The continuity of sands in the Winton Formation, South Australian sector, Eromanga Basin.
Brown, Callan J.
Honours Degree 2009
University of Adelaide
The Winton Formation was studied over the South Australian sector of the Eromanga Basin to determine whether it is likely to be a continuous aquifer and therefore needs specific isolation during well abandonment procedures. The formation itself comprises a non-marine succession and was deposited in a fluvial to lacustrine environment. It consists of interbedded fine to coarse-grained sandstone, carbonaceous and pyritic shale, siltstone and coal seams with intraclast conglomerates.
Data availability and quality severely affected the way the project was undertaken. There has been no core acquired within the formation which meant the majority of the data had to be taken from drill cuttings and petrophysical logs. Drill cuttings are an inaccurate way of studying a formation due to the uncertainty in their true depth. Also, data quality is constrained by the lack of commercial interest in the formation.
The sands were mapped out as best as possible and potential correlations were made. Sand percentage calculations were used to conclude these sandy intervals were unlikely to be interconnected. Seismic data was utilised and it was found the formation has undergone extensive polygonal faulting. The faults are therefore likely to cross cut any permeable sand intervals. XRD analyses were undertaken in Bookabourdie 4 within the Winton Formation. These identified smectite, a swelling clay that is likely to swell when exposed to water. Other procedures such as thin sections, log porosity and sand percent calculations from cuttings were also undertaken to help further understand the permeability and connectivity of the sands within the formation.
The study concludes that even in the low probability occurrence that the permeable sand intervals are interconnected, it is probable that smectite is present that will destroy any permeability if a water of different salinity is introduced. As a result, the formation is not likely to be an aquifer and would therefore not require isolation during well abandonment. Due to the uncertainty in all of the procedures, it is recommended that core should be taken over the Winton Formation to help understand the formation further.