Ancient sedimentary basin potential

Petroleum potential of ancient sedimentary basins
D.I. Gravestock, Principal Geologist, Petroleum Division PIRSA.

The basins under investigation are the Officer (Neoproterozoic to Devonian, western SA), Warburton (Cambro-Ordovician, northeastern SA), and the Arrowie and Stansbury Basins (Cambrian on Neoproterozoic) in the Adelaide fold belt. Funding through the South Australian Exploration Initiative has enabled some key issues to be addressed, namely hydrocarbon source richness and maturity, the timing of trap formation and reservoir quality of these sedimentary rocks.

Reservoir studies were conducted in-house while other research was carried out in collaboration with the University of Adelaide, ACS Laboratories, Amdel Petroleum Services, Geotrack International and the Palaeontological Institute of the Russian Academy of Sciences (PIN). Some highlights of this work are listed below.

Officer Basin

The Officer Basin has received the closest attention because PIRSA rates its petroleum prospectivity as high. The eastern region of the basin is now under licence.

  • Structural style is compressional, with major thrust faulting in the Petermann Ranges Orogeny (540–560 Ma) as well as the Alice Springs Orogeny (300–360 Ma). The timing of trap formation was favourable for the generation and migration of Neoproterozoic and Cambrian oil.
  • The high organic productivity of Ordovician source rocks is a global phenomenon related to the abundance of the cyanobacterium Gloeocapsomorpha prisca (Michaelsen et al., 1995). This organism has now been found in the Early Cambrian Ouldburra Formation in the Tallaringa Trough. There, the source rocks contain Type II kerogen with Hydrogen Indices in the range 360–520.
  • Sufficient methylphenanthrene index measurements have now been obtained to produce regional maps of maturity trends in the Officer Basin for the first time.
  • Apatite fission track data show that thermal events associated with the Alice Springs Orogeny have overprinted earlier thermal histories, even in wells 200 km south of the Musgrave Block. Heating to 90ºC had also occurred prior to cooling during the Cretaceous, suggesting a greater thickness of Permo-Triassic cover over the region.
  • The porosity/permeability characteristics of seven Neoproterozoic-Cambrian sandstone reservoirs up to 300 m in thickness have been documented and related with some success to wireline logs. Porous and permeable dolomite reservoirs in the Early Cambrian Ouldburra Formation have also been studied. With the exception of the low permeability Tarlina Sandstone, the other units average 15 per cent porosity and display measured permeabilities in the 10 millidarcy to darcy range.

Stansbury Basin

The majority of geological and geophysical research in the Stansbury Basin has been carried out by Canyon (Australia) Pty Ltd. PIRSA initiatives relate mainly to sampling for vitrinite reflectance measurements and apatite fission track analyses. Work with PIN is also in progress. Some highlights include:

  • Biostratigraphic documentation of the Early Cambrian-Middle Cambrian boundary at the base of the Coobowie Limestone.
  • Detailed biostratigraphy of much of the Early Cambrian succession using small shelly fossils.
  • Recognition of a major sequence boundary and possible sandstone reservoirs near the top of the Moonan Formation.
  • Sedimentology and depositional environment of the Stokes Bay Sandstone, Kangaroo Island.

Arrowie Basin

The majority of geological and geophysical research in the Arrowie Basin has been carried out by Beach Petroleum and partners in PEL 45 and 51. In the older rocks of the Adelaide Geosyncline, research has been carried out by Frontier Petroleum in PEL 41. MESA contributions include:

  • Geochronology of Early Cambrian tuffs from outcrops and drillcores.
  • Detailed correlations between wells in the Moorowie Syncline.
  • Further sampling of cored drillholes for source rock geochemistry.
  • Additional biostratigraphic work.
  • Gravity and ground magnetic traverse west of the Flinders Ranges (a student project funded by Beach Petroleum NL).

New initiatives

Three new avenues of research are proposed as part of the South Australian Targeted Exploration Initiative.

The first centres on the Warburton Basin beneath the oil and gas-productive Cooper Basin. Several commercial oil and gas discoveries have been made in porous sandstone, leached tuff and fractured reservoirs. The basement consists of Cambro-Ordovician rocks of the Warburton Basin but the oil and gas are Permian sourced. Study in collaboration with the National Centre for Petroleum Geology and Geophysics has mapped potential reservoirs and documented their petrophysical characteristics from cores and cuttings.  Most importantly, the orientation of natural fracture systems have been determined.

The second field of research investigates the link between cyanobacterial (or algal) mats and Neoproterozoic-Cambrian source rocks in the Officer, Stansbury and Arrowie Basins. The work is being carried out by specialists at PIN, Moscow.

A third project, in collaboration with Geotrack International of Melbourne, will examine the thermal history of the Warburton and Officer Basins using Zircon Fission Track Analysis (ZFTA).  This technique will clarify the thermal histories of rocks subjected to temperatures exceeding 300 degrees Celsius and may help constrain the timing of orogenic activity.