METHOD:

Formation Extent

Extents were based on drillhole data (see Main References) and from Hydrogeology of the Great Artesian Basin Australia (Habermehl & Lau, 1997) for Eromanga and Surat sub-basins. For the Carpentaria Basin, Mesozoic Geology of the Carpentaria and Laura Basins (Geoscience Australia, 2013) was used, and for offshore, the broader extent is based on seismic surveys (see Seismic Survey References).

Extent lines were adjusted to envelop all drillhole intercepts of the Hydrostratigraphic unit. This produced some varied and irregular shapes, some patchy regions, and required some interpretation to establish the most likely extent boundary.

Outcrop Extent

Outcrop extents were taken from Hydrogeology of the Great Artesian Basin Australia (Habermehl & Lau, 1997) for the Eromanga and Surat sub-basins. For the Carpentaria Basin, Mesozoic Geology of the Carpentaria and Laura Basins (Geoscience Australia, 2013) was used. Both of these were used minus their Paleogene/Neogene cover.

Isopach Raster

Source point thickness values calculated from drillhole intercepts were extrapolated using the ESRI ANUDEM Topo-To-Raster surface modeller. Zero thickness constraints were applied at the known extent of the aquifer/aquitard, except in cases where the formation extends beyond the GAB boundary (for example the Precipice formation on the eastern side of the GAB, where the formation is quite thick and is exposed as a cliff). In these cases, constraints were not applied and the software was allowed to model a thickness right up to the GAB boundary. Resulting grids were modified using the ESRI Grid Calculator to set the minimum thickness to 0, and clipped to the aquifer/aquitard extent.

Isopach Contours

For a regional picture of the offshore Carpentaria Basin, the thickness of the Normanton Formation and Rolling Downs Aquitard offshore is based on seismic thickness for the entire Rolling Downs Group (RDG) (see references to seismic surveys). Offshore Carpentaria Rolling Downs Group (RDG) cannot be differentiated using seismic surveys, so based on stratigraphy in well completion reports for Duyken 1 (Blake et.al. 1984) and Mornington 1 & 2 (Harrison et al. 1961), we have assumed 25% of RDG total thickness is Normanton Formation.

For the onshore Carpentaria Basin, well completion reports of individual wells (see References section), as well as BMR drill Records (Gibson et al., 1974) have been used as a source of thicknesses.

Isopach contours were calculated from the Winton-Mackunda thickness grid using the ESRI Contour Tool. These were calculated at 50m intervals. In most cases the zero contour lines generated by the tool were replaced by the extent of the aquifer due to the erratic nature of the generated lines. In cases where the aquifer/aquitard is thick at the extent, the zero isoline is outside the extent and is not mapped in that area. Isopachs were clipped to the aquifer/aquitard extent.

Data Point Locations

Data Point Locations have been derived from the bore hole data collected for this project. Only the location has been included.

SOFTWARE:

All modifications/edits and geoprocessing were performed using ESRI ArcGIS 10 software.