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Seismic Interpretation Report

Exploration Permit T/35P SOSN08 Brandt 3D Seismic Survey

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Table of Contents INTRODUCTION....................................................................................................................................................... 1 PERMIT HISTORY ................................................................................................................................................... 1 EXPLORATION HISTORY...................................................................................................................................... 3 GEOLOGY AND HYDROCARBON PROSPECTIVITY...................................................................................... 3 SEISMIC MAPPING.................................................................................................................................................. 4

AREA AND DATA MAPPED ......................................................................................................................................... 4 INTERPRETATION METHODOLOGY ............................................................................................................................. 4 HORIZONS INTERPRETED............................................................................................................................................ 4 DEPTH CONVERSION .................................................................................................................................................. 4 STRUCTURE MAPS PREPARED .................................................................................................................................... 4

DISCUSSION OF SEISMIC RECORD QUALITY ................................................................................................ 5 PLAY TYPES.............................................................................................................................................................. 5 LEADS AND PROSPECTS ....................................................................................................................................... 5 CONCLUSIONS ......................................................................................................................................................... 7 FIGURES..................................................................................................................................................................... 8

PERMIT LOCATION MAP............................................................................................................................................. 8 DATA LOCATION MAP ............................................................................................................................................... 9 STRATIGRAPHIC CHART ........................................................................................................................................... 10 SYNTHETIC SEISMOGRAMS ...................................................................................................................................... 11 SEISMIC DATA QUALITY.......................................................................................................................................... 12 SEISMIC DATA QUALITY COMPARISON.................................................................................................................... 13 INTERPRETED SEISMIC SECTIONS............................................................................................................................. 14

ENCLOSURES - MAPS OF KEY HORIZONS..................................................................................................... 16


SOSN08 Brandt3D T35P Seismic Interpretation Report.doc

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Introduction

The SOSN08 Brandt 3D Seismic Survey was acquired by Santos in April 2008 as part of the work commitment for the T/35P Permit Year 5.

Acquisition occurred between 25 February and 28 March 2008. The survey was acquired by the PGS MV Pacific Explorer seismic vessel. A total of 446 km2 of seismic data was acquired (Figure 2).

All processing was undertaken at the CGGVeritas office in Perth, Western Australia, and commenced in April 2008. Due to the complex seafloor topography and resultant rapid lateral changes in velocity in the shallow section, Pre Stack Depth Migration processing was necessary for this survey.

Permit History

Exploration Permit T/35P is located in the Otway Basin, Offshore Tasmania (Figure 1). The Permit comprises 50 graticular blocks covering an area of approximately 3285km2. The Designated Authority awarded the Permit T/35P effective 30 July 2003 for an initial term of 6 years.

On 1 May 2006, Mittwell Energy Resources Pty Ltd accepted a 25% share of the Santos and Unocal interest, resulting in a reduced Santos and Unocal interest of 37.5%.

In 2005 Unocal was taken over by Chevron Australia (SE Australia) Pty Ltd, although Chevron continued to use the Unocal South Australia Pty Ltd name. On 3 July 2008 notification was received that Perenco (SE Australia) Pty Ltd purchased Chevron Australia (SE Australia) Pty Ltd.

Santos Offshore Pty Ltd is Operator of the Permit. The current interest holders in the permit are:

Company Percentage Interest

Santos Offshore Pty Ltd (Operator) 37.5%

Perenco (SE Australia) Pty Ltd 37.5%

Mitsui E&P Australia 25%



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The work obligations for Exploration Permit T/35P are set out below:

PERMIT YEAR (Commencing) PRIMARY WORK PROGRAM INDICATIVE

EXPENDITURE

One (30 July 2003 – 29 July 2004) 1276 km 2D Seismic Acquisition $2,100,000

Two (30 July 2004 – 29 July 2005) Geological and Geophysical Studies $500,000

Three (30 July 2005 – 29 July 2006)

Geological and Geophysical Studies & Environmental Assessment $400,000

TOTAL FIRM PROGRAM $3,000,000

PERMIT YEAR (Commencing) SECONDARY WORK PROGRAM INDICATIVE EXPENDITURE

Four (30 July 2006 – 29 July 2007)

Geological and Geophysical Studies & Detailed Well Design $800,000

Five (30 July 2007 – 29 July 2010) 410 km² 3D Seismic Acquisition $6,600,000

Six (30 July 2010 – 29 July 2011) One Well $21,000,000

TOTAL SECONDARY PROGRAM $28,400,000



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Exploration History Described below are the results of the offshore wells closest to the T/35P exploration permit. Well completion reports issued by the Operator and the updated interpretation of seismic data provide the primary source of information for detailing the exploration history. No wells to date have been drilled on the permit.

Prawn-A1 (Esso, 1967). Intersected 712m of Waarre Formation equivalent. Measured porosities in the sandstone exceeded 20% with permeabilities up to 235 mD.

Clam-1 (Esso, 1969). Tested structural closure of the basal Tertiary and updip pinchout of Cretaceous sediments against the Clam High. Good reservoir sandstones with porosities up to 20% were intersected. Clam-1 reached a depth of only 1592m, intersecting Devonian conglomerates on Pre-Cambrian metamorphic basement. The penetrated sedimentary section is immature for hydrocarbon generation.

Whelk-1 (Esso, 1970). Drilled an anticlinal closure to test anticipated Waarre Formation sandstones with good reservoir characteristics. Although 94m of Waarre Sandstone was intersected, top seal (Belfast Mudstone) lithologies were not developed.

Thylacine-1 (Origin, 2001). Successfully tested a mapped Waarre Formation closure, with associated amplitude anomaly. It encountered gas within the Thylacine/Flaxman/Waarre section.

Geographe-1 (Origin, 2001). This well was a successful test of a mapped Waarre Formation closure, with associated amplitude anomaly. It encountered gas within the Thylacine/Flaxman/Waarre section.

Geographe North-1 (Origin, 2001). This well drilled as a follow-up test of Geographe-1 and was a dry hole (no full stack amplitude anomaly).

Thylacine-2 (Origin, 2001). This well was an appraisal to follow up success at Thylacine-1.

Jarver-1 (Santos, 2008). Drilled an anticlinal closure at Thylacine Member level. No hydrocarbons were intersected in the borehole, with only minor fluorescence observed in the Paaratte Fm.

Somerset-1 (Woodside, 2009). Not yet open file. No hydrocarbon shows reported.

Geology and Hydrocarbon Prospectivity The Sorell Basin formed during oblique rifting between the Australian and Antarctic continents in the Late Cretaceous and is considered to be a southern extension of the Otway Basin. While relatively open ocean conditions existed to the north and west in the Otway Basin, a restricted marine embayment was formed in the Sorell region, bounded to the southeast by a land-bridge between the two continents, which did not separate until the Eocene. Late Cretaceous reservoir sandstones, belonging to the Waarre Formation, Flaxman Formation and Thylacine Sandstone Member (Figure 3), form the main productive zones within Otway Basin fields such as Casino (Santos-operated), Minerva, La Bella, Thylacine and Geographe, located north of the deep water acreage trend. These sandstones exhibit excellent reservoir quality with average log porosity in the range of 15-28% and permeabilities of up to 8 Darcies. These sandstones are mapped extending southwards into the Sorell Basin permits. Deposition of coarse-grained siliciclastics in the Turonian-Santonian is associated with periodic fluvio-deltaic pulses into an overall upwards-deepening/fining section (Belfast Mudstone), providing an effective reservoir-seal couplet. Regional regression in the Campanian led to renewed coarse-grained, fluvio-deltaic input to the basin as the Paaratte and Timboon sandstones. Potential sealing sequences of the Skull Creek and Timboon mudstones and the Massacre



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Shale are observed to thicken into the basin. Mild structural inversion in the latest Cretaceous was followed by rapid subsidence and transgression resulting in retrogradation of the deltaic systems and deposition of the Wangerrip Group in the Palaeogene. Eventually the marginal sedimentary systems were drowned and, in association with the opening of the seaway in the late Eocene and subsequent formation of the Circum-Antarctic current, deposition came to be dominated by cool-water carbonates that persist to present day. Nine offshore discoveries have been made in the Otway Basin to date proving in excess of 1.6 TCF recoverable gas. These discoveries have CO2 content increasing generally from north to south. Hydrocarbon charge in the basin is considered to come from Albian-aged, Eumeralla Formation source rocks.

Seismic Mapping

Area and Data Mapped The new SOSN08 Brandt 3D seismic survey is located in the northeast of Permit T/35P, with part of the 3D survey extending into Permit T/30P (Figure 2). The survey was designed to image the Brandt and Florey Prospects and mature them to drillable status. The 3D survey is intersected by older vintage 2D seismic data, from DS01, DS02 and SS03 surveys.

Interpretation Methodology Paradigm 3D Canvas was the primary package used in the structural interpretation of this seismic data.

Seismic interpretation was undertaken using primarily the all-offset pre-stack depth migrated volume (scaled to time), with reference to the near, mid and far angle stacks, as well as coherency volumes (which assist with the fault mapping). Mapped horizons on the new SOSN08 Brandt 3D data were tied to mapped horizons on the multiclient DS01 and DS02, and the Santos-acquired SS03 data.

Interpretation on the 2D data had previously been calibrated to synthetics at the Thylacine-1, Prawn-A1 (Figure 4) and Whelk-1 (Figure 5) wells.

Horizons Interpreted Key horizons interpreted include the water bottom, Wangerrip Group, Belfast / Skull Creek Mudstone, Thylacine Member, Waarre Formation and Eumeralla Formation (Figure 3). The Thylacine Member is hard to pick in the south-west part of the survey, which is thought to be related to the absence of sand in this distal facies.

Due to the high density of faults, only those faults with significant lateral extent or vertical throw were mapped. In particular, the Paaratte Formation has high density small-scale polygonal faulting, but only those faults with significant throw or lateral extent were mapped.

Depth Conversion Depth conversion was undertaken in Paradigm using seismic (stacking) velocities derived by the High Density Velocity Analysis (HDVA) process. These provided a detailed velocity field, but of necessity, were un-calibrated due to absence of wells in the survey area. Pre-stack depth migration velocities developed during the iterative pre-stack depth migration process provided a smoother, interval velocity field, more suited to the layer-cake PSDM process.

Structure Maps Prepared Structure maps prepared include the Water Bottom, Wangerrip Group, Skull Creek Mudstone, Thylacine Member, Waarre Formation and Eumeralla Formation (Enclosures 1-6). In addition, amplitude maps at both Thylacine and Waarre Formation (Enclosures 7-8) have been draped with depth structure contours to show possible amplitude relationships to depth. The amplitude at Top Thylacine was extracted from



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the Far Offset Stack in a narrow window around the Thylacine Member horizon (maximum amplitude within 16 ms window centred on horizon). The Waarre Formation amplitudes are normalised by extracting (from the Far Offset Stack) a windowed amplitude around the Waarre horizon and dividing that by an average amplitude extracted in the overlying section. This removes possible dimming related to shallow volcanics, which would otherwise bias the Waarre amplitudes.

Discussion of Seismic Record Quality Weather conditions were good during acquisition of the SOSN08 Brandt 3D. Sail direction during acquisition was parallel to the shelf break to reduce effects of swell noise (predominant swell direction orthogonal to shelf break) and to capture reflections from the seafloor canyons (Enclosure 1). With this acquisition direction some energy was lost due to reflections from the steep dips at the shelf break arriving beyond the 3D spread. This effect was considered to be of relatively minor significance.

Because of the presence of seafloor canyons, the SOSN08 Brandt 3D was originally planned to be processed using partial pre-stack depth migration. This technique consists of depth migration to the water bottom and time migration beneath the water bottom. This processing technique is designed to overcome problems faced by conventional time migration in the presence of rugose seafloor topography, but at much reduced time and cost relative to full pre-stack depth migration. However it was found that the partial pre-stack depth migration did not deal adequately with the rapid lateral changes in velocity in the shallow section. To achieve the required imaging quality, full pre-stack depth migration was carried out on the Brandt 3D. It is recognised that pre-stack depth migration requires significantly more processing time than pre-stack time migration. Extensions to Permit Year 5 were approved to allow this processing to be completed.

Most of the SOSN08 Brandt 3D seismic data is of good-excellent quality. Data quality is reduced beneath the shallow volcanics (Figure 6) and under the shelf break (Figure 7). Key events are generally still mappable in these areas, but with reduced confidence.

Apart from the signal attenuation beneath the shelf break and the shallow volcanics, the SOSN08 Brandt 3D seismic survey is significantly better quality than the older vintage 2D seismic data (Figure 8). This is mostly due to the improvement in imaging achieved by the pre-stack depth migration.

Play Types The key play type targeted in this permit is Cretaceous in age, with Late Cretaceous Thylacine Member, or Waarre Formation reservoirs and Belfast or Skull Creek top seals, and hydrocarbons sourced from the Eumeralla or Waarre coals, within faulted structural traps. These plays have been proven in the Otway Basin fields to the north.

Leads and Prospects Prior to acquisition of the SOSN08 Brandt 3D survey, there were four identified prospects and leads within Permit T/35P, namely Florey, Brandt, Withnall and Ormsby (Figure 2). The Brandt 3D survey was designed to image the Brandt and Florey Prospects and mature them to drillable status. Following acquisition and interpretation of the Brandt 3D survey, the amplitude over the Florey Prospect (previously expected to indicate presence of gas) does not appear to conform to structural closure, or an easily identifiable stratigraphic trap. Studies indicate that this amplitude is more likely to be related to lithology, rather than fluid content. Detailed fault mapping in the Florey area also shows that the faults no longer



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provide a clear trapping mechanism. As a result, the Florey prospect has been downgraded in the prospect and lead inventory to a high risk lead. In the Brandt area detailed fault mapping appears to divide the prospect into two separate prospects, Brandt NE and Brandt SW.

Florey

This prospect was previously interpreted on 2D data as a low-side 3-way fault closure at the Thylacine Member level. The seismic data exhibited elevated amplitude within this structure, which was expected to be a related to possible gas presence in the structure. Based on interpretation of the SOSN08 Brandt 3D seismic data, the faults which set up the trap on the 2D interpretation are clearly en echelon, and no longer provide a trapping mechanism. There is no clearly identifiable structural trap or stratigraphic trap which correlates to the extent of the amplitude anomaly. Based on other studies the observed amplitude is now expected to be related to lithology changes, rather than fluid content.

Brandt NE

Brandt NE is a faulted 4-way dip closure at the Waarre Formation. There is an amplitude anomaly over the prospect on the full-stack volume, but it does not conform to structural closure (Figure 10). Instead, the anomaly is located in the lowside of a growth fault which bounds the structure. Analysis of the gathers indicates this anomaly may be caused by coal. There is no other significant seismic anomaly over the structure, other than a mild increase in far-offset amplitude near the crest of the structure. However, this does not have a clear structural conformance and is not considered likely to relate to fluid content. Without amplitude support, this prospect is considered a high risk gas opportunity. The lack of amplitude could be due to hydrocarbon phase being oil, although this cannot be confirmed on seismic data as the acoustic properties of an oil-filled reservoir are similar to a wet reservoir. No oil discoveries have been made in the offshore Otway or Sorell Basins to date. Cape Sorell 1 intersected live oil shows, but in a shallower interval, in a separate (Strahan) sub-basin.

Brandt SW

Brandt SW is an up-thrown fault closure at the Waarre Formation. Elevated amplitudes on the far offset stack have fair conformance to depth (Enclosure 8) and may indicate a possible gas cap at the top of the structure (Figure 9). Latest, volumetric estimates of this amplitude anomaly indicate it is significantly below the minimum economic pool size. If this amplitude anomaly were hydrocarbon, it would prove the presence of a working petroleum system in this part of the basin. It is possible that an oil leg exists down-dip of this gas cap, although such can not be determined from seismic data. However, due to the depth of burial (nearly 3km of overburden), oil is likely to be difficult to recover from reservoir at this depth (reduced permeability due to burial and diagenesis) and hence a significant reservoir risk is attributed to the oil prospect.

The crestal far offset amplitude anomaly terminates, on the eastern flank (Enclosure 8), where the bounding NE-dipping faults intersect with SW-dipping faults, creating a region of complex faulting and smaller throws. If the crestal amplitude is related to gas-filled reservoir, this transfer zone may be a leak point for the structure, which would increase the seal risk for the postulated oil leg discussed above.

There are several alternative explanations of this gas cap. Since there are intrusive volcanics in the shallow section overlying this prospect, the “gas cap” amplitude anomaly could be caused by CO2 expelled from the volcanics. The amplitude anomaly could also be caused by constructive interference of thin beds at or near the top Waarre event, enhanced by thinning over the crest of the structure. Multiples are also apparent on some gathers near the Waarre Formation over Brandt SW, which may also contribute to the anomalous amplitudes.



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Conclusions The SOSN08 Brandt 3D seismic dataset is of good quality and generally of better quality than the nearby 2D surveys. The improvement in imaging due to the pre-stack depth migration justifies the extra time required for this processing method. Interpretation of the 3D survey is significantly easier than on the previous 2D dataset, largely due to this improved imaging. Pick confidence on the 3D is reduced in localised areas beneath the shelf break and under the shallow volcanics where the data quality is poor.

Interpretation of the SOSN08 Brandt 3D seismic data has found that much of the evidence previously supporting the Florey prospect has been dismissed, and, as a result, the Florey prospect has been downgraded in the prospect and lead inventory to a high risk lead. Interpretation of the 3D dataset has also provided evidence to separate the Brandt prospect into two separate prospects: Brandt NE, which has no amplitude conformance to support the prospect, and Brandt SW which has only a small “gas cap” amplitude anomaly at the crest of the structure. These prospects would be more attractive if an oil case could be proven; although at this depth the reservoir may suffer quality degradation related to depth of burial and possible diagenesis.


Figures

Permit Location Map

Figure 1. Exploration Permit T/35P Location Map.


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Data Location Map

Figure 2. Seismic Survey Location Map


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Stratigraphic Chart

Figure 3. Stratigraphic Chart


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Synthetic Seismograms

Figure 4. Synthetic seismogram at Prawn 1.

Figure 5. Synthetic seismogram at Whelk 1.


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Seismic Data Quality

Figure 6. XLN479 showing the presence of a shallow volcanic (in the Tertiary section) resulting in signal deterioration beneath.

Figure 7. XLN1049 showing the poorer imaging beneath the shelf break.


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Seismic Data Quality Comparison

SS03-21

SOSN08 Brandt 3D

SS03-21

SOSN08 Brandt 3D

Figure 8. Arbitrary traverse through Brandt 3D along path of SS03-21 for data quality comparison. Note vastly improved imaging on Brandt 3D at deeper faulted section.


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Interpreted Seismic Sections

Full stack

Far stack (35-50º)

Skull Creek / Belfast

Waarre

Eumeralla

Wangerrip

XLN1259

intrusives?

“gas cap”

Full stack

Far stack (35-50º)


Waarre

Eumeralla

Wangerrip

XLN1259

intrusives?

“gas cap”

Figure 9. XLN 1259 through Brandt SW. On the full stack volume, the amplitude dims at Waarre level over the crest of the structure. On the far stack, the amplitude brightens over the crest. This bright amplitude shows fair conformance to structural closure, thus indicating possible extent of a “gas cap”.


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Full stack

Far stack (35-50º)


Waarre

Eumeralla

Wangerrip

XLN1039

Thylacine

Full stack

Far stack (35-50º)


Waarre

Eumeralla

Wangerrip

XLN1039

Thylacine

Figure 10. XLN 1039 through Brandt NE. On the full stack volume, the amplitude is anomalously bright at Waarre level, although this unfortunately is not crestal in either time or depth, and is thought to be related to presence of coals. The slight brightening at Thylacine level is thought to be related to lithology.


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Enclosures - Maps of Key Horizons

Enclosure 1. Water bottom depth structure map.


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Enclosure 2. Wangerrip Group depth structure map.


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Enclosure 3. Near top Skull Creek depth structure map.


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Enclosure 4. Thylacine Member depth structure map.


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Enclosure 5. Waarre Formation depth structure map.


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Enclosure 6. Eumeralla Formation depth structure map.


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Enclosure 7. Thylacine Member Far Offset Stack amplitude extraction with depth contours.


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Enclosure 8. Waarre Formation normalised far offset amplitudes, with depth contours. Note termination of amplitudes on eastern flank of Brandt SW is coincident with complex faulting around transfer zone, where NE-dipping faults intersect SW-dipping faults.


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