Geological Survey

Offshore Clarence-Moreton Basin - Geological Overview

The onshore Clarence-Moreton Basin is a major Late Triassic to Jurassic/Early Cretaceous depocentre located in the extreme northeastern portion of New South Wales, and southern Queensland. Its eastern extent lies close to, and forms part of, the present-day coastline. The basin's structural configuration, depositional trends, and outcrop patterns imply little likelihood for significant offshore extensions across the adjacent continental shelf. These perceptions, together with the presence of a narrow margin, thought to be dominated by a relatively thin Neogene to Recent sediment cover, inhibited any serious offshore petroleum exploration following the recording of an initial offshore seismic survey in 1970.

No petroleum wells have been drilled in the offshore Clarence-Moreton Basin. In this context the offshore basin is regarded as a frontier area for petroleum exploration.

Approximately 700km (482.89 statute miles) of seismic data recorded by Teledyne Inc for Murphy Oil in 1969 across the shelf, adjacent to the basin, between Woolgoolga and Evans Head has been reprocessed as part of an on-going evaluation of the petroleum potential of New South Wales. The data set comprises 12 east-west dip traverses and 5 strike lines, recorded as 24-fold coverage.

The offshore Clarence-Moreton Basin is a poorly explored portion of the east Australian continental margin. The extent of this offshore continuation has yet to be established, but it incorporates the shallow waters of the shelf and possibly limited portions of the continental slope. In this area, the shelf is up to 60 km wide and water depths are typically less than 120 m.

Background

The Clarence-Moreton Basin is a major Triassic to Jurassic/Early Cretaceous depocentre located in the extreme northeastern portion of New South Wales and southern Queensland. As mapped its eastern extent lies close to, and forms a minor part of, the present-day coastline. Outcrop patterns, depositional trends and the interpretation of onshore seismic coverage imply little significant offshore extension of the Clarence-Moreton Basin across the adjacent continental shelf. Numerous seismic surveys have been conducted across the onshore Clarence-Moreton Basin since the early 1960's. However, only two seismic surveys have been acquired in the adjacent offshore area. One survey was acquired by the then Bureau of Mineral Resources (now Australian Geological Survey Organisation) as part of its Continental Margin program during 1970 and 1971. Seismic data from this survey has never been processed nor an interpretation of ship's monitors published. The other survey was the reconnaissance Yamba-Evans Head Marine Seismic Survey recorded by Teledyne Inc. for Murphy Oil in 1969 across the shallow (generally less than 120 m) continental shelf in an area roughly adjacent to the segment of coastline located between the townships of Woolgoolga and Evans Head. The main seismic grid of the Yamba-Evans Head Marine Seismic Survey comprises 12 east-west (dip oriented), and 5 north-south (strike), traverses. A further survey was conducted in 1970-71 to infill a small area in the northeast of the previous survey.

There has been strong interest in petroleum exploration in the Clarence-Moreton Basin since the beginning of the Departments Discovery 2000 initiative. Therefore it was decided to review the results of the Yamba-Evans Head Marine Seismic Survey with the possibility of a future acreage release. The original interpretation of the survey suggested the presence of a northeast trending fault bound trough (the Yamba Trough) containing up to 5000 metres of sediments. The Department reprocessed the approximately 700 line kilometres of the Yamba-Evans Head Marine Seismic Survey data. Some acquisition parameters had to be assumed because various navigational and recording specifications logged during the survey could not be located with the original field tapes. Nevertheless, the improvement enabled a more reliable interpretation of the offshore Geology. Although the original data had been recorded using then state of the art digital technology, processing and presentation techniques have progressed a long way since 1969 and previous experience has shown that many older datasets can be greatly improved using modern reprocessing and display techniques.

Seismic Interpretation

Four discrete sedimentary packages, overlying basement, were identified from the reprocessed data. In the absence of any direct offshore well control, the age and nature of the sedimentary units comprising these packages must be inferred from adjacent onshore geology and knowledge of the evolution of this portion of the eastern Australian margin. The shallowest package, lies above a strong, easterly dipping unconformity surface and corresponds to a relatively thin veneer of Neogene (?Miocene - Recent) sediment cover. This package would include Recent sediments derived from contemporary river systems such as the Clarence River.

Underlying this is a package dominated by easterly directed buildout, or progradation. This package includes initial deposition, following the subsidence of the margin below sea-level after breakup of eastern Australia and dispersal of the Lord Howe Rise in the Late Cretaceous to Early Tertiary. Beneath this is a package of well stratified sediments distinguished by an absence of strong easterly progradation. With conspicuous faulting this package is bound at both its top and base by erosional truncation. Its thickness and distribution imply deposition during a period predating formation of the present margin. Reflectors within the package can be correlated across the inner shelf and immediately adjacent to that segment of the coastline where onshore Clarence-Moreton Basin sediments are mapped or inferred. This package corresponds to the offshore extension of the onshore Clarence-Moreton Basin sediments, most likely sediments of the Marburg Subgroup. It includes reflections previously interpreted as comprising part of the Yamba Graben. The graben itself is now considered an artifact of variations in the original data quality due to corresponding variations in seismic penetration and depth of imaging (i.e. the Yamba Graben does not exist as a structural entity).

The deepest package comprises a thick (2000m or more), well stratified, sometimes highly faulted sequence which extends across much of the outer shelf. It is distinguished from underlying basement by the greater coherency of its internal reflectors. Based on its relationship with the overlying package, sediments of this package are interpreted to most likely comprise an early stage of Clarence-Moreton Basin deposition, a �syn-rift� stage equivalent, analogous to those constituting a southern extension of the Esk Trough which is inferred to be present beneath the onshore Clarence-Moreton Basin. These syn-rift sediments would incorporate offshore correlatives of the onshore Evans Head and Ipswich Coal Measure sequences as well as Chillingham Volcanics of Middle and perhaps Early Triassic age.

Underlying this package, and skirting the coastline along the inner shelf adjacent to the coastline south of Brooms Head, is a sequence of reflectors exihibiting very limited lateral coherency, steep dips, folding and faulting. The top of the sequence is irregular and locally protrudes above the surface where it is manifest as outcrops of the Solitary Island Group. Surface mapping confirm that these islands comprise offshore extensions the Coffs Harbour Beds which are extensively deformed over a wide area of the adjacent onshore Coffs Harbour Block. They comprise metamorphosed mudstones, argillites and sandstones which form effective basement.

Summary

In summary, the newly reprocessed seismic data now raises the likelihood that a separate sequence, containing a substantial thickness of inferred Mesozoic aged sediments, and overlain by up to 1000m of post-breakup sediment cover, extends across the shelf adjacent to the onshore Clarence-Moreton Basin. During the Early Mesozoic, syn-rift development, analogous to that occurring onshore in the Esk Trough, may have taken place across what is now the continental shelf. The outboard edge of this development may have coincided to a zone of continental rupture during subsequent breakup. Based on onshore analogs, offshore inferred syn-rift lithologies are likely to include source rock (coals) and reservoir (sandstone) sequences. From onshore trends it is anticipated that potential source rocks would be adequately mature for the generation of oil and gas. Whether this syn-rift sequence was physically connected to that beneath the onshore Clarence-Moreton Basin is presently unknown.

The offshore portion of the Clarence-Moreton Basin is now thought to be considerablely larger and more prospective as a result of reprocessing and reinterpretation of the Yamba-Evans Head Marine Seismic Survey. Encouraged by these findings the Department is currently engaged in transcribing the analog data recorded during the Continental Margin survey of the then BMR in 1970-1971. It is proposed that this data will also be processed and an interpretation incorporated into the preliminary Yamba-Evans Head Marine Seismic Survey interpretation over the coming months, with the potential for an acreage release in the future.

Products & Publications

Exploration Data - Clarence-Moreton Basin (Onshore & Offshore)

Exploration data related to the offshore Clarence-Moreton Basin can be obtained through the online services.

Stratigraphic Table

Offshore Clarence-Moreton Basin Stratigraphic Table To Download (Image)

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Details of the onshore Clarence-Moreton Basin Geology

For further information, please contact petroleum.geoscience@industry.nsw.gov.au