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Mineral Resources
Tasmania
Release of offshore petroleum explorationareas in 1998 — T98-1 and T98-2
Sorell Basin, Tasmania
146°E
146°E
144°E
42°S42°S
40°S40°S
144°E
148°E
Tasmania Adjacent Area BoundaryPermit area
T/30P
T/28P
T/27P
T/RL1
T/18P
T/25P
T98-2
T98-1
Wells drilled in Tasmanian waters
Report 1998/03
Mineral Resources Tasmania
Tasmanian Geological Survey Record 1998/03 2
Sorell Basin, Tasmania, Australia
Release of offshore petroleum explorationareas in 1998 T98-1 & T98-2
by C. R. Calver
Tasmania
Mineral Resources TasmaniaReport 1998/03
Amongst the areas to be released for petroleum exploration under Australia’s competitive work programbidding system in 1998 are two adjoining acreages off the west coast of Tasmania. These areas cover themost prospective parts of the Sorell Basin, an underexplored, Jurassic to Tertiary passive-margin basinwith a history broadly similar to the nearby, prolific Gippsland Basin. Some prospects in T98-1 are maturefor drilling. Both areas overlie continental shelf (mostly <200 m water depth).
Permit area
T/30P
T/28P
T/27P
T/RL1
T/18P
T/25P
T98-2
T98-1
146°E
146°E
144°E
42°S42°S
40°S40°S
144°E
148°E
Tasmania Adjacent Area Boundary
Wells drilled in Tasmanian waters
Cape Sorell–1
Clam–1
TASMANIA
Figure 1. Location of T98-1 and 2, offshore western Tasmania, and othercurrent offshore petroleum tenements in Tasmanian state waters.
SummaryThe Sorell Basin and neighbouring, related coastalsoutheast Australian basins — the Gippsland,Otway and Bass Basins — were produced byextensional tectonics in the Jurassic to Tertiary. TheGippsland Basin is a world-class oil and gas provinceand has produced almost 70% of Australia’s oil.Prospective acreage in the Bass Basin is almost fullyheld, with four or more wells planned for 1998,including at least one appraisal well on the Yollagasfield. The Otway Basin, adjoining the SorellBasin to the north, is a gas producer in onshoremainland Australia, and has recently yielded acommercial offshore gas discovery.
Of these basins, the Sorell Basin remains the leastexplored with only two wells drilled, despite live oilshows, evident parallels with the nearby producingbasins, and the potential for a variety ofstructural/stratigraphic traps in three or more thickdepocentres under the continental shelf.
Petroleum exploration in the Sorell Basin dates backto the 1960s, when a reconnaissance seismic data setwas acquired. More recent exploration activity(Amoco in the early 1980s, Maxus in the early 1990s)has been concentrated in the Strahan Sub-basin inarea T98-1. Only two exploration wells have beendrilled: Clam-1 in the King Island Sub-basin andCape Sorell-1 in the Strahan Sub-basin. Live oilshows were found in Cape Sorell-1 below theCretaceous–Palaeocene unconformity, which hassince been shown to have no closure.
The Sorell Basin is considered to be both oil and gasprone. The deregulation of gas markets in thenearby mainland Australian States of Victoria andNew South Wales could provide gas from the SorellBasin with a number of market opportunities.
The acreage offers a rare opportunity to participatein a promising, nearly unexplored basin, within afavourable economic and political environment.
Sorell Basin: Structureand stratigraphyThe Sorell Basin formed as a result of the latestJurassic to earliest Cretaceous rifting that heraldedthe separation of Australia and Antarctica. Therifting had a left-lateral, transtensional componentthat resulted in a number of small, deep, mainlyfault-bounded depocentres, including a number ofprospective sub-basins under the west Tasmaniancontinental shelf (fig. 3).
The stratigraphy of the Sorell Basin is similar tothat of the better-known, adjoining Otway Basin tothe north (fig. 4) and there are also strong parallelswith the stratigraphy of the prolific GippslandBasin.
The earliest rift fill sediments are correlatives of theEarly Cretaceous Otway Group and restunconformably on Proterozoic and Palaeozoicbasement. The Otway Group has not been reachedby wells in the Sorell Basin, the nearest penetrationbeing Prawn-1 located in the nearby southern part ofthe Otway Basin. However seismic packagespredicted to be Otway Group correlatives areobserved in the deeper sub-basins and onlappingearly rift basement highs. In the Otway Basin, theOtway Group includes low-energy fluvial/lacustrinedeposits and is known to have sourced the Minervacommercial offshore gas discovery.
A prominent mid-Cretaceous unconformity issucceeded by the Cenomanian–MaastrichtianSherbrook Group, consisting of marginal marine tofluvial clastic sediments that include gas and oilprone source rocks where intersected by CapeSorell-1 in the Strahan Sub-basin. Oil shows werealso seen in the Sherbrook Group in this well.
Continental breakup west of Tasmania in theMaastrichtian coincided with a major wrenchingevent, seen in the continental shelf sub-basins asanticlinal flower structures that are potential traps(fig. 5). Wrench movement continued until themid-Oligocene, when the Australian and Antarcticplates finally cleared each other.
The Maastrichtian breakup unconformity issucceeded by the thick fluvial to shallow marinesandy clastic rocks of the Wangerrip Group, ofPalaeocene to early Eocene age. These sedimentsare broadly equivalent to the upper part of theLatrobe Group, the main reservoir interval of theGippsland Basin. A top Palaeocene unconformitywithin the Wangerrip Group, locally displayinghundreds of metres of erosional relief, has led to anerosional remnant stratigraphic play conceptdirectly analogous to several of the giant GippslandBasin fields. At least one such prospect has beenmapped in the Strahan Sub-basin (see below).
The section is completed by Eocene to Mioceneshelfal marls and temperate carbonates of theHeytesbury Group, broadly equivalent to theSeaspray Group, the base of which forms theregional seal in the Gippsland Basin.
Tasmanian Geological Survey Record 1998/03 3
Tasmanian Geological Survey Record 1998/03 4
Prawn 1
Cape Sorell 1
Whelk 1King Island
King Island Sub-basin
Sandy Cape Sub-basin
Strahan Sub-basin
Port DaveySub-basin
TASMANIA
SOUTHEASTINDIANOCEAN
0 50 km
144° 146°142°
40°
42°
44°
DSDP 282
Figure 2.Bathymetry and seismic coverage (solid lines) on the west Tasmanian margin
and location of exploration wells. From Hill et al. (1997).
Tasmanian Geological Survey Record 1998/03 5
Figure 3.Sediment thickness and main structure, Sorell Basin. From Hill et al. (1997).
Tasmanian Geological Survey Record 1998/03 6
Figure 4.West Tasmanian well stratigraphy and correlations. [Diagram provided by P. Hill]
Area T98-1: Strahan Sub-basinArea T98-1 overlies the highly prospective StrahanSub-basin, which is a complex half-graben thatcontains up to 6.5 km of Cretaceous and Tertiarysediments. The Strahan Sub-basin has beenexplored by Esso, Amoco and most recently byMaxus Energy Corporation. A dense 2D seismic grid(1 × 1.25 km spacing) shot by Amoco and Maxuscovers most of the sub-basin. Several prospects andleads have been mapped (fig. 6) and some of theprospects are ready to drill. Amongst these is anerosional remnant play sealed by Eocene canyon-fillshales, resembling the giant Marlin, Halibut andTuna fields of the Gippsland Basin. Anotherprospect exhibits a direct hydrocarbon indicator(flat-spot) on three seismic lines.
Hydrocarbon prospectivityThe only well drilled in T98-1, Cape Sorell-1, wasdrilled close to a basin boundary fault andintersected a succession dominated by a thick, verysandy Palaeogene section. Basinward sections areexpected to be more shale-prone, providing effectiveseals and source rocks. Live oil shows were recordedfrom below the top of the Sherbrook Group(Maastrichtian) to near the bottom of the hole at3445 metres. The Sherbrook Group includes oil andgas-prone source rocks, with TOCs averaging 3%.The unconformity at the top of the Sherbrook Groupdemonstrates no structural closure at the welllocation.
In 1990 Maxus reprocessed Amoco’s (1981) seismicand shot infill lines to give a close (1 × 1.25 km) gridin the Strahan Sub-basin. Several prospects andleads were mapped, and further potential remainswithin the block. Play concepts derive, in part, from
those pursued successfully in the Gippsland Basin.Maxus estimated the recoverable reserves from eachof the Braddon Point, Sloop Point and Trial Harbourprospects to be 150 to 200 million barrels.
The Braddon Point Prospect occurs at the topPalaeocene unconformity, which is a deeplychannelled surface overlain by a shaly canyon fill(the Pember Mudstone equivalent). This prospectdisplays fault-dip closure in an east-west direction,and is closed off by Eocene channel fill to the northand south (fig. 7). The canyons appear to becomparable in age and topographic relief to the earlyto mid Eocene canyons of the Gippsland Basin thatare important in sealing the Marlin, Halibut, Tunaand Mackerel fields.
The Sloop Point prospect consists of two stackedtargets, testable with a single well, on the southernside of the sub-basin. The lower target, at the top ofthe Otway Group, exhibits closure by southward,updip wedgeout of the Otway Group and fault-dipclosure in the east-west direction. The higher target,at the top of the Sherbrook Group, has a largerpotential acreage with a simple dip and fault closureagainst the mid-basin fault.
The Trial Harbour Prospect is a downthrownrollover feature at the level of the top Sherbrookunconformity, with structural dip and fault closureadjacent to the northern boundary fault of thesub-basin (fig. 8). A seismic anomaly, believed to beindicative of vertical gas migration, is associatedwith this location. This prospect is conceptuallysimilar to that penetrated by Cape Sorell-1, but inthe Trial Harbour Prospect the non-closing section,within which the shows were encountered in thewell, does exhibit closure.
Tasmanian Geological Survey Record 1998/03 7
Cape Sorell 1
Palaeozoic
? Wrench Anticline(Cretaceous)
Wrench zone(Palaeogene)
EarlyCretaceous
CretaceousReactivated riftboundary fault
U3
U3
Tertiary
U13
U10
LateTD
Fault surfa
cerefle
ction
0 5 km
1
2
3
4
Ref
lect
ion
Tim
e(s
econ
ds)
Figure 5.An interpretation by Hinz et al. (1986) of Amoco line W-81-12 oriented SW-N through Cape Sorell-1 well.
Note the ‘flower structures’ that signify wrenching. Cape Sorell-1 may have beenbetter sited about 8 km to the SW.
Tasmanian Geological Survey Record 1998/03 8
AUSTRALIA
TASMANIA
T24/P
GENERATIVE SOURCE AREA
SHERBROOK/OTWAY GROUPSEARLY OIL (Ro 0.5)
1.620 Sec., 7000 Ft.
GAS/CONDENSATE (Ro 1.3)
2.520 Sec., 13,500 Ft.
PROSPECTS & LEADS
BASINMOD ANALYSIS
CAPE SORELL-1
MAC
QU
AR
IE
HAR
BO
UR
TRIAL HARBOUR
PROSPECT
BRADDON POINT
PROSPECT
144°30’
42°00’
42°10’
42°30’
144°50’
0 10 km
Figure 6Strahan Sub-basin in T98-1, showing generative source area, and main prospects
and leads mapped by Maxus. Courtesy of Maxus Energy Corporation.
The Eocene Canyon Sand Leads comprise potentialstratigraphic traps within the canyon system wheresands may be sealed by overlying shale and thecanyon wall. One lead is located in a thick (450milliseconds) section of channel fill at the pointwhere two channels merge immediately west of the
Braddon Point erosional high. A recent re-appraisalof the seismic has resulted in the interpretation of adirect hydrocarbon indicator (flat-spot) within thenorthern channel fill, about 1000 acres in extent, onthree seismic lines.
The Northern Strahan Wedgeout Lead is apotentially very large structural-stratigraphic trapthat lies outside the sub-basin and the main seismicgrid, in deep (>1000 m) water.
Geohistory modelling suggests Late Cretaceous toPalaeocene levels are in the oil window, and oldersuccessions are in the gas window in those parts ofthe sub-basin distal to the Cape Sorell-1 well.Significant maturation occurred post-early Eocene(after trap formation).
A 1985 marine seafloor survey recoveredthermogenic hydrocarbons from upper continentalslope sediments over the western StrahanSub-basin. This is consistent with thermalmaturation modelling which indicates thatgenerative conditions have existed — and currentlyexist — within T98-1.
Exploration History
Area T98-1 has been held by Esso Exploration andProduction Australia Ltd (1968–1972), AmocoAustralia Oil Company (1980–1983), and most
Tasmanian Geological Survey Record 1998/03 9
Figure 7Time structure on Palaeocene erosional surface (intra-Wangerrip Group) defining Braddon Point prospect.
Courtesy of Maxus Energy Corporation.
recently by Maxus Energy Corporation (1990–1993).Maxus relinquished the permit after failing toattract farm-in partners. The Strahan Sub-basin iscovered by a 2D seismic grid of 1 × 1.25 km shot byAmoco and Maxus Energy Corporation. Severalprospects and leads have been mapped by Maxus. In1995 the Australian Geological Survey Organisation(AGSO) carried out a deep crustal seismic surveyalong the length of the west Tasmanian margin,with multiple lines over the Strahan Sub-basin.Cape Sorell-1 (Amoco, 1982) is the only well drilledin T98-1.
WellsWell: Cape Sorell-1Operator: Amoco Australian Petroleum
CompanyRig Release date: 1982KB 22 mStratigraphy:
Heytesbury Group 94 m
Nirranda Group ?Wangerrip Group 389 mSherbrook Group 1759 m
TD: 3528 mWater depth: 94 mStatus: P & A dry, minor oil shows
Seismic surveys
Part or all of the following surveys are located inT98-1. Shotpoint maps, seismic sections and reportsare available from Mineral Resources Tasmania.
Line prefix Survey Operator/year
EE Esso Tasmania 1968T69A Esso West Tasmania 1969T70A Esso West Tasmania 1970T70C Esso West Tasmania 1970N ‘Petrel’ survey (Shell) 1973W81 Amoco West Tasmanian Basin 1981MXT 90 Maxus Energy CPN survey 1990148 Circum-Tasmania deep seismicAGSO — 1995
Tasmanian Geological Survey Record 1998/03 10
Figure 8Time structure on top Sherbrook Group reflector defining the Trial Harbour prospect.
Courtesy of Maxus Energy Corporation.
Area T98-2: King Islandand Sandy Cape Sub-basinsArea T98-2 encompasses the King Island and SandyCape Sub-basins. These sub-basins contain up to 3.6and 5 km of sediment respectively, and have onlybeen lightly explored. Recent discoveries in theoffshore Otway Basin enhance the area’sprospectivity. The King Island Sub-basin is coveredby a 7 km grid of 1968 Esso seismic lines, whileAGSO’s 1995 deep crustal seismic survey includedmultiple lines over the King Island Sub-basin.Seismic coverage of the Sandy Cape Sub-basin issparse, but enough to suggest some potential leads.Apparent drape closures over tilted fault blocks arepresent in both sub-basins. The Sandy CapeSub-basin is in a favourable position for updiphydrocarbon migration from thick depocentresunder the continental slope. As in T98-1, traces ofthermogenic hydrocarbons have been recoveredfrom shallow sediment samples from the uppercontinental slope.
Clam-1, drilled in the King Island Sub-basin by Essoin 1969, is the only well drilled in T98-2. Sited on theeastern flank of a large basement high, the wellintersected reservoir-quality sands but reachedbasement at only 1510 metres.
Hydrocarbon ProspectivityThe King Island Sub-basin, lying shelfward of theClam High, is defined by a widely-spaced seismicgrid, mostly of 1960’s vintage. Clam-1, on the flankof the Clam High, penetrated a prograding wedge ofPalaeocene sediment sands overlying a thin sandyUpper Cretaceous sequence containing redbeds.These overlie basement phyllite of probableProterozoic age.
In the relatively shallow depocentre of the KingIsland Sub-basin, the best prospects are offered bythe lowermost Cretaceous sediments, draped overtilted fault blocks in the deepest parts of the trough.
The lower Eocene canyons mapped in the StrahanSub-basin also occur in the southern part of the KingIsland Sub-basin and may help form effective traps.
The Sandy Cape Sub-basin extends for 120 km alongthe margin, partly under the continental shelf, andhas a maximum sedimentary thickness of over 5000metres. Similar sediment thicknesses underlie largeareas of the adjacent continental slope (fig. 3), whichtherefore represents a vast downdip ‘kitchen area’for hydrocarbon generation offshore.
Seismic coverage is sparse but enough to delineatesome potential leads, for example drape over tiltedbasement fault blocks on BMR78-5, and dip andfault closures at the northern end of the sub-basin(fig. 9). The lower Eocene submarine canyons arepresent (line t69A-2) and may form substantialstratigraphic traps, and may act as conduits formigrating hydrocarbons generated in the thickdepocentres under the continental slope intoshallower sandstone targets under the shelf.
Exploration History — WellsWell: Clam-1Operator: Esso Standard Oil (Australia) LtdRig Release date: 1969KB 30 mStratigraphy:
Heytesbury Group 102 mNirranda Group ?Wangerrip Group 387 mSherbrook Group 925 mLower Cretaceous Redbeds 1462 mPz/Late Proterozoic 1510 m
TD: 1592 mWater depth: 102 mStatus: P & A dry
Seismic surveysLine prefix Survey Operator/yearK Cape Grim to Cape Jaffa Hematite/1964–65EO Offshore Otway Basin, 1966 Esso/1966–67EE Tasmania 1968Esso/1968EP Offshore Otway Basin, 1967 Esso/1967–68M West Otway Magellan/1969N Roving Petrel Shell/1969BMR1970–73 Continental Margins BMR/1973BMR 82 Survey 40 BMR/1982
Tasmanian Geological Survey Record 1998/03 11
Clam-1
CLAM HIGH KING ISLANDSANDY CAPE SUB-BASIN
SUB-BASINLINE 78/05
4 km0
0
2
4
Ref
lect
ion
time
(S)
SW NE
INTRUSION
Figure 9
Interpreted seismic line across T98-2,showing fault drape in Sandy CapeSub-basin, the Clam High, and the
western part of the King IslandSub-basin (from Moore et al., 1992).
SO-36 Survey 36 BGR/198578 Survey 78 BMR/1988148 Circum-Tasmania deep seismic AGSO/1995
Work Program Bidding System
Offshore petroleum exploration in Australianwaters is administered jointly by theCommonwealth (Federal) Government and therelevant State Government.
Intending explorers are referred to the brochuresproduced by the Department of Primary Industriesand Energy (Release of Offshore PetroleumExploration Areas Australia 1998) for fullinformation on exploration protocols in Australianoffshore waters.
Under the work program bidding system, theapplicant is required to propose a six-yearexploration program. The first three years of theprogram — the ‘minimum guaranteed dry hole workprogram’ — should comprise work of an exploratory(rather than appraisal) nature. All the work must becompleted to avoid cancellation of the permit. Theapplicant also identifies a ‘secondary work program’to cover the second three years of the permit.
Work program permits are issued for an initial sixyears, and may be renewed for an unlimited numberof five year periods. At each renewal 50% of thepermit area must be relinquished.
Closing date for work program bids for T98-1and T98-2 is Thursday 15 October 1998.
Data availability
Open File Exploration Databases
Australia’s offshore petroleum legislation requirescompanies to submit data and technical reports onexploration activities. Most basic data then becomespublicly available two years after submission, withinterpretative data becoming available five yearsafter submission.
Seismic (and magnetic) survey lines and reports,well completion reports, and logs pertaining to T98-1and 2 are available from Mineral ResourcesTasmania, Hobart, and the Bureau of ResourceSciences, Canberra. Seismic and well survey data onmagnetic tape is stored by Australian Archives inSydney. The Bureau of Resource Sciences managesthe tape archive and the lending service, and hasestablished a computerised database allowingon-line access to information on data held.
Digitised E-logs and well cards are available fromWiltshire Geological Services, Adelaide. Digitisedshotpoint maps are available from PetroconsultantsDigimap, Sydney. BMR and AGSO surveys areavailable from AGSO, Canberra.
Geological and Prospectivity Studies
CONOLLY, J.; GALLOWAY, M. J. 1995. Hydrocarbonprospectivity of the offshore west coast of Tasmania.Tasmanian Geological Survey Record 1995/04. [A34-page report summarising prospectivity of theSorell and southern Otway Basins, including 15figures and 12 key interpreted seismic lines,available for $A250 from Mineral ResourcesTasmania (address below).]
HILL, P. J.; EXON, N. F.; ROYER, J-Y.; WHITMORE, G.;BELTON, D.; WELLINGTON, A. 1997. Atlas of theOffshore Tasmanian Region: Swath-mapping andGeophysical Maps from AGSO’s 1994 Tasmantesurvey. [Available as hard-copy and CD-ROM.Presents the results of swath-mapping of 200 000km2 of seabed off western and southern Tasmania,with most maps at 1:1 000 000 scale. Includesbathymetric contours, gravity and magnetic profilesand contours, seabed sampling location andinformation, sediment thickness and mainstructures, and other data. Available at $A250 fromthe AGSO Sales Centre (tel. +61 2 6249 9519; GPOBox 378, Canberra, ACT 2601).]
HILL, P. J.; MEIXNER, A. J.; MOORE, A. M. G.; EXON,N. F. 1997. Structure and development of the westTasmanian offshore sedimentary basins: results ofrecent marine and aeromagnetic surveys.Australian Journal of Earth Sciences 44:579–596.[A concise summary of the development of the SorellBasin based on new regional seismic, aeromagneticand bathymetric data sets.]
LODWICK, W. 1998. Petroleum prospectivity study,Sorell Basin . [Available on CD-ROM. Acomprehensive account of geology, explorationhistory, and prospectivity, including a 1998interpretation of the last detailed seismic survey(Maxus, 1990) in the Strahan Sub-basin (areaT98-1). Package includes two 8 mm tapes containing1990 Maxus survey in SEGY format; stack,migration and RPF migration; and Landmarkinterpretation. Available from Bureau of ResourceSciences (address below).]
MOORE, A. M. G. 1991. Western Tasmanian margin:seismic interpretation and mapping. Record BureauMineral Resources Geology & Geophysics Australia1991/70. [Interpretation of pre-1990 seismic, withillustrative seismic sections and TWT maps at1:250 000 and 1:100 000 scale.]
MOORE, A. M. G.; WILLCOX, J. B.; EXON, N. F.;O’BRIEN, G. W. 1992. Continental shelf basins on thewest Tasmania margin. APEA Journal 32:231–250.[A detailed account of petroleum prospectivity of theSorell and southern Otway Basins, based on Moore’s(1991) interpretation, incorporating well data andgeohistory modelling.]
[1 May 1998]
Tasmanian Geological Survey Record 1998/03 12
Tasmanian Geological Survey Record 1998/03 13
Further information on these areas is available from:
Ms Carol BaconManaging Geologist,Industrial Minerals, Environment & Tenement ManagementMineral Resources TasmaniaPO Box 56 Rosny ParkTasmania Australia 7018
Telephone: +61 3 6233 8326Fax: +61 3 6233 8338email: [email protected]
Dr Bill LodwickPetroleum Resources BranchBureau of Resource SciencesPO Box E11 KingstonACT Australia 2604
Telephone: +61 2 6272 3740email: [email protected]
While every care has been taken in the preparation of this report, no warranty is given as to thecorrectness of the information and no liability is accepted for any statement or opinion or for any error oromission. No reader should act or fail to act on the basis of any material contained herein. Readersshould consult professional advisers. As a result the Crown in Right of the State of Tasmania and itsemployees, contractors and agents expressly disclaim all and any liability (including all liability from orattributable to any negligent or wrongful act or omission) to any persons whatsoever in respect ofanything done or omitted to be done by any such person in reliance whether in whole or in part upon anyof the material in this report.
