2.1.2.3.6 Characterisation of the bedrock stratigraphic units in the Clarence-Moreton bioregion

The challenges associated with the development of the sedimentary bedrock formation tops are very different from those related to defining the base of the alluvium and basalts. In contrast to the alluvium-bedrock interface, there are far fewer points where the interfaces between different bedrock units are intersected. As outlined in Section 2.1.2.1.1, stratigraphic information from the deeper stratigraphic bores together with information derived from exploration bores and seismic data are the main sources of information.

In addition to the lack of data points, the considerable geological complexity as influenced by structural features such as different depositional centres and the presence of faults, folds or dykes make this task difficult.

A comparison of a previously developed isopach map of the Walloon Coal Measures (Ingram and Robinson, 1996) with the Walloon Coal Measures thickness observed at some more recently drilled exploration bores (Figure 18) demonstrates that the newly obtained information will result in considerable changes of the isopach maps in some areas. Similarly, other formation tops will change, although most newly drilled exploration bores do not extend beyond the top of the Koukandowie Formation. As a result, while the thickness and geometry of all Clarence-Moreton Basin bedrock units from the youngest formation (Grafton Formation) to the top of the Koukandowie Formation will change, the formation tops of the deeper formations still primarily rely on the same data as presented by O’Brien and Wells (1994) and Ingram and Robinson (1996), complemented by seismic data provided by the NSW Department of Primary Industry. However, it is important to note that where formation boundaries are conformable, the geometry of the contacts between deeper formations is also informed by the geometry of shallower formation boundaries.

The horizons of the tops of the different sedimentary basin stratigraphic units are represented by the well log data and picks from seismic lines aided by the grids provided by Metgasco (Dataset 4) and the NSW Department of Trade and Investment (Dataset 5). Where required, control points were digitised in GoCAD™ (Paradigm Geophysical Pty Ltd, version 2009.4) as additional constraints for each horizon using regular sets of cross-sections to avoid model artefacts and prevent horizon crossings. This was required particularly in regions where only few wells intersect the deepest sedimentary basin stratigraphic units and where no seismic data exist (e.g. Brisbane river basin or at the boundary between the Richmond river basin and the Logan-Albert river basin).

The three-dimensional geological models were then developed using the stratigraphic and structural modeling workflow in SKUA® (Paradigm Geophysical Pty Ltd, version 2014.1) petroleum reservoir engineering software. The following steps taken to characterise the bedrock stratigraphic units in the Clarence-Moreton bioregion are: