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, 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:

  • Assignment of input data to geological horizons. Objects such as curves that represent the extent of each model unit or formation top picks derived from cross-sections are assigned to each horizon. Following this, the role of each object is defined (e.g. curves representing the extent of model units are defined as non-erosional outlines).
  • Definition of the three-dimensional geological model domain. The horizontal and vertical extent of the model domain, also referred to as the ‘volume of interest’ (VOI), is defined during this step. This task is required because it defines the volume which ‘seals’ the model. The horizontal extent is determined from a polygon that represents the surface water basin boundaries (for the regional models) or the boundary of the Clarence-Moreton bioregion for the bioregion-wide model. The vertical extent is based on the inferred elevation of the horizons, which will be included in the models.
  • Modelling of horizons. A surface is fitted to the input data, taking into account the location and extension of the other horizons, the stratigraphic column and the defined relationships between different horizons (e.g. erosional contact versus conformable contact). Horizons are first previewed, and then iteratively improved. This procedure, for example, includes the detection of horizons that are crossing and the computation of the mismatch between the input data and the previewed horizons. After all critical issues are resolved, the horizons are developed.
  • Honouring of well log data. During this workflow step, the consistency of the built horizons is compared with the well input data. A mismatch table shows the vertical distance between the horizons and the markers. Where the computed mismatch is considerable, this might indicate that there are errors with the well marker, and the original well logs are then revisited. After checking the mismatch table and revisiting the original well logs where required, the horizons are fit to the input well markers.
  • Building the three-dimensional geological model grid. In the last step of the workflow, the three-dimensional geological grid is built from the horizons representing the stratigraphic units in accordance with the stratigraphic column within the entire VOI.
Last updated:
8 January 2018
Thumbnail images of the Clarence-Moreton bioregion

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