Controls on Polygonal Faulting in the Eromanga Basin, Central Australia
Poonawala, Moyez A.
Honours Degree, 2006
University of Adelaide
Polygonal faults compartmentalize the shale dominated Cretaceous succession of the Eromanga Basin. This study examines control of mineralogy and lithology on fault development. The study also quantifies fault density and velocity variation from four 3D seismic surveys of the basin.
Thirty-eight representative drill cutting samples were selected at regular intervals from three wells of the faulted succession for the determination of mineralogy and lithology profiles. The three wells were selected to sample different intensities of faulting. Fault density was quantified in the vicinity of these wells using the modified Box Technique method on Variance Cube time slices flattened at the Cadna-owie level.
Fault density varies across the stratigraphic units and is most intense at the level of the Coorikiana Sandstone and the middle of the Winton Formation. These faults, although seen throughout the basin, appear to be more intense in the areas of over thrust fault blocks. The variance cube time slices recognizes these faults as non tectonic because they are restricted to within a layer or tier. The fault density varies within stratigraphic units and four tiers of faults are recognized for this succession. Dense faulting is associated with finer grain size and higher smectite proportions. Sandier units show a lower density of faults.
Swan Lake Field has higher velocities throughout the Cretaceous succession which do not correlate with increased quartz and calcite proportion in the rocks. In general velocity increases with depth in the polygonally faulted intervals but is not related to clay diagenesis as no significant smectite-illite conversion is recognized for this part of the succession.
Polygonal faults, which are thought to be formed due to dewatering, are not associated with dewatering due to mineral alteration. The observations from this study are consistent with polygonal fault formation via syneresis (Dewhurst et al., 1999) or the low coefficient of friction in fine grained rocks (Goulty & Swarbrick, 2005). Dewatering due to syneresis may not explain the high density of faults in the non-marine deposits of the Winton Formation.
Although this limited study was inconclusive, more work is needed to analyze the pore fluid chemistry, particularly in comparison between the marine and non-marine sediments.