ViMP

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. Permeabilities of up 1000 md are of thief zones within the cross-fault seal, or present in these producing sandstones in the charge unable to reach the structure. Katnook and surrounding gas fields in South Australia, at depths of 2500-3000 m. Significant gas shows were encountered in Nerita 1 in both the Late Cretaceous – Early In the past, structural risk has been perceived Tertiary Eastern View Group and the Early as a major risk, but with additional infill Cretaceous Eumeralla Formation. The Eastern seismic and state-of-the-art processing, this View Group shows are related to coals but the risk can be expected to be significantly reduced. deeper gas shows within the Eumeralla suggest an active petroleum system may be present in MPV has carried out regional mapping in the the Sub-basin. In addition, sea-bed gas area at Top Basement, Top Otway Group and anomalies have been detected by BMR (now Top Boonah Formation (Late Eocene AGSO) north of Nerita 1 in areas where sandstone). A number of leads have been pockmarks are evident on the sea-bed. mapped using the Top Basement Horizon as a guide. These include a very large structure in There are potentially two petroleum systems the Early Cretaceous beneath Wild Dog 1. working in the Torquay Sub-basin. The 4 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

Other play types, in addition to tilted fault blocks, are:

• onlap traps onto palaeohighs, • fractured basement plays, • hanging wall sandstone plays.

Shallow water depths of approximately 50-90 m, coupled with the proximity to shore and pipeline infrastructure suggest that gas discoveries could be candidates for early commercialisation. Oil could also be relatively cheaply developed. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 5

1 Introduction Significant hydrocarbon accumulations have been found in the offshore Otway Basin in the Minerva and La Bella gas fields to the west of The Commonwealth and Victorian State Joint the gazetted area. These are reservoired in the Authority has released, for application under Cenomanian to Santonian Waare Formation. the work programme bidding system, an area Onshore, in the nearby Port Campbell covering a large part of the offshore Torquay Embayment, commercial gas discoveries are Sub-basin. The area, gazetted as V99-1, present at North Paaratte, Iona, Boggy Creek contains 119 graticular blocks with water (CO2), Mylor, Fenton Creek and Langley. At depths in the order of 50-90 m and is situated Mylor 1 there is also a 2.5 m oil leg, the first approximately 100 km south-southwest of recovery of oil from the Late Cretaceous section Melbourne (Fig. 1). In addition to the relatively in this area. shallow water depth, the gazettal area is close to shore and, more importantly, close to onshore In the Penola and Robe troughs of the western gas pipeline infrastructure and the oil refinery Otway Basin, commercial gas and minor oil has at Geelong. been found in the Late Jurassic-Early Cretaceous Crayfish Subgroup in the Katnook, A seismic grid varying from 1 x 4 km to Ladbrook Grove, Haselgrove and Haselgrove 5 x 7 km is present over approximately half of South gasfields, as well as in the Killanoola 1, Area V99-1 (Fig. 2). The remaining area has a Redman 1 and Nunga Mia 1 wells. The regional seismic grid of approximately structural and stratigraphic setting of these 10 x 10 km. The available seismic data South Australian discoveries is perceived as a includes a number of vintages. Post 1970 possible model for the Torquay Sub-basin. surveys include Haematite HO-2, Portland - King Island (1972), BMR 40 Bass Strait (1982), Confirming the setting of the Torquay Shell OS88A (1988) and OS90A (1990). The Sub-basin in a regionally prospective area seismic data quality is variable between (Fig. 1), there have also been gas discoveries to surveys and across the area. It is anticipated, the southeast in the Bass Basin at Yolla, White with recent developments in seismic acquisition Ibis 1, Pelican 1 and a residual oil column at and processing, that significant improvement in Cormorant 1. Yolla 1 also intersected a small data quality should be possible. oil column in the Eocene section. Three wells (Fig. 2) have been drilled to date in Area V99-1: Nerita 1 (Shell, 1967), Snail 1 (Haematite, 1972) and Wild Dog 1 (Shell, 1992). Snail 1 was not a valid structural test, but Nerita 1 tested a valid Pliocene anticline and Wild Dog 1 tested an Oligocene, faulted anticline. Depths drilled varied between approximately 1200 and 2100 m and all the wells terminated within the upper part of the Eumeralla Formation. Gas shows were encountered in the Nerita 1 well within the Eastern View Group and Eumeralla Formation.

The purpose of this report is to review the prospectivity of Area V99-1 for hydrocarbons. The report draws on unpublished work previously carried out by the Basin Studies Group of Minerals and Petroleum Victoria (MPV) and the Bureau of Resource Sciences in Canberra. The report provides an assessment of the petroleum potential of the deeper stratigraphic section, namely the Jurassic to Early Cretaceous sequence (Otway Group), which has not been tested to date. 6 OFFSHORE TORQUAY SUB-BASIN, VICTORIA 38 S 39 S 40 S CORMORANT 1 YOLLA 1 PELICAN 1 WHITE IBIS 1 145 E 145 E MELBOURNE GEELONG SNAIL 1 NERITA 1 NERITA HINDHAUGH CREEK 1 HINDHAUGH ANGLESEA 1 WILD DOG 1 144 E 144 E V99-1 LORNE COLAC STONEYFORD 1 APOLLO BAY APOLLO 100m 80m IONA 1 PORT CAMPBELL NORTH PAARATTE 1 NORTH PAARATTE 1 MINERVA 143 E MYLOR 1 MYLOR 100 143 E 200m LA BELLA 1 80 LANGLEY 1 GARVOC 1 WOOLSTHORPE 1 PECTEN 1A 60 WARRNAMBOOL TRUE NORTH IS SHOWN FOR THE CENTRE OF MAP TRITON 1 Km BOGGY CREEK 1 40

20 Victoria - Tasmania boundary Tasmania - Victoria 142 E 142 E 0 PORTLAND LINDON 1 141 E SAWPIT 1 141 E HASELGROVE 1

Mt GAMBIER

ROBERTSON 1 REDMAN 1 Victoria - S.A. boundary S.A. - Victoria KATNOOK 1 KATNOOK KILANOOLA 1 LADBROKE GROVE 1 140 E 140 E NUNGA MIA 1 WELL SYMBOLS OIL SHOW (WEAK) DRY GAS WELL OIL SHOW (STRONG) OIL WELL OIL & GAS WELL WEAK OIL & GAS SHOW GAS SHOW WEAK OIL GAS SHOW (STRONG) GAS WELL STRONG OIL SHOW TROAS 1 Figure 1 V99-1. Gazettal Area 38 S 39 S 40 S OFFSHORE TORQUAY SUB-BASIN, VICTORIA 7 50 40 30 TRUE NORTH IS SHOWN FOR THE CENTRE OF MAP Km 20 MELBOURNE 145 E

10 OS88A -12 OS88A 0 Victoria - Tasmania boundary Victoria - Tasmania

BMR 12-7a

OS88A -04 OS88A OS90A -53 OS90A SNAIL 1 NERITA 1 NERITA HINDHAUGH CREEK 1 HINDHAUGH GEELONG OS90A-13 ANGLESEA 1 144 E WILD DOG 1 LORNE V99-1 COLAC APOLLO BAY APOLLO STONEYFORD 1 50m COBDEN IONA 1 PORT CAMPBELL 143 E NORTH PAARATTE 1 NORTH PAARATTE 1 MINERVA 80m MYLOR 1 MYLOR 100m LANGLEY 1 Lines referred to in report Structural well correlation LA BELLA 1 200m GARVOC 1 300m PECTEN 1A BOGGY CREEK 1 WOOLSTHORPE 1 WARRNAMBOOL TRITON 1 38 S 39 S Figure 2 well locations. and Otway Basin: permit 8 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

2 Basin development During the early Tertiary, regional subsidence predominated except for periods of local compression towards the end of the Palaeocene 2.1 Structural history and Oligocene. Since Miocene times, possibly due to the collision of Australia’s northern The offshore Torquay Sub-basin forms part of margin with Indonesia and Papua New Guinea, the eastern Otway Basin, which includes the there has been associated compression in the Otway Ranges and the Colac Trough onshore Torquay Sub-basin. This has resulted in the (Figs 3 & 4). The Torquay Sub-basin lies development of folds, inversion of extensional between the Otway Ranges in the northwest faults and uplift, which can be observed by the and the King Island-Mornington Peninsula elevation of the Otway Ranges and King High in the southeast. Island – Mornington Peninsula High and uplift in the Torquay Sub-basin. The eastern Otway Basin was initiated during the Late Jurassic, with the onset of continental 2.2 Structural elements rifting between Australia and Antarctica. The structural style was extensional faulting with The Torquay Sub-basin consists of two discrete the development of half grabens, that were structural elements already established in the orientated approximately northeast-southwest Early Cretaceous: the Basin Deep and the Snail (Fig. 3). There is a significant strike-slip Terrace, the latter bounded to the east and component associated with the development of south by the King Island – Mornington the extensional faults (Cooper, 1995). Initially Peninsula High (Fig. 3). To the north and west, the half grabens were of limited areal extent. the degree of late Tertiary compressional The rifting phase, which was episodic, folding increases significantly, culminating in continued through the Early Cretaceous with the Otway Ranges which form the western limit two major pulses, in the late Jurassic and of the Torquay Sub-basin (Trupp et al., 1994). Barremian. The regional depocentre for the The Snail Terrace contains a sequence of Otway eastern Otway Basin was in the vicinity of the Group sediments, which were deposited in a Otway Ranges. half graben environment. To the east, the Snail Terrace is terminated by a system of relay By mid-Cretaceous times, continental break-up faults, which developed on a shallow had commenced along the southern margin of detachment surface. The Basin Deep, although Australia. There was local compression in the defined by relatively poor quality seismic data, vicinity of the Otway Ranges with inversion is interpreted to contain a thick sequence of and erosion occurring (Hill et al., 1995). In the Otway Group sediments in a series of half vicinity of the Otway Ranges approximately grabens. 3 km of erosion occurred, which was largely structurally controlled. The uplift effectively prevented the deposition of the Sherbrook Group in the Torquay Sub-basin except possibly in local depressions. Cooper (1995) interprets no erosion in the Torquay Sub-basin based primarily on thermochronological data. However, at wells such as Nerita 1, the youngest Eumeralla Formation sediments are C. striatus in age, the C. paradoxa and P. pannosus zones being either absent or part of a condensed sequence. If the zone is absent, it is likely that minor erosion has occurred, probably less than 300 m, which is within the error bar of the thermochronological data. Both Smith (1986) and Trupp et al. (1994) interpret the King Island – Mornington Peninsula High to lack topographic relief at this time and, therefore, there was communication between the Torquay Sub-basin and the Bass Basin. This clearly existed by the late Cretaceous (Eastern View Group times). OFFSHORE TORQUAY SUB-BASIN, VICTORIA 9

38 39

BRS 96/541E

145

(see Figure 4)

A - B Line of section BASS BASIN

BAY

PORT PHILLIP HIGH MORNINGTON -

Melbourne

KING ISLAND KING

SUB-BASIN SNAIL TERRACE SNAIL VICTORIA TASMANIA 40 km

B

DEEP TORQUAY N BASIN Geelong

KING HIGH

ISLAND 144 GRABEN 0

A

TROUGH DOWNS GELLIBRAND RANGES OTWAY V99-01

COLAC

TROUGH BARWON

SORELL FAULT ZONE

HIGH

STONEYFORD

TROUGH 143 SHIP WRECK

EMBAYMENT

CAMPBELL

PORT Otway Basin Margin (approx.) Campbell Port ZONE

Warrnambool

HIGH FAULT

WARRNAMBOOL

MUSSEL Figure 3 Basin: structural elements. Eastern Otway 10 OFFSHORE TORQUAY SUB-BASIN, VICTORIA B SSE BRS 96/542E HIGH KING ISLAND SNAIL TERRACE sea level datum

Tertiary undifferentiated Tertiary Eastern View Group (Upper Cretaceous - lower Tertiary) BASIN DEEP OTWAY RANGESOTWAY SUB-BASIN TORQUAY GRABEN BARWON DOWNS

P.notensis - P.pannosus P.notensis R.watherooensis P.notensis Eumeralla Formation ( spore/pollen zones) Pre-Mesozoic basement Casterton and Pretty Hill Formations ( - lower spore/pollen zones) COLAC TROUGH A

0 5

10 15 20 Depth (km) Depth NNW Figure 4 Otway Basin. Regional eastern cross-section the across OFFSHORE TORQUAY SUB-BASIN, VICTORIA 11

3Stratigraphy 200 m in thickness. The nearest occurrence to the Torquay Sub-basin is in the Woolsthorpe 1 and Ballangeich 1 wells located north of Although a number of stratigraphic Warrnambool (See Figs 1, 2 & 15) where it subdivisions have been used previously in the varies from 50 m to more than 148 m in Otway Basin, for the purpose of this report the thickness (Lavin & Muscatello, 1998). nomenclature used by Trupp et al.(1994) has been adopted (Fig. 5). Crayfish Subgroup The sediments of the Torquay Sub-basin have two associations. Those older than mid- The Early Cretaceous Pretty Hill Formation Cretaceous in age are more closely aligned with conformably overlies the Casterton Formation. the Otway Basin (Morton, 1990). Sediments The formation is comprised of predominantly younger than the mid-Cretaceous uplift of the fine to medium grained quartz-feldspar-lithic Otway Ranges have characteristics more closely sandstones with minor mudstones and coals aligned with the Bass Basin (Smith, 1986). (Morton, 1990). The unit is interpreted to have been deposited in a fluvial to sheet-flood Basement comprising metasediments, volcanics environment under high-energy conditions and intrusives is interpreted to be of Palaeozoic (Struckmeyer & Felton, 1990). The sandstones age. To the west of Geelong, in the Ceres are possibly derived from the reworking of region, greenstones are present (Coulson, 1930) basement palaeohighs (Constantine, 1995). which have been linked to the Cambrian greenstone on Philip Island (Henry & Birch, Eumeralla Formation 1992). In the Port Campbell Embayment, quartz-mica schists have been recovered from The Eumeralla Formation is one of the thickest core (Bain, 1964). units of the Otway Group, being up to 5 km thick in South Australia and consists of a During the late Jurassic to early Cretaceous monotonous sequence of terrestrial rifting phase, approximately 6700 m of syn-rift volcaniclastic sandstone, siltstone, mudstone, sediments of the Otway Group were deposited conglomerate and coal. Siltstone and coal in the Torquay Sub-basin (Cooper, 1995). dominates the lower section and volcaniclastic Onshore, where it forms the dominant surface sandstones the upper section. A minor lithology, the Otway Group comprises two unconformity with the underlying Crayfish major sedimentary units, namely the Crayfish Subgroup may be present, although a clear Subgroup and the Eumeralla Formation. angular unconformity is only evident in the However, except for the upper part of the western Otway Basin in South Australia. Eumeralla Formation, the Otway Group has Palynologically, the formation spans the not been penetrated in the offshore Torquay P. notensis to P. pannosus zones and is Aptian Sub-basin. to Albian in age. 3.1 Otway Group 3.2 Eastern View Group

The Early Cretaceous Otway Group comprises The Late Cretaceous to Early Tertiary Eastern the Casterton Formation, Crayfish Subgroup View Group comprises interbedded sandstone, and the Eumeralla Formation. gravel, claystone and coal deposited in a dominantly fluviatile environment. Deposition Casterton Formation commenced in the Campanian (T. lilliei zone) as shown by the Nerita 1 and Anglesea 1 wells. The Late Jurassic Casterton Formation is The group is thickest in the Basin Deep and comprised of a sequence of carbonaceous shale, thins dramatically over the Snail Terrace to the feldspathic sandstone, siltstone and east (Fig. 4, Encl. 1). There is an interbedded olivine basalts (Morton, 1990). unconformable relationship with the underlying Deposition is interpreted to have occurred in Otway Group. In the Bass Basin, the Eastern low energy lacustrine conditions (Struckmeyer View Group contains excellent reservoirs with & Felton, 1990). The unit has been porosity up to 30%. encountered in only 10 wells in the Otway Basin. Where fully penetrated, in the western part of the Victorian Otway Basin, it is around 12 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

3.3 Demons Bluff Group

The Demons Bluff Group reflects an increasing marine influence, with deposition in a marginal marine environment. The unit is preserved in the Torquay Sub-basin, northern Otway Ranges and Colac Trough (Reeckman, 1979). Offshore, the Anglesea Siltstone and the regressive, sandy Angahook Formation overlie a basal, transgressive, sandy unit called the Boonah Formation. The Anglesea Siltstone provides the seal for the Tertiary Boonah Formation targets, which have been the focus of exploration drilling to date. The seal unit varies in thickness from approximately 120 m on the Snail Terrace to 300 m in the southwest part of the Sub-basin. For the purposes of this study, the age range of the Demons Bluff Group, as defined by Trupp et al., (1994), namely late mid-Eocene to earliest Oligocene, is used. 3.4 Torquay Group

Torquay Group deposition commenced with the onset of the Oligocene transgression, which resulted in calcareous sediments being deposited under widespread marine conditions. The group onlaps onto the King Island – Mornington Peninsula High which, near its crest, may be condensed or absent. It is thickest in the Basin Deep where approximately 700 m of Torquay Group is present. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 13

SPORE-POLLEN N S MA GROUP / FM LITHOLOGY AGE -VOIR SEAL ZONES ROCK RESER EPOCH PERIOD SOURCE QQ PLIO- T. pleistocenicus CENE M. lipsus L C. bifurcatus -- -- 10 PUEBLA M T. bellus FORMATION

-- -- 20 NEOGENE MIOCENE E

P. tuberculatus L TORQUAY GROUP JAN JUC -- -- 30 FORMATION E OLIGO- CENE ANGAHOOK FM L ANGLESEA SILTSTONE -- -- 40 TERTIARY BOONAH FM N. asperus DEMONS BLUFF M GROUP

-- -- 50 EOCENE P. asperopolus E PALEOGENE M. diversus

L -- -- 60 L. balmei EASTERN VIEW E

PALEO- CENE GROUP

-- -- 70 Ma T. longus

T. lilliei -- -- 80 Ca N. senectus Sa T. apoxyexinus LATE (KU) Co -- -- 90 Tu P. mawsonii Ce A. distocarinatus P. pannosus -- -- 100 Ab C. paradoxa

CRETACEOUS C. striatus EUMERALLA FM -- -- 110 Ap P. notensis Br II/III GROUP Ht F.wonthaggiensis -- -- 120 EARLY (KE) KATNOOK SST ? LAIRA FM Va ? II/III C. australiensis PRETTY HILL SST CRAYFISH SUBGROUP -- -- 130 NEOCOMIAN Be ? ? ? ? R. watherooensis Ti CASTERTON OTWAY I/II FM. JURASSIC

-- -- 400 BASEMENT (MELBOURNE -- -- 420 TROUGH

SILURIAN METASEDIMENTS)

(After Trupp et. al., 1994)

Figure 5 Torquay Sub-basin stratigraphy. 14 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

4 Well failure analysis timing of structural development. The structure is relatively recent and shows no thinning in any of the target horizons, being 4.1 Well results initiated in the late Miocene or Pliocene. It is therefore interpreted that failure resulted from All wells drilled in the gazettal area to date the late development of the feature, which have targeted mid to late Tertiary objectives. A post-dated the main migration phase. structural well correlation section between the major wells in the Torquay Sub-basin, namely The Nerita 1 Stratigraphic well summary sheet Anglesea 1, Nerita 1, Wild Dog 1 and Snail 1, is is presented as Enclosure 2. presented as Enclosure 1. Well results are displayed in Enclosures 2, 3, 4, and 5. Wild Dog 1

Nerita 1 The well was also a shallow test, again designed to evaluate the Eocene Boonah The Nerita prospect was mapped by Shell as a Formation sandstones sealed by the Anglesea large, elongate NE-SW trending anticline Siltstone, with a secondary target being the covering an area of approximately 38 km2 and Eastern View Group. Top reservoir was with about 100 m of closure. The pre-drill prognosed at 920 mSS. The well, drilled in late seismic data quality was poor, with a grid 1992, was planned to be drilled into the upper spacing of about 4.5 km in the dip direction and part of the Eumeralla Formation. few strike lines. Closure was mapped at all horizons but only minor faulting was Structurally, the target was seen as a simple, interpreted to be present. asymmetric, fault-bounded anticline, which formed as a result of localised movement on a The well, drilled in 1967, targeted Boonah major fault (Fig. 7). The structure was mapped Formation, Eastern View Group and Sherbrook as a relatively large feature, approximately Group reservoirs. A significant thickness of 7 km by 5 km, with about 100 m of fault- Tertiary and Upper Cretaceous sediments independent closure and an additional 30 m of consisting of unconsolidated sands, gravels, fault-dependent closure. The structure formed clay bands and abundant coal seams was late, during the Oligocene, and hydrocarbon predicted. Top reservoir was prognosed at charge was anticipated to be from the Lower 600 ft (180 m) with the top of the Sherbrook Cretaceous. Group expected at 2900 ft (885 m). Top Otway Group was prognosed at circa 1675 m. The well The top of the Boonah Formation sandstone was drilled to a total depth of 6000 ft (1825 m). reservoir was intersected at 905 mSS, some 15 m shallow to prognosis. The reservoir Within the basal Eastern View Group and the consisted of medium to very coarse grained, Eumeralla Formation, gas readings up to 18% moderately well sorted, sub-angular to rounded gas (C1 to C3) were recorded. In the Eastern sandstones, with minor silica cement, pyrite View Group, the gas shows are associated with and glauconite. The net to gross ratio was coaly intervals. However, in the Eumeralla 99.6% with an average log porosity of 32.1%, Formation, the high gas readings are associated but no shows were observed. with slightly more permeable intervals. These shows are discussed more fully in section 4.2. In the secondary target Eastern View Group, the sandstones were also water wet and devoid Reservoir quality in the Boonah Formation is of shows with a net to gross ratio of 29.8% and excellent with dominantly loose, coarse-grained average log porosity of 29.8%. Two biozones sandstones interbedded with claystones and were identified: N. senectus (Campanian, occasional thin coals. Porosity is of the order of 1110 m to 1152 mRT), which consisted of 25-31%; however there were no shows at this interbedded siltstone, medium to very coarse level. Based on the later OS88A survey, there sandstone, and minor carbonaceous claystone is closure at the primary reservoir, which is the grading to coal; and L. balmei (Paleocene, top of Boonah Formation (Fig. 6). 1008 m to 1054.5 mRT) comprising interbedded coals, silty claystones and carbonaceous The cause of failure of the well is likely to be claystones. The age of the intervening interval associated with the timing of expulsion and is unknown, but during this phase, deposition migration of hydrocarbons in relation to the OFFSHORE TORQUAY SUB-BASIN, VICTORIA 15

was probably controlled by periodic fault 4.2 Hydrocarbon shows movements. Nerita 1 The Eumeralla Formation is mainly a light grey greywacke, with minor quartz in a Significant gas shows are present within the predominantly silty matrix. It is likely to form Eastern View Group and the Eumeralla an excellent seal. Formation (Encl. 2). The Eastern View gas shows are dominantly associated with coals, The well was interpreted by Shell, the operator, except for a tight sandstone over the interval to have tested a valid structure (Trupp et al., 1262m - 1274 mKB which had up to 5% C1 1994). However the overlying seal contains against a background of around 0.1%. In the sandy intervals and the prospect is possibly Eumeralla Formation, there are several tight dependent on a crestal fault. The juxtaposition sands, each generally less than 5 m in of sandstone units across this fault could thickness, with gas shows up to about 14% total account for leakage of hydrocarbons from the gas. While a significant number of these shows structure. This is believed to be a likely cause are associated with coals between 1810 to of failure for the well; alternatively the main 1829 mKB, a medium to fine, argillaceous migration phase for hydrocarbons may have sandstone with relatively more permeable occurred prior to the formation of the structure. intervals and no associated coal, has significant Further reasons postulated by Shell include the gas peaks of up to 18% (C1 18%, C2 0.15% and lack of an adequate charge and the Eumeralla C3 0.08%). From about 1926 m to 1996 mKB, Formation inhibiting vertical migration. gas peaks up to 8% are also present in tight sandstones. The Wild Dog 1 Stratigraphic well summary sheet is presented as Enclosure 3. Anglesea 1 Snail 1 Anglesea 1, drilled in 1962 by Oil Development NL is located on the coast, north of V99-1, and The well was drilled in 1972 to test the Eastern intersected the top of the Eumeralla Formation View Group in the southern part of the Torquay at about 586 mKB (Encl. 5). From Sub-basin. A broad closure of approximately approximately 1340 mKB to total depth of 50 km2 was mapped with about 60 m of vertical 3064 mKB the gas levels varied from about 5% relief. The structure was orientated NE-SW on to 40%. The well was drilled in 1962 using only a broad terrace and there was little faulting a total gas detector and, therefore, there is no evident at the target horizon. breakdown of the gas components. Good quality reservoirs were expected in the Thirty-three cores were cut during the drilling Eastern View Group. The well also had a of the well. In core #13 (1054.6 - 1057.6 mKB) secondary stratigraphic objective to assess the broken fresh surfaces of arkosic sandstone Otway Group for reservoir quality. showed small irregular brown oil stains, which fluoresced yellow. Although excellent reservoir quality sands were encountered, the current view, based on later seismic interpretation, is that Snail 1 was not a 4.3 Hydrocarbon seepage valid structural test at any reservoir level, including the main target Boonah Formation During the late 80’s – early 90’s, AGSO (Fig. 8). Reservoir quality of the Eastern View (formerly BMR) carried out seabed-geochemical sandstones was, however, good, with up to 35% sniffer surveys in the Torquay Sub-basin porosity measured in core samples. The (Bishop et al., 1992). A total of six bottom Eumeralla Formation is comprised of poorer water hydrocarbon anomalies were detected in quality fine to medium, lithic and argillaceous the 1992 survey. An example of two of these sandstones. There were no shows in the well. anomalies (Seeps 2 and 3) from line 104/022 are shown in Figure 20. Seep 3 was also detected The Snail 1 Stratigraphic well summary sheet in a 1989 survey carried out in the area. is presented as Enclosure 4. Associated with this seep are pockmarks on the seafloor. 16 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

This seep is located just north of Nerita 1 (near SP 300 on line 12-7-A (Encl. 6). Near the pockmarks, 8 vibrocores were taken with the objective of investigating the molecular and isotopic compositions of the light hydrocarbons in the sediments. From these vibrocores, Bishop et al. (1992, p. 15) empirically identified the presence of thermogenic hydrocarbons in the sediments. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 17 Figure 6 top Formation). (Boonah reservoir closure showing at through Nerita 1 OS88A-12 Seismic line 18 OFFSHORE TORQUAY SUB-BASIN, VICTORIA Figure 7 Formation). (Boonah reservoir top at closure Wild 1 showing through Dog OS90A-13 Seismic line OFFSHORE TORQUAY SUB-BASIN, VICTORIA 19 Figure 8 Formation). (Boonah reservoir top at lack closure of note through Snail 1: OS88A-04 Seismic line 20 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

5 Seismic evaluation These processes would address both multiple problems and any structural complexity issues. The two valid structures tested to date by Nerita 1 and Wild Dog 1 are both Late Tertiary 5.3 Interpretation and mapping (Oligocene to Pliocene) compressional features. The Wild Dog structure was initiated during A variety of horizons were interpreted as part the Oligocene, whereas Nerita 1 is a more of the study, including Top Jan Juc Formation, recent feature, with the structure forming in Top Demons Bluff Group, Top Boonah the Pliocene. Formation and Top Eastern View Group within the Tertiary. Attempts were made to map The proposed targets for the next exploration deeper horizons within the Cretaceous such as phase will be inevitably deeper than the Top Otway Group (=Top Eumeralla Formation) structures drilled to date. The top of the and various intra-Eumeralla horizons as well proposed target horizon, the Pretty Hill as Top Basement. Only the Top Basement Formation, cannot be recognised due to the poor (Encl. 8) and Top Otway Group TWT horizons data quality and lack of well ties, but is likely (Encl. 9) were both interpreted regionally and to be similar in form to the Top Basement Map included in this report to highlight the major (Encl. 8), assuming that the Pretty Hill structural elements within the Sub-basin. The Formation was deposited reasonably uniformly. Tertiary Top Boonah reservoir horizon, being a strong event, was also mapped over the entire 5.1 Seismic database area. Structuring at the latter level, however, is not relevant for remaining prospectivity and the map has not been included in this report. A seismic grid varying from 1 x 4 km to The top of the proposed target horizon, Pretty 5 x 7 km is present over approximately half of Hill Formation, has also not been mapped due the area (Fig. 2). The remaining area has a to data quality problems but is considered regional seismic grid of approximately likely to have a generally similar structural 10 x 10 km. The available seismic data form to the Top Basement (Encl. 6). includes a number of vintages. Post 1970 surveys include Haematite HO-2, Portland - There have been a number of Top Basement King Island (1972), BMR 40 Bass Strait (1982), interpretations for the Torquay Sub-basin Shell OS88A (1988) and OS90A (1990). (e.g. Trupp et al., 1994, Cooper, 1995 and Collins, this report). There are differences in 5.2 Data quality the picks, especially north of the Snail Terrace. However, with advances in seismic acquisition The following brief discussion of seismic data and processing technology since the last survey quality is based on the structural interpretation in 1990, it is anticipated that considerable and seismic mapping carried out by MPV. Data improvement in seismic data quality could be quality is variable between surveys and across achieved. This would enable both a more the area, but is generally good on the Snail accurate assessment of depth to basement to be Terrace and poor north of the Snail Fault and made, as well as a more accurate definition of to the east. The poor data quality needs to be likely prospects within the Torquay Sub-basin. addressed in the first instance by reprocessing of existing data. The Top Otway Group is affected both by Late Cretaceous and Tertiary structuring. An example of the potential improvements that Confidence in this event and its major fault could be carried out would be the removal of the trends is generally good. The Basement water-bottom multiple (See Fig. 6). In horizon demonstrates the Early Cretaceous particular, the following processes might be structure, highlights the main depocentres and tried: shows the associated development of potential leads within the Early Cretaceous Otway Group • application of an inner trace mute, (Encls 6 & 8). Confidence in this event away • closer spacing of velocity analyses, from the Snail Terrace, however, is low. Fault • two-phase velocity analysis, correlation in the east and northwest of the • tau-P multiple suppression, and Snail Terrace is interpretative but is believed to • DMO. be representative of overall structure. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 21

In the Early Cretaceous, large, southerly Cretaceous southeast of Wild Dog 1. This dipping half grabens developed, controlled by lead has an area of approximately 90 km2 faults which trended from northwest through with 400 msecs of vertical relief. east-west to northeast. The influence of the underlying northeast to southwest basement 3. The mapping is reliable on the Snail Terrace trends increases towards the Mornington but to the north and east is less reliable due Peninsula. to poorer quality seismic data. Enclosures 6 and 7 highlight the variability in seismic At the end of the Early Cretaceous, an episode data quality. With additional, modern infill of uplift of one kilometre or more occurred, seismic, structures are likely to be centred on the Otway Ranges. As in the Bass unambiguously defined. Basin, there was no deposition during most of the Late Cretaceous. Minor uplift and erosion 5.4 Structural leads in the Oligocene followed the accumulation of up to 700 m of Maastrichtian to Eocene A number of structural leads are present which sediments. Wild Dog 1 was drilled on a have been defined on the basis of the Top structure formed during this Oligocene uplift. Basement TWT map (Encl. 8). These are These sediments are now limited to an area in summarised in Figure 9 and Table 1. The leads the hanging wall of the basin margin fault, are all fault-related and associated with west of Snail 1, and to a broad area oriented structural development in the Late Jurassic – northwest to southeast, running through Early Cretaceous. The leads with higher Nerita 1. During the remainder of the Tertiary structural confidence are located on the Snail there were multiple periods of uplift and Terrace. A dip-line through the T-1 lead is subsidence, with major episodes in the Early shown in Figure 10, which demonstrates onlap Miocene and Pliocene. Nerita 1 is drilled on onto basement. There is also the potential for a one such structure formed during the Pliocene. fractured-basement play at T-1, by analogy Closer to the Otway Ranges, the structure is with the results of the Sawpit 1 well in the increasingly dominated by the Pliocene uplift South Australian onshore part of the basin and structuring. It appears from the relatively (Fig. 2), where oil was recovered from basement poor quality data that there may have been a (Moriarty et al., 1995). The leads in the Basin more E-W to NW-SE trend in the pre-Tertiary Deep, however, are higher risk due to the structure north of Wild Dog 1. The Tertiary poorer seismic data quality in that area. compressional folds cross cut this, trending Examples of the Basin Deep leads are shown in E-W or NE-SW. To the northeast, it appears Figure 11 (B-2 Lead) and Figure 12 (R-1 Lead). that the Tertiary faults are inverted along NE Nevertheless, the model of half graben to SW trending Early Cretaceous extensional development, suggests a high probability of the faults. presence of further leads in the Basin Deep.

There is a phase of intrusive/extrusive volcanics likely to be of early Oligocene age. Its expression is variable but generally the intrusions are onlapped by Jan Juc Formation sediments. The onlap is best seen to the west of Wild Dog 1 and is probably related to the basin margin fault. Other examples can be seen elsewhere in the basin.

There are three key points to note from the TWT maps:

1. There is in the order of 6000 m of mainly Early Cretaceous sedimentary rocks in the Basin Deep and 4000 m on the Snail Terrace. These thicknesses are more than adequate for thermal maturity.

2. There is a potentially very large, structural, tilted fault block lead in the Early 22 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

Table 1 Summary of Pretty Hill Formation Leads

Structural Lead Depth Area Closure Risks Element (msec) (km2) (msec)

Snail Terrace T-1 1400 171 400 Fault dependent

Snail Terrace T-2 1600 9 200 Fault dependent

Basin Deep B-1 3100 71 500 Fault dependent, Structure & Seal

Basin Deep B-2 2800 27 500 Fault dependent, Structure & Reservoir

Basin Deep B-3 3200 19 150 Fault dependent, Structure & Reservoir

Basin Deep B-4 3000 39 600 Fault dependent, Structure & Reservoir

Basin Deep B-5 3300 17 300 Fault dependent, Structure & Reservoir

Basin Deep R-1 1500 32 600 Fault dependent

Basin Deep R-2 2000 5 200 Fault dependent

Basin Deep R-3 1800 14 100 Fault dependent; End of seismic line OFFSHORE TORQUAY SUB-BASIN, VICTORIA 23 38 20 S 38 18 12S 38 30 S 38 40 S 38 50 S 39 00 01S 144 51 47E 144 51 47E

10

6 0 0 8

800 6

0 8 0 TRUE NORTH IS SHOWN

FOR THE CENTRE OF MAP

0 km

0 0 0

0

1 4 6

0 0

2

1 2

1 4 0 0

1 6 0 0 0

0 0 1 8

0 2 0 0

0

2 0 2

0 4 0 2

0

0 0 0

0 0 0 0

0 0 0

0 0 0

144 40 E 0

2 8 6 4 1

0 2

1 144 40 E 0

1 1

0 0

1

6 2 2

0

0

0 2 0 0 2 4 0 0

0 2

4 4 2 2600 2 0 0

0 0

R3 4

6 0 1

1 0

0 0 1

0 8

1

2

2 0

0 0

0 0 0

0 2 0 4 2 0

0 6 2 0

2 0

0 2 0

4 0

R2 2 0 2

0 0

2 0

0 2

4

144 30 E 3

0 (Fig 12) OS90A-53

3 0

144 30 E

0

0

0

0 2

6 2

0

0 0 1 2 8 0 0

8 0

8

1 3

2 0 6 0 T2

0 0

R1 0

0

0 0

3

0 6 4 2 00

4 2

2 0 0

0

0 4

6

1

1 0 0

8 0 1

0

0 4

2 6 0 0 0

0 8 3

0 2

0 0 0 0 0

6 0 0

0 0

2 0 0

2 3 6

0 3 0 0

4 8 1

3 1 2 8 0 0

Snail 1

144 20 E 0

(Fig 10) 12-7-a 144 20 E 0

0

6 0

6 1

1

0 B4

0

6 0 0 3

0 0 0 T1

4

6 0 3 4

3 3 0

0 6 Nerita 1 B5

0

0 8

0 Anglesea 1 3

0

6 0 3

0 6

3 0

0 0 0

3 4 0

2 0

0 0

0 8

4 1 144 10 E

B1 B3 2 144 10 E Wild Dog 1

0 0

0

8 4 0 0 3 3 0

(Fig 11) OS90A-136 0

2 0

0 8 0

2

4

3

0 0

B2

2 0 3 0 0 0

2 0 2

3

0

0

2

2

0

0

4 3 144 E 144 E 6 0 0

0 0 3 2 0

0 10

0

0 0 3

0 0

0

1 143 50 E

143 50 E

Ingleby 1

0

0 0 8 8 0 1

800

0

0

2 1

Olangolah 1 1000 143 38 53E 143 38 53E 38 50 S 38 40 S 38 20 S 38 30 S 39 00 01S 38 18 12S Figure 9 Torquay Sub-basin: map. Leads 24 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

Figure 10 Seismic line BMR 40 12-7A: T1 Lead. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 25

Figure 11 Seismic line OS90A-13: B2 Lead. 26 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

Figure 12 Seismic line OS90A-53: R1 Lead. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 27

6 Hydrocarbon the Pretty Hill Formation, shows significant permeability (up to 1000 md) at depths prospectivity approaching 3000 m (Fig. 19). The relationship between calculated overburden porosity versus 6.1 Reservoir calculated overburden permeability for the Katnook Gas Field channel facies is also shown The proposed new play for the Torquay in Figure 19. Sub-basin comprises sandstones below the Eumeralla Formation, with the primary target 6.2 Seal being Early Cretaceous (F. wonthaggiensis) Pretty Hill Formation. The Pretty Hill Top seal will most likely be provided by the Formation forms part of the Crayfish Subgroup Eumeralla Formation (Otway Group) and/or (Fig. 5), but has yet to be penetrated in the claystones of the Pretty Hill Formation (Figs 14 Torquay Sub-basin. To the west of the Otway & 15). While the gas fields at Ladbrook Grove, Ranges, Pretty Hill Formation reservoirs have Katnook and Haselgrove confirm the presence been intersected in a number of wells. of an effective seal for the Pretty Hill Sandstone Examples of the formation’s log character are reservoirs in South Australia, the development given for Stoneyford 1, Garvoc 1 and of seals in this unit in the Torquay Sub-basin Woolsthorpe 1 (Figs 13, 14 & 15 respectively). remains conjectural. Seismic sections (Encls 6 In all of these wells there is a high net-to-gross & 7) show that the Eumeralla Formation has a sandstone ratio. The sandstones are fine to generally low amplitude character in the upper coarse grained, pebbly in part, with variable section, with discontinuous amplitudes sorting, some lithic fragments and clay matrix. increasing with depth. This is thought to Mineralogically, the sandstones are complex probably reflect the increasing presence of coals and are interbedded with minor amounts of with depth. siltstone, claystone and occasional coal. One of the most complete sections of Eumeralla The sediments are likely to have been deposited Formation penetrated in the Torquay Sub-basin in the half graben setting, in low sinuosity, bed- is at Anglesea 1 (Encl. 5). The section consists load dominated river systems, similar to those predominantly of interbedded to associated with the Katnook area gas fields to interlaminated, tight arkosic sandstones, the west (Scholefield et al., 1996). If the Port siltstones and mudstones. The arkosic Campbell Embayment is considered as an sandstones are described in the well completion analogue, it is estimated that at least 500 m of report as typically “light to medium grey, fine sediments could have been deposited in the grained, compact and tight, composed of sub- Torquay Sub-basin during F. wonthaggiensis angular grains of quartz and brownish-grey times (Fig. 16). The major depocentre during feldspars in a grey argillaceous and moderately these times was in the vicinity of the Otway calcareous matrix; dark grey laminae of Ranges. An assessment of 12 wells in the carbonaceous matter and brown mica are Otway Basin for Eumeralla and Pretty Hill present”. The claystones are described as “dark Formation reservoirs shows significant grey to black, generally carbonaceous, silty in variations in porosity for any depth interval places and grading rarely to argillaceous (Fig. 17). While there is little obvious siltstone; plant remains including Taeniopteris correlation with depth, Little and Phillips are very common through this section and (1995) state that “reservoir quality is controlled rarely are partly pyritised” (Core # 31, by mechanical compaction and the detrital and 2790.7-2796.7 mKB, Anglesea 1 WCR). The authigenic mineralogy”. However, this is a resisitivity logs of this section reflect this simplistic approach, as the influence of variability and show the formation to be depositional environments also needs to be predominantly tight (Encl. 5). The SP log considered when predicting porosity and shows a typical lack of character, with only permeability development. With the limited occasional slightly more permeable intervals, data available, however, it is not possible to which are unlikely to act as thief zones as they comment on probable reservoir quality, nor to are not likely to be laterally extensive. assess a probable depth for economic basement.

A detailed assessment of the Ladbroke Grove, Katnook and Haselgrove gas fields (Fig. 18) (Scholefield et al., 1996), which are hosted by 28 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

MSFL 0.2OHMM 2000 SP LLS -20MV 100 METRES 0.2OHMM 2000 GR LLD DT 0GAPI 200 0.2OHMM 2000 140US/F 40

Figure 13 Stoneyford 1: Wireline logs of Pretty Hill Formation (1000 m TD). OFFSHORE TORQUAY SUB-BASIN, VICTORIA 29

CALI 717INCHES SP SN -20MV 100 METRES 0.2OHMM 2000 GR IND DT 0GAPI 200 0.2OHMM 2000 140US/F 40

Figure 14 Garvoc 1: Wireline logs of Pretty Hill Formation (1300 m TD). 30 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

CALI 717INCHES SP SN -20MV 100 METRES 0.2OHMM 2000 GR IND DT 0GAPI 200 0.2OHMM 2000 140US/F 40

Figure 15 Woolsthorpe 1: Wireline logs of Pretty Hill Sandstone (1000 m TD). OFFSHORE TORQUAY SUB-BASIN, VICTORIA 31 38°30' 39°00' Bay

Port Phillip

? ? 500 - 1000 0 -500 (c.i. = 100 m) >2000 1000 - 2000

TORQUAY SUB-BASIN 0 THICKNESS IN METRES TORQUAY SUB-BASIN

Geelong 100

144°00’

1500

300

1000

2000 500

? ? OTWAY RANGES

?

?

? 0 500

300

100 143°00’ 0 20km EMBAYMENT PORT CAMPBELL

200 Port Campbell Figure 16 Isopach sedimentary of thickness F.wonthaggiensiszone. 32 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

0

1

2 Depth (km)

P. pannosus 3 C. paradoxa Eumerella C. striatus

P. notensis

F. wonthaggiensis Pretty Hill

4 0 5 10 15 20 25 30 35 Porosity (%)

Figure 17 Eumeralla and Pretty Hill Formations: porosity versus depth for 12 wells. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 33 FACIES (m) 2950 2925 2900 TVD 3000 2975

GR (API)

HASELGROVE 2 HASELGROVE 0 200 GWC FACIES (m) TVD 2900 2925 2950 2975 2875 GR (API) 0 200 HASELGROVE 1 HASELGROVE

1 2 3 4 5 6 7 8 9 10 SST LAYER LAYER LAYER LAYER LAYER LAYER LAYER LAYER LAYER LAYER FACIES (m) 2950 2925 TVD 2850 2875 2900 KATNOOK 2 KATNOOK GR (API) 0 200 HILL FACIES (m) 2900 2975 2850 2925 TVD 2875 2950 KATNOOK 3 KATNOOK GR (API) 0 200

GWC PRETTY FACIES (m) 3000 TVD 2975 2925 2950 2900 2875 KATNOOK 4 KATNOOK GR (API) 0 200 TOP GWC FACIES (m) TVD 2575 2600 2625 2550 Core Channel Floodplain Bar GROVE 1 GROVE LADBROKE GR (API) 0 200 Figure 18 1996). al., et Scholefield fields (after gas Trough Penola Stratigraphic correlation 34 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

10000.00

Ladbroke Grove 1 (Lower zone) 1000.00 Ladbroke Grove 1 x +x x Katnook 3 + + Katnook 2 100.00

10.00 x x

x

OKHC (mD) 1.00 x x x x x + .10

xx x + Y=10(-0.31039 + 1.22 log - 0.04297 logx * logx)) .01 x x x x Where Y = Overburden Permeability x x X = Ambient Permeability + .001 xx x .001 .01 .10 1.00 10.00 100.00 1000.00 10000.00 AKHC (mD)

10000.00

Katnook 3 1000.00 + Katnook 2 ++

++ 100.00 +

10.00 +

1.00 + CALCOKHC (mD) .10 +

+ + .01

Y=10(-5.88 + 47.65X) .001 Where Y = Overburden Permeability X = Overburden Porosity

.0001 .000 .060 .120 .180 .240 .300

CALCOPHC (Frac)

Figure 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). OFFSHORE TORQUAY SUB-BASIN, VICTORIA 35

6.3 Source rock quality Sawpit 1. HI vs Tmax data indicates that the Casterton Formation consists largely of Type II There are three dominant source rock intervals to III kerogens, suggesting it is potentially both in the Otway Basin: the Casterton Formation, oil and gas generative (Lavin & Muscatello, the Crayfish Subgroup and the Eumeralla 1998). Formation. These source intervals are expected to be present in the Torquay Sub-basin and the The distribution and quality of potential hydrocarbons for the postulated traps are likely Casterton Formation source rocks in the to be sourced from these units. However, it is Torquay Sub-basin cannot be assessed, due to probable that the dominant source rock for the lack of well penetrations. Whilst it is proposed target Pretty Hill Sandstone acknowledged that lacustrine deposits in early reservoirs will be the Crayfish Subgroup and rift sequences can be highly variable in their possibly the Casterton Formation. development and hence source quality, the likelihood of source presence is considered high. Discussions on the early Cretaceous source rock potential in the Otway Basin include Mehin The petroleum potential of the Crayfish and Link (1994, 1995, 1997), Padley et al. Subgroup is highlighted by the following (1995) and Lovibond et al. (1995). The most discoveries attributed to a Crayfish source: the recent summary of petroleum systems in the Penola Trough onshore gas condensate fields at Otway Basin is found in Edwards et al. (1999). Haselgrove, Katnook, and Ladbrook Grove; An excellent summary of the potential of the offshore gas in Troas 1; gas, condensate and oil Eumeralla Formation to source both oil and gas in Wynn 1; and oil in Sawpit 1. At Redman 1, in the Otway Basin is also found in Geary and oil was recovered from below the gas-water Reid (1998). In addition to the economic gas contact and an oil column is present in accumulations in the Waare Formation and Killanoola 1 (Edwards et al., 1999). In occasional oil shows within the Eumeralla Robertson 1 (Fig. 2), shales within the Crayfish Formation in the onshore Port Campbell Subgroup have a Total Organic Carbon (TOC) Embayment, the offshore gas discoveries at content of up to 7%, Hydrogen Index (HI) of 515 La Bella and Minerva, located in a similar and genetic potential of 37.26 kg/tonne Early Cretaceous structural setting, are (Lovibond et al., 1995). A van Krevelen sourced from the Eumeralla Formation. diagram for the Otway Group is shown in Figure 21. The HI vs. Tmax plot (Fig. 22) The Eumeralla Formation is stratigraphically shows that there is also some potential for type higher than the primary target, Pretty Hill II (oil and gas prone) and type III (gas prone) Sandstone. It may, however, have the potential source rocks which may have reached early to charge Pretty Hill Sandstone fault block maturity at depths of 1500 m. traps, by face-loading across the bounding faults or directly into intra-Eumeralla Edwards et al. (1999) showed there were at reservoirs. least three distinct oil types in the Crayfish Subgroup which can be related to three Crayfish Subgroup and Casterton different depositional environments, namely freshwater fluvial-lacustrine, lacustrine and Formation playa lakes.

According to Lovibond et al. (1995), the best In the Torquay Sub-basin, similar source rocks source rocks in the Penola Trough are in would be associated with the axis of the half lacustrine shales of the Casterton Formation at grabens and would be well situated to feed the base of the Otway Group, where they are Pretty Hill Sandstone traps (Encl. 1). widely regarded as the major source of Using the Otway Basin to the west of the hydrocarbons in the trough. This Otway Ranges as an analogue, gas is interpretation is based largely on geochemical considered to be the most likely hydrocarbon, data (Padley et al., 1995) which indicate that it but with some potential for oil. and the overlying Pretty Hill Formation are the source of condensate in the Katnook Field and Eumeralla Formation oil in Wynn 1. This is further enhanced by the presence of oil within the Casterton Formation The dominant source interval in the Eumeralla in Digby 1 and the occurrence of oil in fractured Formation of the eastern Otway Basin is the basement below the Casterton Formation in P. notensis zone (Geary & Reid, 1998, Mehin & 36 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

Link, 1994 & 1995). In the Port Campbell 1994; Cooper. 1995). At Nerita 1 (Fig. 24b), ° Embayment, economic gas fields occur at Boggy modelling has suggested temperatures of 180 C Creek, Iona, Grumby, North Paaratte, Wallaby at about 3000 m during the Early Cretaceous Creek, Mylor and Fenton Creek. Offshore gas (Duddy, 1994). From the Late Cretaceous to discoveries occur at Minerva, La Bella, Troas 1, Early Miocene deposition predominated. Since and Pecten 1A and minor gas shows were the Early Miocene, uplift has occurred and encountered in Triton 1. These accumulations, Duddy (1994) has modelled approximately 1 km with the exception of Troas 1, are sourced from of uplift and erosion at Nerita 1. The modelling coals and coaly shales near the base of the indicates the Otway Group sediments in the Eumeralla Formation (P. notensis). Troas 1 is early Miocene were at approximately the same believed to be sourced from a more distal facies, temperatures as they were prior to the mid- either Eumeralla Formation or Crayfish Cretaceous uplift and erosion event. This Subgroup (Padley et al., 1995, Pickavance & model suggests hydrocarbon generation and Oke, 1994). expulsion would have occurred effectively prior to mid-Cretaceous uplift. Timing of generation Source rock data from the P. notensis zone and expulsion therefore, becomes a major risk shows average TOC values of between 0.85% and would tend to downgrade the Torquay and 18.74%. The HI vs. Tmax plot (Fig. 23) Sub-basin. Other authors also lend support to again indicates potential for type II and type III the high heat flow model in the Early source rocks to have reached early to middle Cretaceous, (e.g. Trupp et al., 1994), although maturity at depths of 2500-3500 m. Seismic there are differences in detail. mapping indicates thick sequences of F. wonthaggiensis and P. notensis zone Cooper (1995), on the other hand, has proposed sediments can be expected to have been an alternative heat flow model based on deposited in half grabens in the Torquay vitrinite reflectance data from 21 wells in the Sub-basin. The depth of these intervals could Otway Basin. Cooper found there was be expected to be between 2000 and 3000 m, significant variation in the palaeothermal particularly within the Basin Deep and possibly gradient across the basin during the Early also within parts of the Snail Terrace. In the Cretaceous (Fig. 25) and believes the heat flow ° deeper parts of the Basin Deep, however, the in the Torquay Sub-basin (35 C/km) was much source interval is likely to be over-mature. less than previously thought (60°C/km). Cooper argues there is little evidence to suggest there While data from the C. striatus through to was major uplift and erosion in the Torquay P. pannosus zones also shows favourable Sub-basin during the mid-Cretaceous. At average levels of TOC, ranging from 0.19% to Nerita 1, Cooper’s burial history (Fig. 24a) 4.13%, the HI values and levels of thermal shows both no erosion and that the calculated maturity suggest these zones are unlikely to maturity also has a reasonable fit to the provide suitable generative source rocks within measured vitrinite reflectance data. This model the Torquay Sub-basin. Again, gas is suggests the sediments were at their most considered the most likely hydrocarbon type, mature just prior to the Miocene-Recent uplift with there being some potential for oil. phase.

6.4 Source rock maturation In both of the models presented, generation and expulsion of hydrocarbons since the Hydrocarbon maturation (in simplistic terms) is Miocene-Recent uplift would have been dependent on the relationship between heat minimal. All late-formed structural features, flow, depth of burial and thermal conductivity such as Nerita, are therefore unlikely to contain of the rocks present in the sequence, as well as hydrocarbons. Wild Dog 1, on the other hand, maceral type (I, II or III). Heat flow in an is interpreted to have formed during the under-explored area is difficult to assess Oligocene. Trupp et al. (1994) argued that lack accurately, but has a major influence on timing of charge is the most likely cause for the lack of of generation and expulsion of hydrocarbons. hydrocarbons in the prospect. However, the presence of sandstones within the fault sealed In the Otway Basin, a significant heat pulse section of the well suggests that alternative has been interpreted during the mid- interpretations for failure are possible. Cretaceous, with palaeothermal gradients in excess of 60°C/km and approximately 3 km of In summary, the range of published data on section eroded in the Otway Ranges (Duddy, source rock modelling suggests there is OFFSHORE TORQUAY SUB-BASIN, VICTORIA 37

insufficient information to accurately determine the heat flow in the Torquay Sub-basin during the Cretaceous. Consequently, the application of a rigid heat flow model for the entire area is not necessarily valid and could be misleading. The authors suggest the published results outlined above are best viewed as end members for a sensitivity analysis of the data and suggest that generation and expulsion post-early Cretaceous uplift is likely to have occurred.

38 OFFSHORE TORQUAY SUB-BASIN, VICTORIA 104042

104023 104024

104041 (from Bishop et al, 1992)

104040 104020

SNAIL -1 104028 104027 104025 NERITA-1

104026 Total Hydrocarbon Methane

104019

104029 WILDDOG -1

SEEP2 104043 104021

104018

104017 104018 104022

SEEP3 104038

104015 120 140 160

Seep2 Shotpoint

Seep 3

LINE 104022 THC, Methane 0 20 40 60 80 100

2 4 0 6 8

12 14 10 20 16 18 yrcros(ppm) Hydrocarbons Figure 20 Bottomwater light hydrocarbon anomalies: Line BMR 104/022. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 39

1000

Type I I 900 Type II Type III Eumeralla Formation 800 X Crayfish Subgroup Casterton Beds X 700

II 600 X X

X 500 X

X X 400

Hydrogen Index X X X X 300 X X X X X X 200 X X X X XX X X XX X XXX X X X X 100 X X X III

0 020406080 100 120 140 160 180 Ol (From Trupp et al, 1994)

Figure 21 Otway Group source rocks: van Krevelen diagram. 40 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

1000 I

900

OIL VR = 0.5% 800

II 700

PEAK 600 HYDROCARBON GENRATION

500

400 MIXED Hydrogen Index OIL & GAS

300

VR = 1.35% 200

100 III GAS

0 380 400 420 440 460 480 500 520 Temperature (max °C)

Figure 22 HI vs. Tmax plot: Crayfish Subgroup (F. wonthaggiensis zone). OFFSHORE TORQUAY SUB-BASIN, VICTORIA 41

1000 I

900

OIL VR = 0.5% 800

II 700 PEAK HYDROCARBON 600 GENRATION

500

400 MIXED Hydrogen Index OIL & GAS

300

VR = 1.35% 200

III 100 GAS

0 380 400 420 440 460 480 500 520 Temperature (max °C)

Figure 23 HI vs. Tmax plot: Eumeralla Formation (C. hughesii = P. notensis zone). 42 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

Nerita - 1 K P E O M P 0 0 Demons Bluff

500 500 EV 20ºC Palaeocene

EV 1000 40ºC Campanian 1000

60ºC Eumeralla 1500 1500 et (m) Depth 2000 2000 80ºC Depth (m) 2500 2500 100ºC

3000 3000

3500 3500

Early Cretaceous 4000 Gradient = 32ºC/km 4000 Measured vitrinite reflectance 4500 4500 120 100 80 60 40 20 0 0.5 1.0 Time (Ma) t=0 Maturity (% Ro) (From Cooper, 1995)

Nerita - 1 K P E O M P 0 0 20ºC Torquay Group Demons 500 40ºC Bluff 500

60ºC Eastern View 1000 1000 80ºC Sherbrook Group 1500 1500 100ºC Otway 1

120ºC (m) Depth 2000 2000 Otway 140ºC 2 Depth (m) 2500 2500 160ºC

180ºC Otway 3 3000 3000

3500 3500

Early Cretaceous 4000 Gradient = 32ºC/km 4000 Measured vitrinite reflectance 4500 4500 120 100 80 60 40 20 0 0.5 1.0 Time (Ma) t=0 Maturity (% Ro)

(From Duddy, 1994)

Figure 24 Comparison of possible burial history plots for Nerita 1, with isotherms and calculated vitrinite reflectance (VR) profile match to measured VR. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 43 Snail-1 32 30 55 40 50 Nerita-1 32

Anglesea-1 44

40

50 60

Hindhaugh Ck-1 oqa Embayment Torquay

Warracbarunah-2 43 ta Ranges Otway

Olangolah-1 60 Krambruck-13 50 oa Trough Colac TM

Otway Basin 40 Tirrengowa-1 50 Cape Otway Stoneyford-1 54 SORREL FAULT ZONE Basinmod Vitrinite Modelling Ferguson Hill-1 28 Ross Creek-1 50 42 Prawn-1 32 Purrumbete-1 Port Campbell-4 40 Embayment

Early Cretaceous Geothermal Gradients Port Campbell Mussel-1 32

MORTLAKE DISCONTINUITY 51 Pecten-1 32 43

Garvoc-1 30 Flaxmans-1 50 30

40 40

50 30 Pretty Hill-1 70

60

80 Warrnambool

50 32

40 Eumeralla-1 Fault affected Wells (n) = 21

30 42 Contour Interval - 5ºC/km 0 50km Portland Mocamboro-11 Southern Ocean 39º00’ 38º00’ 141º00’ 142º00’ 143º00’ 144º00’ Figure 25 1995). Cooper, wells (after Basin 21 Otway from VR data modelling of on kinetic Early Cretaceous based gradients palaeothermal 44 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

7 Petroleum systems

The petroleum systems defined in this section are named after the potential source interval. As discussed above, the Torquay Sub-basin is considered likely to have two main petroleum systems, sourced from the Crayfish Subgroup and Eumeralla Formation. It is not known if the Casterton Formation is present, so its source potential cannot be addressed.

7.1 Crayfish Subgroup

The distribution of the active petroleum system is summarised in Figure 26. In the Basin Deep, the source rocks are assumed to be present. However, in view of the sediment overburden, it is interpreted that the section will be overmature and effectively in economic basement. This petroleum system, therefore, is considered to be mainly viable on the Snail Terrace and the northeastern flank of the Basin Deep. The distribution of the system to the south and east is limited by the King Island - Mornington Peninsula High.

The most likely trap types are tilted fault blocks and onlaps onto basement highs. An additional high risk, high reward target on the Snail Terrace could be fractured basement, recognising that oil has been recovered from fractured basement at Sawpit 1 and shows were encountered in Gordon 1 in the western part of the onshore Otway Basin. 7.2 Eumeralla Formation

The distribution of this petroleum system is summarised in Figure 27. As with the Crayfish petroleum system, the source rocks are interpreted to be present in the Basin Deep. In the deepest part of the basin it is probable the basal Eumeralla section is overmature. The structural dip of the beds suggests that the dominant migration direction will be in an arc towards the north and northeast.

Reservoirs will be dominantly sandstone units within the Eumeralla Formation sealed by claystones. The presence of good gas shows within the Eumeralla Formation in Nerita 1 suggest that this play has significant potential. The most likely traps are sandstone units on the hanging wall of faults developed during syndepositional faulting. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 45 38 18 12S 38 20 S 38 30 S 38 40 S 38 50 S 39 00 01S 144 51 47E 144 51 47E 10 8 6

TRUE NORTH IS SHOWN km FOR THE CENTRE OF MAP

0 0 1 0 4

2 No Pretty Hill reservoir

5 1 0 0

0

0 0 0 2

0 0

0

5

0 0 0

2 0

0 0 0 0 0

0

144 40 E 2

5

0 1 144 40 E 1

5 0 2 2

0 250

0

0

0 5

0

0

0

1

0

5 1 2

0 0

0

2

0

0

5 2

0

0

0

0

144 30 E

0 144 30 E 5

2 2

0 0 0 0

5 5

2 0 3

0 0 3

0 0 0 25 0

5

1

4

0 0 0

0 0 2

0 00 0 5 2 3 0

0 0

0

3 5

Snail 1

144 20 E 144 20 E

0

0

5 3 00 5

1 Nerita 1

Anglesea 1

0

0

5 0 3 0 0

2

0

144 10 E 0 144 10 E 5 0 Wild Dog 1 0 3 5 2 Approximate limit of current oil/gas window OVERMATURE

0 0

No Pretty Hill reservoir 0

0 2

0

0 5 0 3

35

0 0

144 E 144 E 5 3 Approximate limit of Pretty Hill sandstone

0

1 00

0

0 0 3

0 0

0 143 50 E 1 143 50 E Ingleby 1

Olangolah 1 1000 143 38 53E 143 38 53E 38 50 S 38 30 S 38 40 S 38 20 S 39 00 01S 38 18 12S Figure 26 Crayfish system distribution. petroleum 46 OFFSHORE TORQUAY SUB-BASIN, VICTORIA 38 18 12S 38 20 S 38 30 S 38 40 S 38 50 S 39 00 01S

144 51 47E

144 51 47E

0 0 1 0

5 1 0 0

0 0 0 2

10

0 0 0

5

0 0 0

2 0 8

0 0 0 0 0

0

144 40 E 2

5

0 1 144 40 E 1

5 0 2 2 6 TRUE NORTH IS SHOWN FOR THE CENTRE OF MAP km 4 0 250

2

0

0

0 5

0

0

0 1

0

5

1 0 2

0 0

0

2

0

0

5 2

0

0

0

0

144 30 E

0 144 30 E 5

2 2

0 0 0 0

5 5

2 0 3

0 0 3

0 0 0 25 0

5

1

4

0 0 0

0 0 2

0 00 0 5 2 3 0

0 0

0

3 5

Snail 1

144 20 E 144 20 E

0

0

5 3 00 5

1 Probably immature migration shadow Nerita 1

Anglesea 1

0

0

5 0 3 0 0

2

0

144 10 E 0 144 10 E 5 0 Wild Dog 1 0 3 5 2

0 0

0

0 2

0

0 5 0 3

35

0 0

144 E 144 E 5 3

0

1 00

0

0 0 3

0 0

0 143 50 E 1 143 50 E Ingleby 1

Olangolah 1 1000 143 38 53E 143 38 53E 38 50 S 38 30 S 38 40 S 38 20 S 39 00 01S 38 18 12S Figure 27 distribution. Eumeralla petroleum system OFFSHORE TORQUAY SUB-BASIN, VICTORIA 47

8 Play fairways encountered in Cretaceous or older sediments. Hence, it is believed that in view of the emphasis on Tertiary prospectivity to date, the 8.1 Trap types source rock and migration potential of the Torquay Sub-basin has not been adequately Four potential trap types are recognised. These tested. The lack of firm data on are summarised in Figure 28 and brief palaeo-heatflow does, nevertheless, indicate comments are made below: that there is currently some risk associated with the timing of expulsion of hydrocarbons. Tilted fault block In addition, the proposed reservoir target has The proposed targets are sandstones of the not been penetrated in the Torquay Sub-basin. Pretty Hill Formation and/or the basal section However, by analogy it can be interpreted that of the Eumeralla Formation in tilted fault suitable reservoirs are likely to be encountered blocks. These potential reservoirs are likely to at moderate depths. be sealed by intra-Eumeralla shales both vertically and laterally juxtaposed across faults. Top seal for the primary reservoir is also not perceived to be a problem, although a minor Onlap onto basement high risk is possible with cross-fault seal. This risk would be more clearly understood, with This target comprises the basal sandy units of additional seismic, which would better define the Pretty Hill Formation and Eumeralla the fault system and the use of Allan diagrams Formation onlapping onto basement in the to illustrate possible cross-fault seal problems. eastern part of V99-1. As with the tilted fault block play, top seal would be provided by the With additional infill seismic acquisition and Eumeralla Formation. reprocessing, together with further mapping, it should be possible to the significantly reduce Fractured basement the structural risk.

On the Snail Terrace, there is significant amount of mapped closure at basement. At Sawpit 1, oil has been recovered from fractured basement and shows were present in Gordon 1.

Hanging wall traps

Within the Eumeralla Formation, hanging wall sands shed from the foot wall block and sealed laterally by shales juxtaposed by faults, or as intra-formational lenses sealed laterally by changes in facies. This is a high-risk target with the current seismic quality in the basin. 8.2 Risks

The limited number of shows encountered in the wells drilled to date has led to the suggestion that source potential is limited or that timing of charge is a problem (Trupp et al., 1994). However, the wells can be demonstrated to have either targeted structurally young Tertiary features which post-date charge, or to have been drilled on invalid closures. In the Otway Basin further west, not only have working petroleum systems been demonstrated to occur, but most hydrocarbon accumulations, certainly the economic discoveries, have been 48 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

SSE Potential reservoirs Source Trap types Fractures

3

4

2

1 Pretty Hill Formation Casterton Formation Basement

3

1

1

4 BASIN DEEP SNAIL TERRACE KING IS HIGH

2

1 Undifferentiated Tertiary / Quaternary Eastern View Group Eumeralla Formation

1

NNW

1 2 0 3 prxmt et (km) depth Approximate Figure 28 play-types. Schematic potential section showing OFFSHORE TORQUAY SUB-BASIN, VICTORIA 49

9 Commercial Issues

9.1 Infrastructure

The Torquay Sub-basin lies 100 to 150 km south-southwest of Melbourne and 50 to 100 km southwest of Geelong. The Shell oil refinery is located at Geelong along with a significant industrial base. The location of existing and proposed transmission pipelines is shown in Figure 1. The construction of the Southwest gas pipeline to connect the onshore Port Campbell gas fields to Geelong and the wider SE Australian market has been recently completed. 9.2 Markets

There are opportunities for the supply of gas into the growing SE Australian gas market. ABARE 1997 predicted the domestic Australian market to grow at a rate of 5.5% pa until 2009/10. Victoria uses about 250 PJ of gas a year with peak winter use of around 1100 TJ/day, (about twice that in the summer months of 600 TJ/day). Until 1998, 98% of Victoria’s gas was supplied from the Gippsland Basin to the east of Melbourne. With the privatisation of the gas industry in Victoria virtually completed and new legislative changes, other options are being opened up for the supply of gas. A pipeline link has been built to the north of Melbourne to connect with the central Australia Cooper Basin fields via New South Wales, and a pipeline from Gippsland along the east coast to Sydney is planned to be built in the latter part of 1999. The size of the Sydney market is of the order of 110 PJ of gas a year. In September 1998, a major fire at the Longford processing plant in Gippsland left Melbourne with only emergency supplies of gas for two weeks. The incident brought home the vulnerability of the gas system due to the dominance of supply from the Gippsland Basin. There now exists, therefore, the opportunity for discoveries to the west of Melbourne to have a premium over those to the east in terms of the security of supply they offer. The Torquay Sub-basin is thus strategically well placed as a potential source of supply to fill this need. 50 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

References Report 55, Department of Natural Resources and Environment. BAIN, J.S., 1964. Well completion report, HENRY D.A. & BIRCH, W.D., 1992. Cambrian Ferguson Hill No. 1, southwest Victoria. greenstone on Philip Island, Victoria. Frome-Broken Hill Company Pty Ltd. Australian Journal of Earth Sciences, 39, Open File Report. Department of Energy pp. 567-577. and Minerals, Victoria. HILL, K.C., HILL, K. A., COOPER, G.T., BISHOP, J.H., BICKFORD, G.P., & HEGGIE, O’SULLIVAN, A.J., O’SULLIVAN, P.B. & D.T., 1992. South-eastern Australia RICHARDSON, M.J., 1995. Inversion surface geochemistry II: light around the Bass Basin, SE Australia. In hydrocarbon geochemistry in bottom- Buchanan, J.G. & Buchanan, P.G., (eds), waters of the Gippsland Basin, Eastern Basin Inversion, Geological Society Otway Basin, Torquay Sub-basin and the Special Publication, No. 88, pp. 525-547. Durroon Sub-basin. Bureau of Mineral Resources Record 1992/54, Vols. 1 and 2. LAVIN, C.J. & MUSCATELLO. T., 1998. The Casterton Formation – Otway Basin, CONSTANTINE, A.E., in prep. Sedimentology Victoria. Victorian Initiative for Minerals and stratigraphy of the early Cretaceous and Petroleum Report 53, Department of Strzelecki Group, southeastern Australia. Natural Resources and Environment. Unpublished Ph.D. thesis, Monash University. LITTLE, B.M. & PHILLIPS, S.E., 1995. Detrital and authigenic mineralogy of the COOPER, G.T., 1995. Structural Geology, Pretty Hill Formation in the Penola Thermochronology, and Tectonic Trough, Otway Basin: Implications for evolution of the Torquay Embayment - future exploration and production. Eastern Otway Basin. Unpublished Australian Petroleum Exploration Ph.D. Thesis, Department of Earth Association Journal, 35 (1), pp. 538-557. Sciences, Monash University, Australia. LOVIBOND, R., SUTTILL, R.J., SKINNER, COULSON, A.L., 1930. On the relationship of J.E. & ABURAS, A.N., 1995. The epidiorite and the granite at Barrabool hydrocarbon potential of the Penola Hills and Dogs Rocks near Geelong, Trough, Otway Basin. Australian Victoria. Proceedings of the Royal Petroleum Exploration Association Society of Victoria, 42, pp. 99-109. Journal 35 (1), pp. 358-371. DUDDY, I.R., 1994. The Otway Basin: MEHIN, K. & LINK, A.G., 1994. Source, Thermal, structural, tectonic and migration, and entrapment of hydrocarbon generation histories. In hydrocarbons and carbon dioxide in the D.M. Finlayson, (ed) NGMA/PESA Otway Basin, Victoria. Australian Otway Basin Symposium, Melbourne, Petroleum Exploration Association April 20th, 1994: Extended Abstracts. Journal, 34 (1), pp. 439-459. AGSO Record 1994/14, pp. 35-42. MEHIN, K. & LINK, A.G., 1995. Early EDWARDS, D.S., STRUCKMEYER, H.I.M., Cretaceous source rocks of the Victorian BRADSHAW, M.T. & SKINNER, J.E., onshore Otway Basin. Victorian 1999. Geochemical characteristics of Initiative for Minerals and Petroleum Australia’s southern margin petroleum Report 22, Department of Agriculture, systems. Australian Petroleum Energy and Minerals. Production and Exploration Association Journal 39 (1), pp. 297-321. MEHIN, K. & LINK, A.G., 1997. Kitchens, kettles and cups of hydrocarbons, GEARY, G.C., & REID, I.S.A., 1998. Victorian Otway Basin. Australian Hydrocarbon prospectivity of the offshore Petroleum Production and Exploration Eastern Otway Basin, Victoria, for the Association Journal 37 (1), pp. 285-300. 1998 Acreage Release. Victorian Initiative for Minerals and Petroleum OFFSHORE TORQUAY SUB-BASIN, VICTORIA 51

MORIARTY, N.J., TAYLOR, R.J., & DANEEL, interpretation: A case study from the G.J., 1995. The Sawpit structure- Otway Basin, Australia. Australian evaluation of a fractured basement Journal of Earth Sciences, 37 (3), reservoir play in the Otway Basin. pp. 351-364. Australian Petroleum Exploration Association Journal 35 (1), pp. 558-578. TRUPP, M. A., SPENCE, K.W. & GIDDING, M.J., 1994. Hydrocarbon prospectivity of MORTON, J.G.G., 1990. Revisions to the the Torquay Sub-basin, offshore Victoria. stratigraphic nomenclature of the Otway Australian Petroleum Exploration Basin, South Australia. Quarterly Association Journal, 34 (1), pp. 479-494. Geological Notes, Geological Survey of South Australia, 116, pp. 2-19 (unpublished).

PADLEY, D., McKIRDY, D.M., SKINNER, J.E., SUMMONS, R.E. & MORGAN, R.P., 1995. Crayfish Group hydrocarbons - implications for palaeoenvironment of early Cretaceous rift fill in the western Otway Basin. Australian Petroleum Exploration Association Journal, 35 (1), pp. 517-537.

PARKER, G.J., 1995. Early Cretaceous stratigraphy along the northern margin of the Otway Basin, Victoria. Victorian Initiative for Minerals and Petroleum, Report 23, Department of Agriculture, Energy and Minerals.

PICKAVANCE, D. & OKE, B., 1994. Troas- 1/ST1 Well Completion Report. Interpretation. BHP Petroleum Pty Ltd (unpublished).

REECKMAN, S.A., 1979. Detailed stratigraphy of the Tertiary Sequence, Torquay, Victoria - facies, environment and diagenesis. Unpublished Ph.D. thesis, University of Melbourne.

SCHOLEFIELD, T., NORTH, C.P. & PARVAR, H.L., 1996. Reservoir characterisation of a low resisitivity gas field - Otway Basin, South Australia. Australian Petroleum Production and Exploration Association Journal, 36 (1), pp. 62-81.

SMITH, G.C., 1986. Bass Basin geology and petroleum exploration. In R. C Glenie, (ed) Second Southeastern Australian Oil Exploration Symposium. Petroleum Exploration Society of Australia, Melbourne, pp. 257-284.

STRUCKMEYER, H.I.M. & FELTON, E.A., 1990. The use of organic facies for refining palaeoenvironmental 52 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

Victorian Initiative for 13 BUSH, M.D., CAYLEY, R.A. & ROONEY, S., 1995. The geology and prospectivity of Minerals and Petroleum the Glenelg region, North West VIMP (VIMP) report series area. 14 SLATER, K.R., 1995. An appraisal of Petroleum Reports in bold typeface new airborne geophysical data over the Glenelg region, North West VIMP area, 1 BUCKLEY, R.W., BUSH, M.D., O’SHEA, Victoria. P.J., WHITEHEAD, M. & 15 RYAN, S.M., KNIGHT, L.A. & VANDENBERG, A.H.M., 1994. The PARKER, G.J., 1995. The geology and prospectivity of the Orbost stratigraphy and structure of the Survey area. Tyrendarra Embayment, Otway 2 VANDENBERG, A.H.M., WILLMAN, C., Basin, Victoria. HENDRICKX, M., BUSH, M.D. & 16 KNIGHT, L.A., McDONALD, P.A., SANDSTONES, B.C., 1995. The geology FRANKEL, E. & MOORE, D.H., 1995. and prospectivity of the 1993 Mount A preliminary appraisal of the pre- Wellington Airborne survey area. Tertiary infrabasins beneath the 3 HOLDGATE, G., 1995. The Murray Basin, Northwestern exploration potential of the Permian Victoria. Numurkah Trough and Ovens 17 PERINCEK, D., SIMONS, B.A., Graben, Victoria. PETTIFER, G.R. & GUNATILLAKE, 4 BUSH, M.D., CAYLEY, R.A., ROONEY, K., 1995. Seismic interpretation of R., SLATER, K. & WHITEHEAD M.L., the onshore Western Otway Basin, 1995. The geology and prospectivity of Victoria. the southern margin of the Murray 18 LAVIN, C.J. & NAIM, H.M., 1995. The Basin. structure, stratigraphy and 5 ROONEY, R., 1995. Mineral exploration petroleum potential of the Portland history of the North West VIMP area. Trough, Otway Basin, Victoria. 6 WILLOCKS, A.J., 1995. An appraisal of 19 SIMPSON, C.J., SIMS, J.P. & the new airborne surveys over the North ORANSKAIA, A., 1995. The geology and West VIMP area. prospectivity of the Mt Elizabeth area, Eastern Highlands VIMP area. 7 WHITEHEAD, M.L., 1995. Geological interpretation of geophysical data over 20 ORANSKAIA, A., 1995. A geological the Dunolly 1:100 000 sheet. interpretation of geophysical data over the Mallacoota 1:250 000 sheet, Eastern 8 VANDENBERG, A.H.M., CALUZZI, J., Highlands VIMP area. WILLOCKS, A.J. & O’SHEA, P.J., 1995. NOT RELEASED The geology and prospectivity of the Mallacoota 1:250 000 sheet, Eastern 21 SARMA, S., 1995. Seismic Highlands VIMP area. interpretation of the offshore Otway Basin, Victoria. 9 SANDS, B.C., 1995. A geological interpretation of the geophysical data 22 MEHIN, K. & LINK, A.G., 1995. Early from the Orbost 1994 airborne survey. Cretaceous source rocks of the Victorian onshore Otway Basin. 10 OPPY, I.D., CAYLEY, R.A. & CALUZZI, J., 1995. The geology and 23 PARKER, G.J., 1995. Early prospectivity of the Tallangatta 1:250 000 Cretaceous stratigraphy along the sheet. northern margin of the Otway Basin, Victoria. 11 CALUZZI, J., 1995. Mineral exploration history of the Tallangatta 1:250 000 24 MOORE, D.H., 1996. A geological sheet. interpretation of the geophysical data of the Horsham 1:250 000 map sheet area. 12 SIMONS, B.A., 1995. An appraisal of new airborne geophysical data over the Tallangatta 1:250 000 map area, Victoria. OFFSHORE TORQUAY SUB-BASIN, VICTORIA 53

25 VANDENBERG, A.H.M., HENDRICKX, 39 MOORE D.H., 1996. A geological M.A., WILLMAN, C.E., MAGART, interpretation of the geophysical data of A.P.M., ORANSKAIA, A.N., ROONEY, S. the Ouyen 1:250 000 map sheet area. & WHITE, A.J.R., 1996. The geology and 40 BROOKES, D.J., 1996. Mineral prospectivity of the Orbost 1:100 000 map exploration history, Ararat and area, eastern Victoria. Grampians 1:100 000 map areas. 26 HENDRICKX, M.A., WILLMAN, C.E., 41 LAVIN, C.J., & MUSCATELLO, T., MAGART, A.P.M., ROONEY, S., 1997. The petroleum prospectivity of VANDENBERG, A.H.M., ORANSKAIA, the Casterton Petroleum System in A. and WHITE, A.J.R. The geology and the Victorian Onshore Otway Basin. prospectivity of the Murrungowar 1:100 000 map area, eastern Victoria. 42 CHIUPKA, J.W., MEGALLAA, M., JONASSON, K.E. & FRANKEL E., 27 BOYLE, R.J., 1996. Mineral exploration 1997. Hydrocarbon plays and play history of the Omeo 1:100 000 map area. fairways of four vacant offshore 28 HAYDON, S.J., 1996. An appraisal of Gippsland Basin areas, 1997 acreage airborne geophysical data from the 1995 release. Omeo survey, Victoria. 43 MEHIN, K. & LINK, A.G., 1997. Late 29 MAHER, S., 1996. Mineral resources of Cretaceous source rocks offshore the Dunolly 1:100 000 map area. Otway Basin, Victoria and South Australia. 30 CHIUPKA, J.W., 1996. Hydrocarbon play fairways of the onshore 44 WILLOCKS, A.J., 1997. An appraisal of Gippsland Basin, Victoria. airborne geophysical data from the Castlemaine-Woodend survey, Victoria. 31 MEHIN, K. & LINK, A.G., 1996. Early Cretaceous source rock evaluation 45 HUTCHINSON, D.F., 1997. Mineral for oil and gas exploration, Victorian exploration history of the Heathcote and Otway Basin. Nagambie 1:100 000 map areas. 32 SLATER, K.R., 1996. An appraisal of 46 MAHER, S., VANDENBERG, A.H.M., new airborne geophysical data over the McDONALD, P.A. & SAPURMAS, P., Dargo region, Victoria. 1997. The Geology and prospectivity of the Wangaratta 1:250 000 map sheet 33 McDONALD, P.A., 1996. An appraisal of area. new airborne geophysical data over the Corryong region, northeastern Victoria. 47 ORANSKAIA, A.N., 1997. Geological interpretation of geophysical features 34 TWYFORD, R., 1996. An appraisal of Bendoc 1:100 000 sheet. airborne geophysical data from the Murrindal survey, Victoria. 48 ORANSKAIA, A.N., 1997. Geological interpretation of geophysical features 35 HUTCHINSON, D.F., 1996. Mineral Cann, Mallacoota and Victorian part of exploration history of the Dunolly Eden 1:100 000 sheets. 1:100 000 map area. 49 WILKIE, J.R. & BROOKES, D.J., 1997. 36 BROOKES, D.J. & BOYLE, R.J., 1996. Mineral exploration history of the Mineral exploration history of the Wangaratta 1:250 000 map area. Bairnsdale 1:250 000 map area. 50 McDONALD, P.A., 1997. An appraisal of 37 MAHER, S., HENDRICKX, M.A., airborne geophysical data from the Yea BOYLE, R.J. & BROOKES, D.J., 1996. survey, Victoria. Geology and prospectivity of the Bairnsdale 1:250 000 map sheet area. 51 EDWARDS, J.E., WILLMAN, C.E., McHAFFIE, I.W., OLSHINA, A. & 38 McDONALD, P.A. & WHITEHEAD M.L., WILLOCKS, A.J., 1997. The geology and 1996. Geological interpretation of prospectivity of the Castlemaine, geophysical data over the Ararat Woodend, Yea and part of Bacchus Marsh 1:100 000 map sheet. 1:100 000 map sheets. 54 OFFSHORE TORQUAY SUB-BASIN, VICTORIA

52 MAHER, S., MOORE, D.H., CRAWFORD, A.J., TWYFORD, R. & FANNING, F.M., 1997. Test drilling on the southern margin of the Murray Basin. 53 LAVIN, C.J. & MUSCATELLO, T. 1998. The Casterton Formation - Otway Basin Victoria. 54 MEHIN, K. & BOCK, M.P., 1998. Cretaceous source rocks of the onshore Gippsland Basin, Victoria. 55 GEARY, G.C. & REID, I.S.A., 1998. Geology and prospectivity of the offshore eastern Otway Basin, Victoria, for the 1998 Acreage Release. 56 MEGALLAA, M., BERNECKER, T. & FRANKEL, E., 1998. Hydrocarbon prospectivity of the Northern Terrace, offshore Gippsland Basin, for the 1998 Acreage Release. 57 LAVIN, C., 1998. Geology and prospectivity of the western Victorian Voluta Trough - Otway Basin, for the 1998 Acreage Release. 58 EDWARDS, J., SLATER, K.R. & PARENZAN, M.A., 1998. Bendigo and part of Mitiamo 1:100 000 map area geological report. 59 RADOJKOVIC, A., 1998. Mineral exploration history of the Ballarat and Creswick 1:100 000 map areas.

60 MESSENT, B.E., COLLINS, G.I. & WEST, B.G., 1999. Hydrocarbon prospectivity of the offshore Torquay Sub-basin; Victoria: Gazettal Area V99-1.

61 SMITH, M.A., 1999. Petroleum systems, play fairways and prospectivity of the Gazettal Area V99-2, offshore southern Gippsland Basin, Victoria.

62 MEHIN, K. and CONSTANTINE, A.E., 1999. Hydrocarbon potential of the western onshore Otway Basin in Victoria: 1999 Acreage Release. . MPVINERALSand ETROLEUM ICTORIA P.O.BOX 500 EMAST ELBOURNE VIC.3002 250 VVPICTORIA P ARADE EMAST ELBOURNE VIC. 3002 PP03HONE: 03 9412 5084 FF03AX: 03 9412 5156