GSV Report 103 - The stratigraphy, structure, geophysics and hydrocarbon potential of the Eastern Otway Basin
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Product description:Download The downloadable version of this report is supplied in PDF format. Abstract Background Oil and gas exploration is a high cost, high risk venture that is developed around existing knowledge. The Department of Energy and Minerals is in a unique position to synthesize data on sedimentary basins. The Department has access, via the relevant Acts, to the latest company data as well as to its own extensive data sets. The Eastern Otway Basin Project was designed to take advantage of this position, and with the cooperation of operating companies, make use of recent data to produce information packages and databases in readily accessible and interpreted format. The Eastern Otway Basin Project objective was to establish comprehensive geological and geophysical databases and provide comprehensive technical information packages for industry. In addition to this, Geological Survey of Victoria geologists and geophysicists provided a synthesis and interpretation of this data. The project collated all public and private sector geological and geophysical data, including drilling, seismic, aeromagnetic and gravity data. Lithological, biostratigraphic, geological and seismic mapping, aeromagnetic, gravity and topography interpretation and structural data were synthesized to focus on areas of hydrocarbon potential and guide future exploration strategies. This information package is the second part of a three part information package. Part 1 consists of the the Port Campbell Embayment 1:100 000 Geological Map and Report, and Part 3 the Colac 1:250 000 Geological Map and Report. In Summary: 1. This study is concerned with geological interpretation and hydrocarbon potential of the onshore Eastern Otway Basin in Victoria. 2. Lithological, biostratigraphic, geological and seismic mapping, aeromagnetic, gravity, topography and structural data have been synthesized into a consistent geological interpretation. Using this framework new models for hydrocarbon potential have been developed. 3. A uniform stratigraphic nomenclature has been established for the Eastern Otway Basin, and applied to 285 key wells in the area. 4. A systematic programme of lithological logging has been undertaken on most boreholes, including petroleum, groundwater and stratigraphic bores. This data has been integrated with geophysical logs into a subsurface structural and facies interpretation. Basin wide cross-sections show the lateral extent of the formations. 5. Available palynology has been compiled into a single dataset. A number of inconsistencies with the data have been resolved, and new analyses have contributed to determining the ages of the various formations. 6. A history of geological mapping, geophysical surveying and petroleum exploration in the basin has been compiled, including a listing of boreholes and previous seismic, gravity and magnetic surveys. 7. As a result of the mapping component of this project new geological maps of Colac 1:50 000, Port Campbell Embayment 1:100 000 and Colac 1:250 000 have been, or are currently being, published. 8. Formation Type Sections have been redefined, with thin sections and hand specimen for key cores described. 9. New airborne geophysics has been obtained in conjunction with the Federal Australian Geological Survey Organisation (AGSO), and has been used to determine the nature and structure of the basement to the Otway Basin. 10. New techniques of data integration and image processing have been used to develop a new approach to basin analysis. The datasets include formation tops, geophysical logs, palynology, gravity, magnetics, seismic, topography and borehole data. Where appropriate, this data is available as located and gridded data suitable for image processing. 11. The stratigraphy has been considered in terms of modern concepts of sequence stratigraphy. Six sequences, representing major transgressive/regressive cycles, have been proposed, two in the Sherbrook Group, one covering the Wangerrip Group, two in the Nirranda Group, and one covering the Heytesbury Group. 12. Isopach maps for all formations have been produced from borehole information. The borehole information has been combined with seismic two-way time data to produce isopach maps for the Tertiary and Upper Cretaceous which take into account structures at depth. 13. Structure contour maps for the top Otway Group, base Tertiary and top Nirranda Group have been produced from borehole and surface mapping data. Base Tertiary, top Otway Group and top basement depth maps have also been produced from borehole and seismic two-way time data. 14. Geological mapping, in addition to gravity stripping, seismic fault mapping and topographic analysis have confirmed the structural complexity of the basin. Surface topographic features are clear indicators of subsurface structure, with the topographic data set providing considerable structural information. 15. It has been shown that gravity is a valuable exploration tool when used in conjunction with other data, particularly seismic and borehole information. Gravity has contributed significantly to the understanding of the broad scale geometry of the Eastern Otway Basin, which consists of northeast trending gravity highs separated by embayments of thick sediment sequences. The gravity data also allows smaller, more local features, to be distinguished. 16. The northern margin of the Otway Basin has been redefined further north. A deep stratigraphic hole, Warracbarunah 2, was drilled to confirm the presence of thick Pretty Hill Formation in the previously unknown Gellibrand Trough. This sedimentation is interpreted to represent an aborted earlier rift sequence. In addition, a number of shallow holes were drilled to map the basin margin. 17. Two major structural zones have been recognized - Lorne Zone and Port Campbell Embayment Zone - on the basis of structural style. The Lorne Zone is characterized by Jurassic - Early Cretaceous north-northwest extension, followed by late Tertiary to Recent northwest compression. The Port Campbell Embayment Zone is characterized by northeast extension in the middle to late Cretaceous. The two zones are bounded by the Stoneyford Gravity high. 18. Hydrocarbon shows from all the bores of the Otway Basin have been compiled, and vitrinite reflectance and total organic carbon plots for key wells generated. 19. The presence of nine gas fields, two sub-commercial oil fields and numerous oil shows indicate that hydrocarbons have been generated in the basin. 20. Source rock analysis indicates that the non-marine Eumeralla Formation and marine/marginal marine Belfast Mudstone are likely to be the major source rocks in the Otway Basin. The Casterton Formation also has marginal oil generation potential. 21. Studies of the depth of burial with time and development of structural traps for the major source rock formations indicate that traps are unlikely to have developed while the Otway Group was still in the oil generation window, but may have developed in time to trap gas. A second stage of hydrocarbon generation occurred after the deposition of the Wangerrip Group, with increased depth of burial moving the Otway Group into the gas window and the Belfast Mudstone into the oil window. 22. Studies of the reservoir potential of the formations indicate that the Pebble Point Formation, Waarre Formation and Pretty Hill Formation are the most likely hydrocarbon reservoirs in the study area. 23. A number of exploration targets and play types have been suggested for the study area. These include pinchouts, rollovers and fault blocks in the Pretty Hill Formation on the northern margin of the basin. Within the Port Campbell Embayment play types include: fault blocks and rollovers in the Waarre Formation; a small area with potential fault block traps in the Nullawarre Greensand; pinchouts, rollovers and fault blocks in the Base Tertiary sands, including Pebble Point Formation; and rollovers and fault blocks in the Dilwyn Formation. 24. The faulted rifts of the northern margin of the Otway Basin are identified as a major new play type for the onshore areas of the basin. Key Recommendations: Use detailed geophysical data, including seismic, gravity and geophysical logs, to further refine possible drill targets outlined here. The studies carried out here be extended to the western Otway Basin, covering the area between this study and the South Australian border. These should include:
Bibliographic reference Geological Survey of Victoria, 1995. The stratigraphy, structure, geophysics and hydrocarbon potential of the Eastern Otway Basin. Geological Survey of Victoria Report 103. | ||||||
