Geophysical Investigations In The Moorowie Syncline, Arrowie Basin, South Australia.
McLean, Ben F.
Honours Degree, 1999
University of South Australia
The Arrowie Basin is a saddle-shaped basin containing clastic and carbonate sediments of Early to Middle Cambrian age, located in central-eastern South Australia and north-western New South Wales. While drilling has encountered encouraging hydrocarbon shows, no commercial discoveries have been made to date.
In this study I investigate of a pair of anomalously high-amplitude reflections which are located within the Moorowie Syncline of the Arrowie Basin. The Moorowie Syncline is a gently north-plunging depression bisected by a set of north-south trending wrench faults known as the Poontana Fracture Zone. The high amplitude reflections have been previously investigated but no study of the polarity of the data used was made. Such an investigation is the first step necessary in order to understand the cause of the reflections.
Seismic data ranging from 1970 to 1987 are available for the study area, but data shot earlier than 1981 is of poor quality. The Moorowie #1 well is the only nearby well of sufficient depth to intersect the strata of interest. Using the Moorowie #1 sonic log it was possible to check the polarity of reflections against known jumps in acoustic impedance, and so establish the polarity of individual lines. After phase and amplitude balancing of the dataset, I tied Moorowie #1 to the seismic data, allowing an interpretation to be made of the area. This revealed the high amplitude reflections to be a trough followed by a peak, indicating a fast or dense interval.
My interpretation is that the high amplitude reflections arise from the upper and lower boundary of an equivalent to the Moorowie Formation. This is mostly a highstand carbonate unit in Moorowie #1, but 24 ft of anhydrite were also intersected. By approximate backstripping of the structure seen in overlying horizons, I have shown that the area generating the high amplitude reflections underwent subsidence in a graben prior to deposition of the Moorowie Formation. Following this deposition the structure was inverted. The time immediately prior to the reversal of stresses coincides with a highstand regression and the deposition of the Moorowie Formation, and I hypothesise that the waning subsidence of the graben created a local restricted marine environment in which thick anhydrite developed. It is this anhydrite that generates the high amplitude reflections.
The scope of this project was expanded upon the recognition of a stratigraphic interval, present throughout much of the study area, that wedges out at the Moorowie #1 well location and hence cannot be identified in core or cuttings. The age of this unit is poorly constrained, and the possible stratigraphic range is large. By backstripping later structure I show that the interval has a characteristic channel profile, and channels are also seen in time thickness maps of the interval. The channels are underlain by faults which appear to control channel orientation and extent. Evidence for exposure and channeling coeval with extension exists in several places in the Flinders Ranges, at the stratigraphic level of the Adelaidean Wonoka Formation, and also at the boundary between Cambrian sequence sets C1.1 and C1.2. These were activated by extension related to wrench faulting, which was probably active in the study area during similar conditions of exposure, resulting in constrained incision. The channeling observed in the Flinders Ranges provides analogues for the prediction of the nature of the channel fill, and reveals this may contain sediments with potential reservoir porosity, such as limestone conglomerates.