NORTHERN TERRITORY GEOLOGICAL SURVEY

NORTHERN TERRITORY GOVERNMENTDepartment of Mines and Energy

For more information visit www.minerals.nt.gov.au/hylogger

Using the HyLogger in mapping stratigraphic boundaries in sedimentary basins: examples from the McArthur Basin, Northern TerritoryBelinda Smith, Pierre-Olivier Bruna, Tania DhuNorthern Territory Geological Survey, GPO Box 4550 Darwin NT 0801 Australia

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Bukalara Sst

Quartz

Sly Creek Sst McDermott Fm

Carbonate

Calcite

Calcite

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HORIZONS BUILDING AND HONOR WELL MARKERS

GRID BUILDING

HYLOGGER DATA

DATA INTEGRATION AND FAULT MODELLING

FACIES PROPERTIES SIMULATIONS

Paradigm Geo®

AbstractResearchers that compile and interpret basin-wide stratigraphic data are faced with the issue of inconsistent 3D stratigraphic mapping. One factor that has hindered this process is stratigraphic logging of drillcore by many different geologists in different mineral and petroleum companies over several decades, with little relogging or comparison between drillholes. Another factor is changes in stratigraphic nomenclature over time, which may lead to redundancy of some stratigraphy. NTGS is using the HyLogger to assist in highlighting inconsistencies in stratigraphic logging, by scanning its core facility drill core with the HyLogger. Initial comparisons of logged stratigraphy with HyLogged mineralogy show inconsistencies both within holes and between neighbouring holes. For example, logged stratigraphic boundaries (particularly unconformities) often do not show expected sharp mineralogical or textural contrasts, or the dominant mineralogy does not match that described for a particular stratigraphic interval. These results assist in targeting zones that require further geological re-logging and validation (Figure 1).HyLogger outputs can highlight mineralogy and other physical changes that may be unique to a particular stratigraphic interval, such as changes in the composition of mineralogy (Figures 2, 3) changes in albedo, core colour changes and cyclic changes in mineral ratios (such as quartz:carbonate ratio with depth). HyLogger imagery may highlight fracture / breccia zones, which can delineate areas of structural deformation. These results can be used in conjunction with other externally generated data (gamma logs, TOC, petrography, XRD) to gain a better understanding of lithological changes within various stratigraphic units. Understanding the regional stratigraphic framework is key to developing an effective exploration strategy. Currently the NTGS is addressing this issue in the McArthur Basin by bringing together disparate one- and two-dimensional datasets into a 3D environment (Figure 4). This allows disparate datasets to be viewed concurrently and aids in developing a coherent understanding that fits all data. The HyLogged datasets are a key input in identifying lithological boundaries that constrain the surfaces within the model.

Figure 2. Drillhole MY5; plotting the carbonates by wavelength of the 2310–2350 nm absorption wavelength shows that there are two distinct populations of carbonate minerals; dolomite at 2321 nm and calcite/ankerite at around 2338 nm. Colouring the spectra by depth also shows that there are two different areas, with the calcite/ankerite occurring only around 335–355 m in the W-Fold Shale. The imagery of the carbonates also changes with both composition and depth in hole (see above).

Figure 3. Logged Proterozoic Barney Creek Formation (Glyde package) from drillholes MY5 (a) and GRNT79-8 (b). X-axis shows drillhole depths and Y-axis plots (smoothed) albedo. Plots are coloured by the dominant mineral group in the TIR. The tuffs are compositionally distinct (orange colour is feldspars, light pink is quartz) and have an albedo contrast to the rest of the Barney Creek Formation lithologies (particularly the dark shales, which have low albedo). The khaki colour shows spectral matches to gypsum, which is interpreted to be a weathering product after pyrite (post-drilling effect) in the dark shales.

Figure 4. Diagram of the modelling workflow being applied to the greater McArthur Basin Project. The workflow utilises classical 1D (wells) and 2D (seismic, cross sections, mapping) geological data to constrain the model. HyLogged datasets are being integrated into the model to improve well-marker locations, rectifying errors in stratigraphic boundaries. These hard constraints are used to model the architecture of the subsurface resulting in a more accurate model. Once the geometry of the horizons has been validated a 3D grid can be generated. Homogenous, non-homogeneous, stratigraphic and non-stratigraphic grids can be generated and used to precisely target different intervals of the model. At this stage mineralogical information derived from the HyLogger interpretation can be used to assist in geostatistical analysis and facies simulation improving confidence in the results.

Two carbonate populations (centred around 2321 nm and 2338 nm coloured by hole depth [see right side for colour of depth (m) legend]

Figure 1. HyLogger output from drillhole GSD7, showing logged stratigraphy (top row), dominant SWIR mineral (row 2), dominant TIR mineral (row 3) and core colour (row 4). Drillhole depths increase from left to right. The stratigraphic log came from a company report. The logged stratigraphic boundary between the Sly Creek Sandstone and the McDermott Formation is clearly defined by a sharp change in the mineralogy (from quartz-rich to carbonate-rich in the TIR) and in core colour. However, the logged unconformity between the Cambrian Bukalara Sandstone and the Proterozoic Sly Creek Sandstone is indistinguishable from the HyLogger results. The core colour and mineralogy appear to be continuous across this boundary. This contact requires further checking.

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