Otway Basin

Note: refer to the Acreage Release basins page for updated information on the Otway Basin.

Basin Details and Geological Overview

The Otway Basin is a north-west to south-east striking, divergent margin, rift and drift basin. It is approximately 500km long from Cape Jaffa in South Australia to north-west Tasmania and forms part of the 4 000km long Jurassic-Cretaceous Australian Southern Rift System.

Geological Summary

The Late Jurassic-Cenozoic Otway Basin is a large, northwest trending on/offshore basin on the southern Australian passive margin. Exploration is mature onshore and immature offshore, with >200 wells in South Australia, Victoria and Tasmania. Commercial gas discoveries include industrial grade CO₂. No commercial oil discoveries have been identified.

The basin formed by multi-stage rift-sag and inversion phases. Late Jurassic to Early Cretaceous rifting resulted in the east-west trending Inner Otway Basin. Late Cretaceous rifting, culminating in continental breakup in the Maastrichtian, produced northwest-southeast trending depocentres beneath the outer shelf and slope. Multiple phases of compression in the Cretaceous-Recent resulted in inversion and wrenching of pre-existing structures.

The basin contains five major depocentres, the mainly onshore Inner Otway Basin, the offshore Morum, Nelson and Hunter Sub-basins and eastern Torquay Sub-basin. The Latest Jurassic-Early Cretaceous Otway Supergroup comprises up to 8km of continental and fluvio-lacustrine sediments that accumulated in grabens and half-grabens of the first rifting event. Coastal-plain, deltaic and marine sediments of the Late Cretaceous Sherbrook Group are up to 5km thick. The Paleocene-middle Eocene Wangerrip Group sediments were deposited in coastal plain, deltaic and inner shelf settings and are separated from the open marine, mixed carbonates/siliciclastics of the Eocene-Miocene Nirrandra and Heytesbury groups, by a major unconformity.

The main exploration targets in the Otway Basin are the Waarre Sandstone at the base of the Sherbrook Group and sandstones of the Pretty Hill Formation and Katnook Sandstone/Windermere Sandstone Member in the Early Cretaceous section. The main source rocks occur in the Early Cretaceous section. Regional and intraformational seals exist in the Pretty Hill, Laira, Eumeralla and Flaxman formations, the Belfast, Skull Creek and Pember mudstones and mudstones and marls of the Wangerrip, Nirrandra and Heytesbury groups. Play types include faulted anticlines, large anticlinal features and tilted fault blocks.

Stratigraphy

In lithostratigraphic terms, the basin fill of the Otway Basin is divided into five major sedimentary successions: The Otway Supergroup and the Sherbrook, Wangerrip, Nirranda and Heytesbury groups. In a recent, regional basin analysis study of the entire Otway Basin, Geoscience Australia authors have proposed a new sequence stratigraphic framework for the Otway Basin (Krassay et al. 2004). In the new tectonostratigraphic framework, seven major basin phases and their eight component supersequences (2^nd order sequences) are recognised as follows;

1. Tithonian?-Barremian rifting that resulted in the deposition of thick continental and fluvio-lacustrine sediments of the Crayfish Supersequence,
2. Aptian-Albian post-rift, volcaniclastic, fluvio-lacustrine deposition of the Eumeralla Supersequence,
3. mid-Cretaceous compression and inversion,
4. Late Cretaceous rifting and associated coastal-plain, deltaic and marine deposition of the Shipwreck and Sherbrook Supersequences,
5. latest Maastrichtian to Middle Eocene basin reorganisation and early thermal subsidence of the Wangerrip Supersequence (coastal plain, deltaic and shallow marine deposition),
6. local inversion and thermal subsidence of the Nirranda Supersequence (Middle Eocene to Early Oligocene near-shore to offshore, mixed clastic and carbonate deposition) followed by thermal subsidence and progressive compression of the Heytesbury Supersequence (Late Oligocene to Late Miocene, progradational open-marine carbonate deposition) leading to Late Miocene uplift and erosion, and,
7. Plio-Pleistocene deposition of the Whalers Bluff Supersequence (mixed siliciclastic-carbonate succession).

The main exploration targets in the Otway Basin are the Waarre Sandstone at the base of the Sherbrook Group (Shipwreck Supersequence), and sandstones of the Pretty Hill Formation (Crayfish Supersequence) and Katnook Sandstone/Windermere Sandstone Member in the Early Cretaceous section. The main source rocks are coals and coaly shales of the Aptian to Albian aged Eumeralla Formation (Eumeralla Supersequence). Regional and intraformational seals exist in the Pretty Hill, Laira, Eumeralla and Flaxman formations, the Belfast, Skull Creek and Pember mudstones, and mudstones and marls of the Wangerrip, Nirranda and Heytesbury groups. Play types include faulted anticlines, large anticlinal features and tilted fault blocks.

Petroleum Exploration

Petroleum systems

Hydrocarbons sourced from basins along the southern margin of Australia have been assigned to the Austral Petroleum Supersystem (Bradshaw, 1993; Summons et al., 1998). Within this supersystem, three petroleum sub-systems related to the Otway Basin and other southeastern margin basins have been recognised (Edwards et al., 1999). Each sub-system correlates geochemically to distinct oil families and related source rock facies with differences primarily related to the type of depositional environment. The three sub-systems are:

• Austral 1 petroleum system - Late Jurassic to earliest Cretaceous fluvio-lacustrine shales. The Austral 1 petroleum system is recognised as the source for most hydrocarbons in the western, onshore Otway Basin.
• Austral 2 petroleum system - Early Cretaceous fluvial and coaly facies. The Austral 2 system is recognised as the source for the majority of gas and minor oil discoveries made in the Otway Basin, aside from the Penola Trough.
• Austral 3 petroleum system - Late Cretaceous to Earliest Tertiary fluvio-deltaic facies. Otway Basin hydrocarbons sourced from the Austral 3 system are uncommon to date.

Oil Families - Otway Basin

Six Austral Petroleum System oil families (A1F1, A1F2, A1F3, A1F4, A2 and A3) have been previously identified in the Otway Basin (Edwards et al., 1999). However, a recent integrated geochemical study by Boreham et al. (2004) shows that the A1F1 oil family (e.g., bitumens from Crayfish A1 and Zema 1), originally interpreted as being sourced from saline lacustrine source rocks, and the A3 oil family (e.g., Wilson 1), originally interpreted as being sourced from marine source rocks, are most likely drilling contaminants, rather than true oil families. Therefore, indigenous, uncontaminated oils in the Otway Basin are interpreted as belonging to four families (A1F2, A1F3, A1F4, and A2) within the Austral 1 and Austral 2 Petroleum Systems (Boreham et al., 2004).

There are strong stratigraphic and geographic controls on oil families within the Otway Basin. Oils in the west and onshore belong to Austral 1 families and were sourced from Late Jurassic — Early Cretaceous, syn-rift, dominantly fluvio-lacustrine organic facies. Oils in the east belong to an Austral 2 family derived from Early Cretaceous, post-rift coaly organic facies. Oils in the central part of the basin have a mixed source affinity, but are predominantly from Eumeralla Supersequence sources (Boreham et al., 2004).

Gas Families - Otway Basin

Natural gases in the Otway Basin show clear geochemical differentiation between those from the western and eastern parts of the basin. The western gases (e.g., Jacaranda Ridge 1, Katnook 2, Ladbroke Grove 2, Redman 1 and Troas 1) belong to the Late Jurassic — Early Cretaceous, Crayfish Supersequence-sourced Austral 1 Petroleum System (Boreham et al., 2004). The eastern gases (e.g., Thylacine 1, Geographe 1, La Bella 1, Minerva 3, Casino 1, Casino 2) belong to the Aptian-Albian, Eumeralla Supersequence-sourced Austral 2 Petroleum System (Boreham et al., 2004). Gases from the central Otway Basin (e.g., Port Fairy 1, Caroline 1) are the products of mixing from both sources within local depocentres.

Multiple charge histories in the natural gas reservoirs are evident from the widespread influx of overmature, dry gas to an initially in-place wet gas, particularly in the western Otway Basin. Both gas charges have the potential to displace and/or alter the composition of any reservoired oil. In the east, however, most natural gases (e.g., Geographe 1, Thylacine 1, La Bella 1, Lavers 1) are interpreted as the result of a single gas charge (Boreham et al., 2004).

Otway Basin natural gases show a strong geochemical association with their respective oils, suggesting that both are generated from the same source. Also, the gases and oils and their effective source rocks have a strong stratigraphic and geographic relationship, indicating mainly short- to medium-range migration distances from source to trap (Boreham et al., 2004).

Key References