Organic matter in sedimentary rocks changes physical properties and composition in an irreversible and often sequential manner after burial, diagenesis, catagenesis and metagenesis with increasing thermal maturity. Characterising these changes and identifying the thermal maturity of sedimentary rocks is essential for calculating thermal models needed in a petroleum systems analysis.
In the Isa Superbasin, the thermal history of the sediments is difficult to model due to erratic thermal maturity profiles, which are often inverted with depth (e.g. Glikson et al. 2006; Gorton & Troup, 2018). In previous studies, these erratic profiles have been attributed to multiple fluid flow events through the basin (Glikson et al. 2006). However, another reason to explain some of these results may be due to low statistical significance and quality control of legacy data. The Australian Standard for reflectance measurements Australian Standard AS2856.3-1998. Coal petrography: Method for microscopical determination of the reflectance of coal macerals requires a minimum of 30 reflectance measurements to be taken on a sample for statistical significance and to maintain confidence in the results. However, Barker & Pawlewicz (1993) suggest a minimum of 20 measurements in sedimentary rocks which may have fewer macerals than coals. The numbers of reflectance measurements are not always provided with legacy data, however some core samples have very low values (n < 5) suggesting low confidence in some results.
In order to maintain confidence in the legacy data, Geoscience Australia contracted CSIRO Energy to conduct a thorough organic petrological analysis of 22 shale samples from two drill cores; Amoco DDH 83-4 and Desert Creek 1 from the Fickling and McNamara groups of the Isa Superbasin. These two wells were selected as Geoscience Australia has recently conducted a full suite of organic geochemistry on these wells and there is legacy reflectance data available.
The estimated organic matter (OM) content of the samples analysed ranged from <0.1% to 30% by volume. The majority of the OM is bitumen that occurs as fine disseminations throughout the mineral matrix in addition to infilling inter-granular porosity of carbonates and other minerals. The abundance of bitumen resulted in reflectance measurements consistent with Australian Standards for most samples, ensuring high confidence in the results.
In Amoco DDH 83-4, the reflectance data generated in this study show a broadly linear increase with depth down core, ranging from thermally mature to overmature. The outliers in the down core trend represent samples with low OM, a minimum amount of bitumen to conduct reflectance measurements on and hence, low statistical significance and low confidence in the results. These results highlight the need to work within the guidelines specified by the Australian standard to maintain confidence in the data. In Desert Creek-1, samples studied are mature for dry gas generation. Although still broadly consistent with previously published work, the down well reflectance profile produced for this study is much less erratic compared with reflectance profiles generated from legacy data. This is likely due to the careful analysis of the same OM type in the samples. For the legacy Desert Creek 1 data, neither reflectance histograms nor the number of reflectance measurements are provided and therefore reasons for the differences between results are not certain.
The results of this study have major implications in a petroleum systems modelling context, as thermal and burial history modelling requires reliable equivalent vitrinite reflectance data for calibration purposes. In the Fickling Group, the new results show that hydrocarbon generation has occurred. As the thermal maturity in the previous study was largely immature, the hydrocarbon prospectivity of the area has been upgraded. The statistically significant results of this study provide a more robust calibration dataset for use in petroleum systems models in the Isa Superbasin. Similar studies on other wells in the basin may be necessary to further reduce uncertainty.
