GABATLAS - Rolling Downs Aquitard - Thickness and Extent

## **Abstract** 

This dataset and its metadata statement were supplied to the Bioregional Assessment Programme by a third party and are presented here as originally supplied.

The Rolling Downs Aquitard - Thickness and Extent data sets, are part of a set that represents the hydrostratigraphic units of the Great Artesian Basin, which include five major aquifers, four intervening aquitards, and the Cenozoic cover to the GAB. 



There are five layers in the Rolling Downs Aquitard map data. 

A: Formation Extent 

B: Outcrop extent 

C: Isopach Raster 

D: Isopach Contours 

E: Data Point Locations 



The datasets have been derived from the lithostratigraphic intercepts in drillhole data from petroleum exploration wells, water bores, and stratigraphic wells. Seismic correlation and assessment of hydrogeological character based on electrofacies have not been used. The working datasets for this study has been derived primarily from the following databases: 

1. PEPS-SA (Petroleum Exploration and Production System - South Australia) (Department of Primary Industries and Regions SA, 2011) 

2. WaterConnect Groundwater database (Govt. of SA, 2011) 

3. QPED (Queensland Petroleum exploration database) (Geological Survey of Queensland, 2010). 

4. GABLOG (Great Artesian Basin Well Log Dataset) (Habermehl, 2001) 

5. Additional supplementary information was derived from published reports listed in the following section. 



This is a regional interpretation for mapping at approximately 1:1 000 000 to produce a broad scale overview, and examination of small areas by collecting extra data is most likely to produce results that differ from this regional interpretation. 



This dataset and associated metadata can be obtained from www.ga.gov.au, using catalogue number 81677. 



Associated report reference: 

Ransley, T., Radke, B., Feitz, A., Kellett, J., Owens, R., Bell, J. and Stewart, G., 2014. Hydrogeological Atlas the Great Artesian Basin. Geoscience Australia. Canberra. \[available from www.ga.gov.au using catalogue number 79790\]

## **Dataset History** 

SOURCE DATA:

Data was obtained from a variety of sources, as listed below: 

1. WaterConnect Groundwater database (Govt. of SA, 2011) 

2. Great Artesian Basin Well Log Dataset (GABLOG) (Habermehl, M. A., 2001). 

3. Petroleum Exploration and Production System - South Australia (PEPS-SA) (Department of Primary Industries and Regions SA, 2011). 

4. Queensland Petroleum Exploration Database (QPED) (Geological Survey of Queensland, 2010). 

5. Well completion and drill log reports (see references in abstract) 

6. Other reports (see references in abstracts) 

7. Seismic surveys and associated reports (see seismic references section in abstract) 



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. 



QAQC: Data sets were searched for errors such as negative thickness, missing data, incorrectly calculated thickness, aquifers/aquitards with missing formations, and false XY data. The data was given a second Q&A after the thickness grids had been calculated. This involve plotting the points and the thickness grid and looking carefully for bad values. Sometimes a false outlier value would cause a 'bullseye' effect on the grid. To check the veracity, nearby data would be compared, and if necessary the original data would be searched check the value. Some petroleum fields would have wildcat picks at certain bore holes and these were compared with nearby boreholes and adjusted or deleted. Additionally, if whole subregions had suspect values the data was check to ensure the relevant data had all been included. Finally, data sets were also checked to ensure the bore whole data recorded the full thickness of the Aquifer. In many cases water bores only go down until a suitable water source is found and often will not penetrate the whole aquifer. This data was considered on a case by case basis, in areas where plenty of suitable data was available they were removed, and in areas of sparse borehole data they were included to establish the occurrence of the formation albeit as a minimum thickness value. 



Data has undergone a QAQC verification process in order to capture and repair attribute and geometric errors.



See "Metadata.pdf" for complete metadata

## **Dataset Citation** 

Geoscience Australia (2015) GABATLAS - Rolling Downs Aquitard - Thickness and Extent. Bioregional Assessment Source Dataset. Viewed 10 December 2018, http://data.bioregionalassessments.gov.au/dataset/0c4f0e0e-2d1d-4dee-9a57-36ecdd1d9a1f.

Tags: Cooper subregion, Galilee subregion, Namoi subregion, geoscientificInformation
Organization: Bioregional Assessment Program