EXPLORATION Study upgrades Australia's Otway basin outlook

Sundar Sarma
Geophysical consultant
Melbourne

Complex faulting and poor quality seismic data have hindered offshore Otway basin exploration. The Victoria Department of Agriculture, Energy and Minerals chose Sarma & Associates Pty. Ltd. in July 1994 to conduct a regional seismic interpretation over the offshore Otway basin.1

Sarma, using a Schlumberger Charisma seismic workstation, interpreted 5,000 km of high quality seismic data from BHP Petroleum's 1991 seismic survey and from the 1980-81 surveys recently reprocessed by Geco-Prakla and BHP.

The study revealed that a large number of structural and stratigraphic plays of significant size are present in less than 200 m of water.

All eight wells drilled prior to 1987 were located either on the downthrown side of major faults or on the flank of faulted structural closures at the main objective level. At this level the seismic events were masked by noise and multiples in the old seismic data.

The center of the study area (Fig. 1 [58366 bytes]) is 300 km southwest of Melbourne, Australia's second largest city. The study covered vacant area V95-01 and permits VIC/P30, VIC/P31, and VIC/P35 (Fig. 2 [75323 bytes]).

The Otway basin is almost equally divided between South Australia and Victoria states (Fig. 1 [58366 bytes]). The Victoria share of the basin covers about 18,500 sq km onshore and 26,000 sq km offshore.

In the onshore areas, North Paaratte and Iona producing gas fields are present, and a number of wells had significant oil shows. Otway offshore is still in the very early stage of exploration. Only 15 exploration wells have been drilled offshore, eight of which were drilled between 1967-86.

The offshore Otway basin received exploration impetus from two BHP discoveries in 1993-Minerva (gas/condensate, commercial) and La Bella (gas, subcommercial) in the east of the study area.

Geco-Prakla in 1993 took the initiative to reprocess 2,500 km of seismic data from the 1980 and 1981 regional seismic surveys as a nonexclusive data package. Reprocessing considerably improved data quality, sharpening fault planes and removing multiples and noise at depth.

Geology, horizons

Rifting along Australia's southern margin started in late Jurassic to very early Cretaceous, forming a series of basins including the Otway.

In existing literature the continental breakup of Australia from Antarctica is inferred to have progressed eastward from within the Great Australian bight in the west, through the Otway basin, eventually jumping south along the Sorrel-Tasman fracture zone, leaving Tasmania attached to the Australian craton. The breakup possibly did not reach the Otway/West Tasmania region in the east (Fig. 1 [58366 bytes]) until mid-Eocene (44.5 Ma).

The Otway basin contains a sequence of Mesozoic to Tertiary sediments more than 7 km thick overlying a Paleozoic igneous and metamorphic basement. The stratigraphic table for the Otway basin (Fig. 3 [127030 bytes]) was recently compiled by the Department of Agriculture, Energy and Minerals2 for the onshore part of the basin.

In the offshore permits VIC/P30 and VIC/P31, BHP Petroleum3 has subdivided the Sherbrook Group into two sequences, the Cenomanian to early Santonian sequence is informally named the Shipwreck Group, and the late Santonian to Maastrichtian sequence has been named as the Sherbrook Group.

Of offshore Otway's main structural elements (Fig. 2 [75323 bytes]), the Voluta trough, Mussel platform, Normanby terrace, Sorell fault complex, and Shipwreck trough are the major features dominant during Cretaceous These elements were better defined as a result of the study. Normanby terrace and Shipwreck trough are new names developed during the present study.

Four major sequence boundaries-the base of the Heytesbury Group, top of the Wangerrip Group, top of the Sherbrook Group, and top of the Eumeralla formation-were mapped over the entire area. The wireline log characteristic associated with the various geological formations at the well Normanby 1 is shown in Fig. 4 [68662 bytes]. The interpreted seismic line OE80A-1056 (Fig. 5 [176283 bytes]) extends from the Minerva structure in the north to the La Bella structure in the south. The locations of seismic lines OE80A-1056 and OP80-09 are shown in Fig. 2 [75323 bytes].

The extensive faulting observed at the top of the Eumeralla formation map (Fig. 6 [89427 bytes]) marks a later phase of the rifting episode. The Eumeralla formation sediments are believed to be the latest synrift sediments before the marine incursion took place in the Otway basin. The Eumeralla formation is a thick lacustrine/fluvial sequence and consists of claystones, siltstones, shales, coals, and minor sands. The base of the Eumeralla formation is seen only on a few seismic lines in the far northern Mussel platform, near the coast.

The top Sherbrook unconformity (Fig. 7 [87539 bytes]) is an erosional surface and is further cut by Tertiary to recent channels in the deepwater regions to the southwest close to the present day shelf edge. Seismic sections indicate low amplitude folding due to Late Tertiary mild compression and uplift. The faulting shown on Fig. 7 [87539 bytes] includes rejuvenated top Eumeralla faults as well as syndepositional relatively high angle Sherbrook Group faults. The faulting within the Sherbrook Group is minimal in the shallow eastern half and extensive in the deeper western half.

Late Cretaceous movements along the Mussel fault system and along the Sorell fault complex significantly controlled Sherbrook Group deposition. The Sherbrook Group time interval indicates an almost east-west trending very thick depocenter (2.7 sec) known as Voluta trough located south of the Mussel platform and an almost north-south oriented moderately thick depocenter (1.5 sec) shown in (Fig. 2 [75323 bytes]) as Shipwreck trough located west of the Sorell fault complex.

The Wangerrip Group sediments grade from shallow marine at the base to continental at the top. Due to the sediment starvation and rapid rise in sea level, the Wangerrip Group sediments onlap onto the top Sherbrook unconformity at the southern edge of the study area. Following a rapid marine transgression, the Wangerrip Group prograded and was deposited as a regressive sequence. Channeling within the sequence is common. Little faulting took place during Wangerrip Group time. The depocenter of the Wangerrip Group lies in the northwestern end of the Mussel fault system and shifted to the north of the Voluta trough, depocenter of the Sherbrook Group.

The Nirranda Group represents a large marine transgression and unconformably overlies the Wangerrip Group. Tectonic activity increased during earliest Oligocene. Mild uplift and associated fall in sea level created an erosional unconformity on which the Heytesbury Group was deposited. The Base Heytesbury horizon shows little faulting. The faults are mostly earlier faults rejuvenated later.

The sediments of the Heytesbury Group were deposited during the most extensive marine transgression within the Otway basin. Late Miocene to Pliocene basalts and Pleistocene to Holocene surficial deposits unconformably overlie the Heytesbury Group. The volcanic activity was more common in the eastern part of the study area.

Source

The Eumeralla formation is the major source rock in the Otway basin. It is a nonmarine sequence and is widely distributed throughout the Otway basin.

Source rock studies by Mehin and Link4 in the Otway basin's onshore wells indicate that the maturity of the Eumeralla formation increases towards the coast and extending the maturity trend offshore the Eumeralla formation is predicted to be within the oil generating window at the present time in the Voluta trough region.

The Eumeralla formation has not been penetrated in any of the four offshore wells drilled in the western area within the Voluta trough. The Eumeralla formation may be several kilometers thick in the Voluta trough and may contain additional sedimentary sections with different burial and thermal histories to that encountered in the onshore wells. In the eastern area the Eumeralla formation is believed to be the source for the gas discoveries in the Minerva and La Bella structures.

The Belfast mudstone of the Sherbrook Group and the Pember mudstone of the Wangerrip Group may also have potential for sourcing hydrocarbons in the offshore Otway basin.

Reservoirs

The study area contains several potential reservoir units within the Wangerrip Group and within the Sherbrook Group (Fig. 4 [68662 bytes]).

The major reservoir units within the Wangerrip Group are the Dilwyn and the Pebble Point formations. Sands within Dilwyn have porosities of up to 40%. Porosities of the Pebble Point formation sands show much variation from 10-30%.

The major reservoir units within the Sherbrook Group are the Waarre formation and the Timboon sand member. Sands are also present within the Paaratte formation. The Timboon sands and the intra-Paaratte sands have porosities ranging from 25-30%.

Based on studies carried out in the onshore wells, the Waarre formation was divided into four units A, B, C, and D, unit D being the uppermost unit,5-6 with generally poor reservoir quality.

Unit C has the best reservoir quality sands with up to 30% porosities and up to 2 darcies of permeability. Unit B contains interbedded fine to medium sands, silts, mudstones, and wispy coals. The sands have fair to good reservoir quality. The silts and mudstones may form an effective seal for the reservoir beneath. Unit A has fair to good reservoir quality sands, with up to 20% porosity and up to 100 md of permeability.

The complete Waarre formation has not been tested in any of the offshore wells drilled in the Voluta trough.

Seal

The Belfast mudstone (Fig. 4 [68662 bytes]) constitutes the major regional seal for the Waarre formation sands.

Additional sealing potential is provided by the shales of the Flaxman formation, which immediately overlies the Waarre formation. The Pember mudstone provides the regional seal for Pebble Point formation sands. A basal siltstone unit of the Pebble Point formation provides seal for Timboon sands.

Dilwyn formation sands are sealed by Nirranda Group marls and claystones. The Nirranda Group's sealing capability is more effective if the lower sequence, the Mepunga formation, is absent.

Leads and play types

The main offshore Otway exploration targets are the sands of the Waarre formation, which are found at the base of the Sherbrook Group sealed by the Belfast mudstone.

The Waarre formation directly overlies the unconformity surface at the top of the Eumeralla formation. A large number of fault-closures are present at this unconformity surface. This faulting also facilitated the migration of hydrocarbons into the Waarre sandstone reservoirs from the major source rocks present within the Eumeralla formation.

The reprocessed data and the seismic data from the 1991 regional survey offered improved fault definition and seismic event resolution at depth, especially at the Top Eumeralla unconformity surface. These data clearly indicate that many of the syndepositional Sherbrook Group faults die out within the Belfast mudstone and do not intersect the Waarre formation. Similarly many of the older faults at the Top Eumeralla unconformity surface that were rejuvenated later do not extend far into the Sherbrook Group and die out within the Belfast mudstone.

On 1982 and older seismic data the seismic events at the Top Eumeralla unconformity and below were masked by multiples generated by high amplitude shallow seismic events. No well drilled in the vacant area V95-O1 lies within structural closure on the top Eumeralla formation time structure map.

Seismic line OP80-09a (Fig. 8 [163382 bytes]) shows that Normanby 1 was drilled on the downthrown side of a major fault and left the huge Normanby structure to the north untested in less than 50 m of water. It has up to 100 sq km in areal closure and 450 m (0.3 sec TWT) of vertical closure at the top of the Eumeralla formation (Fig. 9 [45159 bytes]).

In the vacant area V95-01, more than 10 faulted closures have been delineated which have the Waarre formation sands as the main objective. The Belfast mudstone forms a regional seal for all of these structures. The Flaxman formation may provide additional vertical seal.

Sandstones within these fault closures are expected to be laterally sealed by the juxtaposition of the downfaulted Belfast mudstone. The mapping of the top of the Belfast mudstone would be essential in order to verify the lateral sealing potential of these faulted closures.

A large number of closures are also present on the downthrown side of faults. In such fault blocks, the lateral seal to the Waarre formation sands across the fault could be provided by the shales, mudstone, and siltstones of the Eumeralla formation. However the characteristics of the Waarre formation sand in the downthrown side of large faults is difficult to predict. It is possible that turbidite sand deposits may be present within the Waarre formation on the downthrown side of major faults showing large vertical displacements. Good quality seismic data with high frequency content are required to identify such subtle geological features.

Intra-Paaratte sand plays are high risk plays. They depend on local intraformational vertical seal. Lateral seals across these faults are difficult to establish.

The Top Sherbrook unconformity is a significant erosional surface in the western region. The Timboon sand member, which underlies this unconformity, may be stratigraphically sealed by the Pember mudstone above in areas where the Pebble Point formation is absent. The Pebble Point formation is missing according to well completion reports at Discovery Bay 1 and Normanby 1.

Reflectors within or at the base of the Timboon sand member (Fig. 4 [68662 bytes]) would need to be mapped for delineating such plays. The Timboon sands may also be laterally sealed by the Pember mudstone across the untested Normanby fault block.

The Top Sherbrook horizon (Fig. 7 [87539 bytes]) shows another structural closure in the southwestern corner of the mapped area that has been generated by channel erosion. On this closure, the Timboon sands may be sealed by both the Pember mudstone and the Narrawaturk marl.

The paleoshelf edge at the top Wangerrip Group time was close to the Mussel fault system. Therefore, possibilities exist for outer shelf fan deposits within the Wangerrip Group to be present in the Voluta trough region. Similarly during the Oligocene lowstand period, deposition of recycled sands as turbidite or deep marine fans close to the shelf margin may generate stratigraphic plays within Tertiary. Additional well control and detailed mapping of individual sequence boundaries are required for delineating this type of stratigraphic play types.

Extensive channeling of the Tertiary sequence is common in the western part of the study area. These channels have been active at various times extending from the Eocene to the recent days. Possibilities therefore exist for plays within the Tertiary sequences generated by these channel activities. The channel fills include both high velocity as well as low velocity materials. Shallow Tertiary plays are dependent on long distance lateral and vertical migration paths for sourcing from the Eumeralla formation.

Conclusions

The Otway basin is complexly faulted. Extensive lateral velocity variations generated by the Tertiary to recent channels add additional difficulties in delineating hydrocarbon plays.

BHP's recent success in the eastern part of the study area is largely due to the availability of high quality recent seismic data. The Otway basin contains mature source rocks as well as reservoirs and seal. The study has revealed the potential for the presence of a large number of structural and stratigraphic plays of significant size, many of which are in less than 200 m of water.

In spite of significant oil shows encountered in various onshore wells, the offshore Otway basin is currently considered to be essentially a gas prone region. The present exploration stage may be compared to the early 1970s in the Northwest Shelf regions of Australia, when only gas discoveries were made.

The availability of good quality seismic data coupled with abundant subsurface data has enabled the industry to discover more and more oil fields since 1980s in the Northwest Shelf.

The results of the complete study are available as VIMP Report 21 from the petroleum unit of the Department of Agriculture, Energy and Minerals, 115 Victoria Parade, Fitzroy, Australia 3065. The interpreted seismic, open file seismic, and well data are also available from the department.

Acknowledgment

The author specifically thanks the Victoria Department of Agriculture, Energy and Minerals for permission to publish this article and providing access to computer facilities and services.

References

1. Sarma, S., Seismic interpretation of the offshore Otway basin, Victoria, Victorian Initiative for Minerals and Petroleum Report 21, Department of Agriculture, Energy and Minerals, 1995.

2. Geological Survey of Victoria, The stratigraphy, structure, geophysics, and hydrocarbon potential of the eastern Otway basin, GSV Report 103, 1995.

3. Luxton, C.W., Horan, S.T., Pickavance, D.L., and Durham, M.S., The La Bella and Minerva gas discoveries, offshore Otway basin, APEA Journal, Vol. 35, No. 1, 1995, pp 405-417.

4. Mehin, K., and Link, A.G., Early Cretaceous source rocks of the Victorian onshore Otway basin, Victorian Initiative for Minerals and Petroleum Report 22, Department of Agriculture, Energy and Minerals, 1995.

5. Buffin, A. J., Waarre sandstone development within the Port Campbell embayment. APEA Journal, Vol. 29, No. 1, 1989, pp 299-311.

6. Mehin, K., and Link, A.G., Source, migration, and entrapment of hydrocarbons and carbon dioxide in the Otway basin, Victoria, APEA Journal, Vol. 34, No. 1, 1994, pp 439-459.

The Author