AusGeo News September 2007 Issue No. 87
Samples show excellent source rock potential
Initial analysis of data from a new geological sampling survey by Geoscience Australia in the Great Australian Bight provides new evidence for the presence of potential petroleum source rocks in the Bight Basin.
Exploratory drilling in the Bight Basin has historically concentrated on the inboard margins of the basin. Apart from Woodside's Gnarlyknots 1A well, which was drilled in the Ceduna Sub-basin in 2003, only the more proximal parts of the Cretaceous depositional systems have been sampled.
Geoscience Australia's previous basin analysis study (1999–2004) identified a series of potential source rock intervals at different stratigraphic levels, including marine or marine-influenced sediments whose source rock character was predicted to improve further basinward.
A study of all available seismic data indicated that these rocks cropped out on the seafloor at the seaward edge of the Eyre Terrace where canyon formation, slumping and faulting have exposed the mid-Cretaceous section. This area was the prime dredging target for the survey conducted earlier this year.
The seismic study also identified a range of sites where, if other geological conditions were met, seepage of hydrocarbons was most likely to occur. These included areas where recent reactivation of deep-seated faults was evident at the seafloor, basin margin areas where seal facies thin sufficiently to allow seal failure, and areas high-graded by the presence of slicks detected by synthetic aperture radar.
Conducted by the Marine National Facility vessel RV Southern Surveyor (survey SS01/2007), the survey gathered geological samples and geophysical data via deep sea dredging and gravity coring techniques.
Southern Surveyor departed Port Lincoln, South Australia on 22 February 2007 for a three-week survey, with a crew of 12 Geoscience Australia scientists and technicians, a scientist from the Geological Survey of Western Australia, and two CSIRO technicians. Good sea and weather conditions enabled sampling to be completed in all nine target areas (figure 1). The survey resulted in the collection of thirty seven dredge samples, 69 gravity cores and 15 grab samples, as well as 4600 kilometres of swath data and 2400 kilometres of sub-bottom profile data.
With the physical material now sorted, catalogued and subsampled, a detailed analysis program has been developed to incorporate sedimentological, geochemical and biostratigraphic analyses.
The organic geochemical analysis program is designed to assess the source rock characteristics of the dredge samples, and to investigate whether the gravity core samples reveal any indication of hydrocarbon seepage. While initial results from the gravity cores have proved disappointing, early results from the source rock characterisation study have been highly encouraging.
Preliminary organic geochemical analysis has shown that some of the samples from Area 5 on the Eyre Terrace (figure 1) have excellent source rock potential, with high organic carbon contents and the potential to generate liquid hydrocarbons. These are the best source rock results reported from the Bight Basin, and indicate that careful targeting of appropriate depositional systems is vital for understanding hydrocarbon source potential.
The results can be summarised as follows:
More sophisticated geochemical analysis is now being applied to a high-graded subset of these samples, most of which come from Area 5, the prime dredging target on the edge of the Eyre Terrace (figure 3).
If the seismic interpretation is correct, the best samples are likely to be from the upper White Pointer and/or Tiger supersequences, which have been preliminarily dated to be of Cenomanian–Turonian age (figure 4). Palynological analysis of the samples is now underway, with results from the first batch of samples expected by early November.
The results of the survey are being compiled as a Geoscience Australia record, which is expected to be available in mid 2008.
For more information phone Jennifer Totterdell on +61 2 6249 9407 (email firstname.lastname@example.org)