METHOD:

Formation Extent

Extents were based on drillhole data (see 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.

Extent lines were adjusted to envelop all borehole 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 came 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 (Ransley TR & Smerdon BD, Eds., 2012) was used. Both of these were used minus the Winton-Mackunda Aquifer and the Cenozoic 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 we have assumed 75% of RDG total thickness is Rolling Downs aquitard, based on stratigraphy in well completion reports for Duyken 1 (Blake et.al. 1984) and Mornington 1 & 2 (Harrison et al. 1961).

For the onshore Carpentaria Basin, well completion reports of individual wells (see References), as well as BMR drill Records (Gibson et al., 1974) have been used as a source of thicknesses. Also within the onshore Carpentaria region, lithostratigraphic revision based on electrofacies (McConachie et al., 1994) has enabled distinction of lithostratigraphic units within the onshore Rolling Downs Group north of the Staaten Sub-basin.

Isopach contours were calculated from the Rolling Downs Aquitard 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.