2.1.2.1.1 Lithological and stratigraphic data


Bore logs or well logs (also commonly known as drill logs) are the main source of data used to construct the three-dimensional geological model, and to reliably assign screened intervals (corresponding to the interval where the casing is slotted to allow intake from the aquifer) to discrete stratigraphic units (e.g. Quaternary alluvium or Walloon Coal Measures in the Clarence‑Moreton bioregion). In this product, ‘bore’ or ‘bore log’ is used where a reference is made to groundwater bores, whereas ‘well’ is used with regards to exploration wells or deep stratigraphic wells. Groundwater bore logs and associated groundwater level measurements are also essential for constructing potentiometric surface maps that show groundwater flow direction and for characterising spatial and/or temporal water chemistry patterns within aquifers.

The major bore/well log data sources used in the Assessment are:

  • groundwater bores, sourced from the Queensland Department of Natural Resources and Mines (DNRM) groundwater database (Queensland Department of Natural Resources and Mines, Dataset 1) and the National Groundwater Information System (NGIS) groundwater database (Bureau of Meteorology, Dataset 2)
  • deep stratigraphic wells, sourced from publications, including O’Brien and Wells (1994)
  • exploration wells, including CSG exploration wells, coal exploration wells, mineral exploration and petroleum exploration wells, sourced from company well completion reports accessed through the Queensland DNRM Interactive Resource and Tenure Maps (IRTM) system and the NSW Digital Imaging Geological Systems (DIGS®).

Geotechnical wells exist in the eastern part of the basin, but have not been used to date as they are mostly shallow and only record lithological rather than stratigraphic information.

A typical bore/well log includes the following information:

  • a registration number that uniquely identifies the well
  • a location (easting and northing, or longitude and latitude)
  • the total depth
  • bore/well type (e.g. groundwater, exploration, appraisal or production)
  • an elevation of the natural ground surface or the top of the casing
  • lithological and stratigraphic descriptions with their associated depth intervals.

Typically, this information is collected by drillers (for groundwater bores) and/or geologists (for exploration wells) when the bore/well is first installed, and then passed on to state agencies for archiving in an electronic database.

While the different bore and well log data are very useful, there are some challenges associated with the use of data from multiple bore/well types in regional-scale syntheses such as the Bioregional Assessment Programme. For example, there is often limited consistency in the ways in which bores/wells are completed, there is no generally adopted standard template, and different companies have different style sheets and purposes. In addition, the groundwater bores and exploration wells were drilled over a very long time period (more than 100 years), and terminology and standards have changed substantially over time. Three-dimensional geological models, potentiometric surface maps and many other geological, hydrochemical or hydrogeological applications are dependent on the accuracy and consistency of the input data from which they are developed. The quality of the bore log data in the groundwater databases in Queensland (tables ‘Stratlogs’ and ‘Stratigraphy’ in Queensland Department of Natural Resources and Mines (Dataset 1)) and NSW (tables ‘Drillers logs’ and ‘Geologist logs’ in Bureau of Meteorology (Dataset 2)) is highly variable and often very poor, particularly for bores which were drilled several decades ago when drillers had little or no experience in geological logging.

Some common problems with lithological and stratigraphic descriptions in these databases are:

  • no lithological and/or stratigraphic information available for many bores or wells
  • omission of important information such as the colour of sandstones
  • use of incorrect geological terms
  • no depth information
  • misspellings.

Consequently, all well log data were subjected to extensive data quality checks prior to use in construction of the three-dimensional geological model or for hydrogeological or hydrochemical applications.

Data preparation

The main steps and procedures used to check, simplify and unify lithological bore log data descriptions were:

  • Consistent use of terminology and spelling. The first stage of checking the bore log data involved editing the lithological and stratigraphic descriptions to ensure consistent use of terminology and spelling. This was done for each individual bore log and also across the entire dataset. For example, superseded stratigraphic unit names (e.g. Helidon Sandstone instead of Woogaroo Subgroup) are common in the Queensland groundwater database (Queensland Department of Natural Resources and Mines, Dataset 1). Furthermore, in the NSW groundwater database (Bureau of Meteorology, Dataset 2), descriptions such as ‘blue metal’ or ‘metal’ are used to describe basalts. The use of such terms to describe basalts is widely known in geology, and they can therefore be converted to ‘basalt‘. Spelling mistakes were also corrected, e.g. replacement of ‘course sandstone’ with ‘coarse sandstone’.
  • Identification of geological inconsistencies or geological errors. In the second stage of data checking, the bore log data were examined for geological inconsistencies that may have represented errors in the lithological descriptions. Table 3 shows a hypothetical example where ‘granite’ is reported to occur below and above sandstone. The lithological description, ‘granite’, is very common in the database, but granites are generally absent within the extent of the Clarence-Moreton bioregion (although present outside the Clarence-Moreton bioregion at the basin margins) (Rassam et al., 2014). This description is therefore geologically unlikely, and thus it was assumed with a high level of confidence that the original description refers to another bedrock type such as shale. The description was corrected in the database to ‘sedimentary bedrock’. The cause of such errors may be related to poor geological knowledge of the logger or difficulties in distinguishing different rock types from cuttings. Other typical geological errors for basalt in the lithological descriptions in the NSW and Queensland groundwater databases include:
    • Basalt is incorrectly described as ‘sandstone’, ‘hard’, ‘shale’, ‘black shale’, ‘red shale’ or ‘volcanic shale’ or simply described as ‘rock’.
    • Basalt is commonly included in bore log descriptions in areas where basalts are likely to be absent according to geological maps or the lack of magnetic anomalies in geophysical data.
    • Sandstone is often described as ‘shale’, ‘soft shale’, ‘hard’ or ‘rock’.
  • Verification of bore elevation data. The source and accuracy of elevation information in a bore log database are generally unknown. For example, this can be because many of the bores contained in the database were drilled a long time ago and methods to determine the elevation have evolved significantly since then. Hence, a digital elevation model (DEM) was used to provide independent verification of the ground elevation reported for each individual bore log. As the source of ground elevation reported in bore logs is often unknown, the elevation estimate from the DEM was generally favoured to ensure consistency across the whole dataset. The DEM used in the Assessment has an approximate resolution of 1 arc second or 30 m, which is derived from the Shuttle Radar Topography Mission (SRTM) dataset (Geoscience Australia and CSIRO Land and Water, 2010). Data for the ground surface elevation of each bore were extracted from the DEM in Global MapperTM.
  • Simplification of lithological logs. Due to the size of the dataset (>30,000 Excel rows), with hundreds of different lithological descriptions, the dataset was simplified into a smaller subset of lithological descriptions. These lithological descriptions formed the basis for the preliminary assessment into simple units such as ‘alluvium’, ‘sedimentary bedrock’ or ‘basalt’.
  • Conversion of lithological logs into stratigraphic logs. No stratigraphic information exists for the bores in the NSW groundwater database, and only nine bores out of several thousand bores within the Clarence-Moreton bioregion in NSW have an aquifer assigned in the National Groundwater Information System (Bureau of Meteorology, Dataset 2). The three-dimensional geological model depicts stratigraphic units, rather than lithological data, as it is not possible to model lithological variation at the scale of the entire bioregion. Furthermore, many of the stratigraphic units in the Clarence-Moreton bioregion are composed of variable sequences of sandstone, mudstone, siltstone or other rock types. As a result, rock types are not necessarily characteristic markers of different formations, and the modelling of the three-dimensional distribution of the different units in the subsurface is based on stratigraphy rather than rock types. Without stratigraphic information, it would not be possible to determine the stratigraphy at the screened intervals of groundwater bores. This severely limits the usefulness of groundwater level measurements or hydrochemical data for further use in the BA. To overcome this limitation, an attempt to assign stratigraphy based on lithology was done based on geological knowledge and the expert scientific judgment of the Assessment team. An example of the procedure is shown in Table 3. In this example, the Koukandowie Formation is the shallowest bedrock unit and the bore is located in an area where the alluvium is present. Consequently, the shallow sandstone at this location is considered to be the Koukandowie Formation. The terms ‘black clay’ and ‘sandy clay’ are common lithological descriptions of alluvial sediments, and the sediments which overlie the sandstone are therefore likely to be alluvium. Therefore, the lithological description of granite from 12 to 15 mBGL (metres below ground level) is likely to be incorrect. For deeper bores that are likely to intersect multiple bedrock stratigraphic units, it is often impossible to differentiate with sufficient confidence between the stratigraphic units in the vertical column based on the lithological records as descriptions are often ambiguous. For example, sandstone or shale can occur in different units. In such cases, the term ‘unknown’ was assigned to the respective depth interval.

Table 3 Example of a hypothetical bore log showing the cleaning and simplification procedure used to convert lithological logs to stratigraphic logs


Bore

ID

Bore depth (mBGLa)

From (mBGL)

To (mBGL)

Lithological log

Corrected and simplified lithological description

Shallowest bedrock geological unitb

Surface geological unitb

New stratigraphic log

99999

48.7

0

1.2

Black clay

Alluvium

Within Koukandowie

Within alluvium

Alluvium

99999

48.7

1.2

4

Sandy clay

Alluvium

Within Koukandowie

Within alluvium

Alluvium

99999

48.7

4

12

Rock

Sandstone

Within Koukandowie

Within alluvium

Koukandowie

99999

48.7

12

15

Granite

Sedimentary bedrock

Within Koukandowie

Within alluvium

Koukandowie

99999

48.7

15

48.7

Sandstone

Sandstone

Within Koukandowie

Within alluvium

Koukandowie

amBGL corresponds to metres below ground level

bWhen a bore is located within the extent of the alluvium, the bedrock unit underlying the alluvium is also considered. In this example, the bore is located within the alluvium, and the bedrock unit underlying the alluvium is the Koukandowie Formation.

  • Data validation using geological judgment and reasoning. Although the data checking procedure starts prior to the development of the three-dimensional geological model, considerable ambiguity may still remain. However, it often becomes evident that certain bore log observations or stratigraphic assignments contradict those of neighbouring bores when the lithological logs are viewed in three dimensions using a package such as GoCAD (Paradigm®). For example, a frequent observation in NSW was that rocks are commonly described as ‘basalt’ in lithological logs in areas where basalts are not present on geological maps and not shown as magnetic anomalies on airborne geophysical images. This becomes very evident when all bores are displayed together in three dimensions, and in such cases, the original data in the database were revisited and consequently corrected. This process will be ongoing throughout the three-dimensional geological model development.

Last updated:
11 July 2017
Thumbnail images of the Clarence-Moreton bioregion

Product Finalisation date

6 October 2016
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