Otway Basin

Summary

Age

 ?Jurassic - Late Cretaceous
Onshore Area in South Australia 9650 km2 (3 730 sq. miles)
Offshore Area in South Australia 50 000 km2 (19 300 sq. miles)
Depth to target zones 1000 - 4000+ m
Thickness > 9 km
Hydrocarbon shows Onshore commercial gas from Pretty Hill Formation and Windermere Sandstone member, 2000 bbl oil recovered on test from Sawpit sandstone; offshore oil shows and non-commercial gas
First commercial discovery 1987 gas (Katnook 1)
Identified reserves 76.9 PJ (72.8 x 1012 Btu) sales gas (in 5 fields)
Undiscovered resources (50% prob.) 1630 PJ (1545 x 1012 Btu) sales gas (PIRSA estimate, April 1995) - not including deep water
Production (12 months to December 2005) 5.27 PJ sales gas; 3 748 kL condensate (Katnook, Ladbroke Grove); 13 052 t saleable CO2 (Caroline)
Basin type Rift
Depositional setting Fluvial - lacustrine - marginal marine - deep water marine
Reservoirs Braided and meandering fluvial, deltaic and slope fan sandstones
Regional structure Early half graben, late shelf collapse
Seals Marine and lacustrine shales
Source rocks Early restricted lacustrine, later marginal marine to marine?
Number of wells 84 (8 development / appraisal)
Seismic line km 10 373 2D, 485 km2 onshore; 21 943 2D, 371 km2 offshore

Structural setting

The Otway Basin comprises one of a series of Jurassic to Cretaceous basins with a Tertiary cover (Gambier Basin) that occur along the southern coast of Australia, and which were formed as a result of rifting between the Antarctic and Australian plates. The northern limit is defined by outcropping early Palaeozoic metamorphic and igneous rocks (the Padthaway Ridge) and sediments may occur up to 160 km offshore. To the west, the basin passes into the Duntroon Sub-basin, and in the east continues into Victoria. Offshore it is locally bound by a Palaeozoic outer-margin high or volcanics.

Separate grabens and half grabens filled with a Jurassic to Early Cretaceous rift sequence are interpreted to exist across most of the basin but have only been penetrated onshore. The rate of rifting slowed during the Barremian to Albian but rapidly increased before the end of the Albian in the northern part of the basin. Over 4 km of late Albian to Late Cretaceous sediments occur within the Morum Sub-basin offshore, and the widespread Cenomanian unconformity is predicted not to occur towards the north.

Two major sedimentary sequences are targets for petroleum and geothermal exploration in South Australia.

(i) The Berriasian to Hauterivian sequence (Crayfish Group, early rift) is known only from the northern area, where E–W and NW–SE trending half-grabens (Robe, Penola, St Clair and Tantanoola Troughs; contain fluvial to lacustrine sediments that are proven gas reservoirs.

(ii) The Late Cretaceous sequence (Sherbrook Group) occurs as a deltaic to deep-water wedge south of the Tartwaup Hinge.

Stratigraphy

The oldest unit is the Casterton Formation, a volcanic and shale unit that occurs in some wells on the northern flank of the Penola Trough and in Victoria, and may occur in the undrilled deeper parts of the Robe and Penola Troughs. The oldest sequence known in South Australia is the Crayfish Group .which fills half grabens that can be identified in the depth to basement structure map. The first unit is the Pretty Hill Formation, a braided fluvial sandstone that occurs in the deepest parts of the troughs. This is followed by fluviolacustrine shale and siltstone (Laira Formation), which in turn is overlain by the braided fluvial Katnook Sandstone. The Katnook Sandstone thickens to the NW, essentially as a sandy facies of the Laira Formation. On the extreme northern margin, both the Katnook Sandstone and Pretty Hill Formation are absent, and the Crayfish Group comprises only the shaly Laira Formation. The Crayfish Group is unconformably overlain by the Eumeralla Formation, which is a fluvial siltstone – shale sequence with some minor coal and meandering fluvial sandstone units. The Windermere Sandstone is a regionally extensive transgressive sand unit which overlays the Crayfish unconformity and thickens within the Early Cretaceous troughs. The Eumeralla Formation comprises extensive fluviolacustrine volcanogenic sediment deposited during the sag phase of the basin.

The Late Cretaceous Sherbrook Group overlies the Otway Supergroup as a deltaic wedge that rapidly thickens to the south offshore, accounting for most of the sediment in the Morum Sub-basin. In the northern part of the basin, where the group is thin, it comprises a coarse sandstone that represents a condensed equivalent of the Copa, Waarre, Flaxman, Belfast, Paaratte and Timboon units found to the south. From Belfast Mudstone to Timboon Sandstone the sequence represents a prograding delta, with early marine influence and deep water submarine slope-fans along the outer margin. A regional section across the Otway Basin is shown in structural elements map and sediment thickness if shown in the sediment thickness map.

Facies

The source for the gas in the Katnook and Ladbroke Grove fields is either an unpenetrated shale beneath the Pretty Hill Formation (?Casterton Formation equivalent), or possibly the Laira Formation.  The Eumeralla Formation is not deep enough in the northern portion of the basin to be a source and the high density of E–W faults may impede migration and groundwater flow. However to the south, where thick Sherbrook sediments occur, the Eumeralla could be a source for Waarre and Flaxman targets.  The Belfast Mudstone is a relatively poor gas-prone source rock, and is only marginally mature (VR = 0.6%), even in the deepest wells drilled to date. The CO2 in Caroline Field has a volcanic source, assumed to be from the Holocene Mt Gambier volcanic chain, which trends NW through the Tantanoola Trough. Carbon dioxide from a magmatic source has also been noted in Ladbroke Grove Field and Kalangadoo 1.

The Pretty Hill Formation has porosities in excess of 25%, permeabilities in excess of 1000 mD, and has flowed over 451 000 m3 (16.01 mmcf) gas per day on test in the Katnook Field. Reservoir permeability does not appear to decrease with increasing depth of burial, although only the upper part of the Pretty Hill Formation is generally of good quality. Reservoir quality in the Windermere Sandstone Member is excellent, although the low relief structures can make it difficult to avoid water production problems.  Victorian production and limited South Australian log data indicate that reservoir quality is excellent for the Waarre, Flaxman and intra-Belfast units.

A separate map of the Wells and seismic lines is available.

More information on the Otway Basin

Key reference
Boult, PJ and Hibburt, JE (Eds) 2002. The petroleum geology of South Australia. Volume 1: Otway Basin. 2nd edn. South Australia. Department of Primary Industries and Resources. Petroleum Geology of South Australia Series.