offshore the Early Cretaceous half grabens, thin Late Cretaceous section and shallow basement of the Mornington-King Island High are clearly imaged, as well as some large, south dipping, normal faults in the northwest of the Bass Basin. A prominent feature in the mid offshore is the Late Tertiary Nerita Anticline. The reverse faults at the eastern margin of the Mornington-King Island High are also clearly imaged. Regional Line F This line extends WNW-ESE parallel to the coast in the west, where it is largely parallel to the strike of the Late Cretaceous faults, to south of Cape Otway, tying Voluta-1, Pecten-1A, Minerva-2A and Loch Ard. SE of Pecten the line crosses the offshore extension of the Otway Ranges. It then continues NE across the Torquay Sub-basin through Wild Dog-1 to Nerita-1, and ENE to the Mornington Peninsula, oblique to strike. Total length is 470 km but only the eastern 2/3rds will be restored. The line clearly shows the nature of the fold belt associated with the Otway Ranges, the shallow basement platform in the SW of the Torquay Sub-basin, the underlying Early Cretaceous structure and young folds in the Torquay Sub- basin and the young reverse faults and folds at the eastern (Mornington Peninsula) edge of the sub-basin. 55
  63. 63. Figure B1: Regional Seismic Line A Figure B2: Regional Seismic Line B 56
  64. 64. Figure B3: Regional Seismic Line C 57
  65. 65. Figure B4: Regional Seismic Line D Figure B5: Regional Seismic Line E 58
  66. 66. 59
  67. 67. Figure B6: Regional Seismic Line F 60
  68. 68. APPENDIX C Regional Well defined Facies Maps Figure C1. Facies map, lower part of Waarre Formation                         Legend       Sediment Transport    Facies Boundary                          ‐‐‐‐‐‐‐‐‐‐‐‐  DT  Delta Top  SM  Shallow Marine  SF  Shore Face  OS  Outer Shelf  61
  69. 69. Figure C2. Facies map, Waarre Formation Figure C3. Facies map, Flaxman Formation 62
  70. 70. Figure C4. Facies map, Nullawarre Formation Figure C5. Facies map, Paaratte Formation 63
  71. 71. Figure C6. Facies map, Timboon Formation 64
  72. 72. APPENDIX D Seismic Sequence Facies Maps Figure D1: Seismic Facies Map – Basal Otway Group 65
  73. 73. Figure D2: Seismic Facies Map – Mid Otway Group 66
  74. 74. Figure D3: Seismic Facies Map – Upper Otway Group 67
  75. 75. Figure D4: Seismic Facies Map – Turonian – Waarre-Flaxman Fm. 68
  76. 76. Figure D5: Seismic Facies Map – Belfast Fm./Shipwreck Sequence Figure D6: Seismic Facies Map – Sherbrook Sequence 69
  77. 77. Figure D7: Seismic Facies Map – Wangerrip Sequence 70
  78. 78. Figure D8: Seismic Facies Map – Nirranda Sequence 71
  79. 79. APPENDIX E Figure E1: Seismic Sequence Well Log Picks 72
  80. 80. APPENDIX F Figure F1: Regional Structural Restoration Section D 73
  81. 81. APPENDIX F Figure F2: Regional Structural Restoration Section E 74
  82. 82. APPENDIX F Figure F3: Regional Structural Restoration Section F 75

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