Vi MP VICTORIAN INITIATIVE Natural Resources and Environment FOR MINERALS & PETROLEUM AGRICULTURE RESOURCES CONSERVATION LAND MANAGEMENT HYDROCARBON PROSPECTIVITY OF THE OFFSHORE TORQUAY SUB-BASIN, VICTORIA: GAZETTAL AREA V99-1 VIMP REPORT 60 B.E.MESSENT, G.I. COLLINS and B.G. WEST April 1999 MELBOURNE GEELONG WOOLSTHORPE 1 HINDHAUGH CREEK 1 GARVOC 1 COBDEN COLAC STONEYFORD 1 WARRNAMBOOL ANGLESEA 1 MYLOR 1 BOGGY CREEK 1 NORTH PAARATTE 1 IONA 1 LANGLEY 1 PORT CAMPBELL NERITA 1 MINERVA 1 PECTEN 1A APOLLO BAY WILD DOG 1 SNAIL 1 TRITON 1 LA BELLA 1 V99-1 0 8 16 24 32 40 Km . VIMP Report 60 Hydrocarbon prospectivity of the offshore Torquay Sub-basin, Victoria: Gazettal Area V99-1 B. E. Messent, G. I. Collins and B. G. West April 1999 Bibliographic reference: may arise from you relying on any information MESSENT, B.E., COLLINS, G.I. AND WEST, B.G., in this publication. 1999. Hydrocarbon prospectivity of the offshore Torquay Sub-basin, Victoria: Gazettal Area V99-1. Victorian Initiative for Minerals and Petroleum Report 60. Department of Natural Resources and Environment. Crown (State of Victoria) Copyright 1999 Petroleum Development ISSN 1323 4536 ISBN 0 7306 9442 9 This report may be purchased from: Minerals and Petroleum Business Centre Department of Natural Resources and Environment 8th Floor, 240 Victoria Parade East Melbourne Victoria 3002 For further technical information contact: Manager Petroleum Development Department of Natural Resources and Environment PO Box 500 East Melbourne Victoria 3002 Authorship and Acknowledgments: Geoff Collins of DNRE carried out the seismic interpretation. Barry Messent, consultant, compiled the report, in part based upon work carried out by Barry West of DISR. The authors acknowledge the assistance of DNRE staff, particularly the Basin Studies Group. Andrew Constantine provided valuable comments. Eddie Frankel provided technical support for the project, including organising seismic and well data, obtaining technical reports, base maps and the final preparation of figures and enclosures. Karina Jonasson prepared the stratigraphic well summary sheets and Geolog cross-section. Halya Sztynda of One Tree Graphics Pty Ltd drafted the majority of supporting figures and enclosures. The report was edited by Dr Mike Woollands, Andrew Constantine and Karina Jonasson. Laiyee Mok is much appreciated for preparing the text for publishing. Disclaimer While this publication may be of assistance to you, the author and 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 consequence which OFFSHORE TORQUAY SUB-BASIN, VICTORIA 1 Contents Executive Summary 3 1 Introduction 5 2 Basin development 8 2.1 Structural history 8 2.2 Structural elements 8 3 Stratigraphy 11 3.1 Otway Group 11 Casterton Formation 11 Crayfish Subgroup 11 Eumeralla Formation 11 3.2 Eastern View Group 11 3.3 Demons Bluff Group 12 3.4 Torquay Group 12 4 Well failure analysis 14 4.1 Well results 14 Nerita 1 14 Wild Dog 1 14 Snail 1 15 4.2 Hydrocarbon shows 15 Nerita 1 15 Anglesea 1 15 4.3 Hydrocarbon seepage 15 5 Seismic evaluation 20 5.1 Seismic database 20 5.2 Data quality 20 5.3 Interpretation and mapping 20 5.4 Structural leads 21 6 Hydrocarbon prospectivity 27 6.1 Reservoir 27 6.2 Seal 27 6.3 Source rock quality 35 Crayfish Subgroup and Casterton Formation 35 Eumeralla Formation 35 6.4 Source rock maturation 36 7 Petroleum systems 44 7.1 Crayfish Subgroup 44 7.2 Eumeralla Formation 44 8 Play fairways 47 8.1 Trap types 47 Tilted fault block 47 Onlap onto basement high 47 Fractured basement 47 Hanging wall traps 47 8.2 Risks 47 9 Commercial Issues 49 9.1 Infrastructure 49 9.2 Markets 49 References 50 Victorian Initiative for Minerals and Petroleum (VIMP) report series 52 2 OFFSHORE TORQUAY SUB-BASIN, VICTORIA List of figures 1 Gazettal Area V99-1 6 2 Otway Basin: permit and well locations 7 3 Eastern Otway Basin: structural elements 9 4 Regional cross-section across the eastern Otway Basin 10 5 Torquay Sub-basin stratigraphy 13 6 Seismic line OS88A-12 through Nerita 1 showing closure at top reservoir (Boonah Formation) 17 7 Seismic line OS90A-13 through Wild Dog 1 showing closure at top reservoir (Boonah Formation) 18 8 Seismic line OS88A-04 through Snail 1: note lack of closure at top reservoir (Boonah Formation)) 19 9 Torquay Sub-basin: Leads map 23 10 Seismic line BMR 40 12-7A: T1 Lead 24 11 Seismic line OS90A-13: B2 Lead 25 12 Seismic line OS90A-53: R1 Lead 26 13 Stoneyford 1: wireline logs of Pretty Hill Formation (1000 m-TD) 28 14 Garvoc 1: wireline logs of Pretty Hill Formation (1300 m-TD) 29 15 Woolsthorpe 1: wireline logs of Pretty Hill Sandstone (1000 m-TD) 30 16 Isopach of sedimentary thickness: F. wonthaggiensis zone 31 17 Eumeralla and Pretty Hill Formations: porosity versus depth for 12 wells 32 18 Stratigraphic correlation Penola Trough gas fields (after Scholefield et al., 1996) 33 19 Cross-plots of ambient vs. overburden permeability and calculated overburden porosity vs. permeability in Pretty Hill Formation channel sands, Penola Trough gas fields (after Scholefield et al., 1996) 34 20 Bottom water light hydrocarbon anomalies: BMR Line 104/022 38 21 Otway Group source rocks: van Krevelen diagram 39 22 HI vs. Tmax plot: Crayfish Subgroup (F. wonthaggiensis zone) 40 23 HI vs. Tmax plot: Eumeralla Formation (C. hughesii = P. notensis zone) 41 24 Comparison of possible burial history plots for Nerita 1, with isotherms and calculated vitrinite reflectance (VR) profile match to measured VR 42 25 Early Cretaceous palaeothermal gradients based on kinetic modelling of VR data from 21 Otway Basin wells (after Cooper, 1995) 43 26 Crayfish petroleum system distribution 45 27 Eumeralla petroleum system distribution 46 28 Schematic section showing potential play-types 48 List of tables 1 Summary of Pretty Hill Formation Leads 22 List of enclosures 1 North – South geological cross-section showing schematic structure and stratigraphic correlation 2 Nerita 1 Stratigraphic well summary sheet 3 Wild Dog 1 Stratigraphic well summary sheet 4 Snail 1 Stratigraphic well summary sheet 5 Anglesea 1 Stratigraphic well summary sheet 6 NNW-SSE Seismic Line BMR 12-7A, showing major structural elements 7 WSW-ENE Seismic Line OS88A-12, showing major structural elements 8 Top Basement Seismic Horizon - Two Way Time Structure Map (1:100,000) 9 Top Otway Group Seismic Horizon - Two Way Time Structure Map (1:100,000) OFFSHORE TORQUAY SUB-BASIN, VICTORIA 3 Executive Summary Crayfish Subgroup petroleum system is sourced by claystones of the Casterton and Pretty Hill Formations and is likely to work mainly in the Invitations to explore for hydrocarbons, under vicinity of the Snail Terrace. The Eumeralla the work program bidding system, in Area petroleum system is sourced by coals in the V99-1 of the eastern Otway Basin have been basal section of the Eumeralla Formation and is gazetted in April 1999. V99-1 is a large more closely aligned with the Basin Deep. (7,200 km²), under-explored, shallow water area comprising virtually all of the offshore Torquay With only limited data available on Sub-basin. Applications for the acreage close palaeotemperature gradients, there is on 14 October 1999. This re-evaluation of the significant scope for variations in heat-flow and hydrocarbon potential, carried out by Minerals therefore the timing of hydrocarbon expulsion. and Petroleum Victoria, shows that there is The presence of gas fields further west in the likely to be significant untested potential in the basin show the claystones of the Pretty Hill Sub-basin. Formation and Eumeralla Formation to be effective seals. Geochemical analysis of the The offshore part of the Sub-basin has two Early Cretaceous Eumeralla, Pretty Hill and main structural elements, namely the Snail Casterton Formations throughout the Otway Terrace and the Basin Deep, the latter bounded Basin demonstrate that they contain source to the northwest by the onshore Otway Ranges rocks capable of generating both oil and gas. and flanked to the southeast by the shallower Thicknesses of the Early Cretaceous are more Snail Terrace. The region has a tectonic history than adequate for thermal maturity. By that reflects two basins. The Late Jurassic to analogy with the Otway Basin further west, Early Cretaceous rift sequence is similar to the where there is significant gas in the onshore western Otway Basin, whereas the Late Port Campbell Embayment and offshore at Cretaceous to Recent sequence is more akin to Minerva and La Bella, gas is the most likely the Bass Basin. hydrocarbon type. However, on the Snail Terrace, the chance of encountering oil is Three wells have so far been drilled in the Sub- higher because part of the source interval is basin: Nerita 1 (1967), Snail 1 (1972) and Wild still probably within the oil window. Dog 1 (1992), all of which targeted the Tertiary section and were dry. Nerita 1 can be shown to The principal remaining play types in the be a late compressional feature which Torquay Sub-basin are tilted fault block traps post-dated expulsion and migration of within the Early Cretaceous, and it is these hydrocarbons. There is no closure at Snail 1 play-types, which are successful elsewhere in and Wild Dog 1 is a relatively late the basin, that are proposed for the next phase compressional feature, which is partly fault of drilling. One of the major targets in the dependent but appears to be a valid trap. The western part of the Otway Basin is the Pretty cause of failure, although not clearly Hill Formation, which is thought to be present understood, may be due to the structure post- at depths of around 2000 m on the Snail dating the main migration phase, the presence Terrace.