Geomechanical rock properties of the Officer Basin

Keywords: Exploring for the Future, geomechanics, laboratory testing, Neoproterozoic Basins, Officer Basin, Officer-Musgrave Project, Paleozoic Basins, petrophysics, Poisson’s ratio, rock properties, static to dynamic, unconfined compressive strength, unconventional hydrocarbons, Young’s modulus.

The Australian Government’s Exploring for the Future program (EFTF) launched in 2016 with A$100.5 million of funding provided to Geoscience Australia to explore Australia’s resource potential and boost investment in northern Australia. In 2020, an additional A$125 million of funding was announced to expand the program nationwide until 2024. The EFTF energy component aims to attract industry investment by delivering a suite of new precompetitive geoscience data and knowledge of the hydrocarbon prospectivity of Australian sedimentary basins. The Officer Basin of South Australia and Western Australia is one of the key study areas under investigation as part of this expanded EFTF program, and is currently the focus of a regional stratigraphic study.

The Officer Basin remains a frontier basin for hydrocarbon exploration with significant uncertainties around prospectivity and stratigraphy that arise from the lack of precompetitive geoscience data. While numerous hydrocarbon shows have been observed in wells throughout the basin, no accumulations have yet been found. Geoscience Australia’s Onshore Basin Inventory (Hashimoto et al. 2016) identified several key knowledge gaps in the Officer Basin that if addressed could impact prospectivity. One area was the need for a better understanding of basin sediment rock properties through new data analysis, particularly looking at the reservoir and seal potential of postulated unconventional and conventional targets. Geomechanical studies to understand the recoverability of hydrocarbons from low-permeability reservoir rocks in the basin were also recommended.

To address these knowledge gaps, Geoscience Australia undertook a series of petrophysical and geomechanical analyses on legacy core samples from five Officer Basin stratigraphic and petroleum exploration boreholes that intersected Cambrian and Neoproterozoic successions. This work contributes to a larger-scale regional stratigraphic study, which also includes new chemostratigraphy, geochemistry, fluid inclusion stratigraphy, thin section petrography, conventional wireline log interpretation and neural network identification of petrophysical parameters (Carr et al. 2022). Further detail on the petrophysical work is included in this volume (Wang et al. 2022).

The Neoproterozoic to Paleozoic Officer Basin is an intracratonic sedimentary basin that covers approximately 525 000 km² over South Australia and Western Australia. It is a remnant of the Neoproterozoic Centralian Superbasin that is bound by the Yilgarn Craton to the west, Musgrave Block to the north and Gawler Craton to the southeast (Hashimoto et al. 2016). The Officer Basin contains up to 10 km of marine and non-marine siliciclastic and carbonate rocks, with minor volcanics. Sediments were primarily deposited within a series of sub-basins that run parallel to the northern basin margin. While no hydrocarbon accumulations have been identified, numerous hydrocarbon shows have been recorded in a range of formations from the Neoproterozoic to the overlying Permian sequences. The basin is likely prospective for both conventional and unconventional hydrocarbons, with salt tectonics providing numerous trapping mechanisms and extensive carbonate intervals that may host stratigraphic traps (Apak and Moors 2001; Carlsen et al. 2003). To date, a total of 75 petroleum and stratigraphic wells have been drilled in the Officer Basin. Hence, while core, cuttings and basic wireline data suites are available, data density is very low and significant uncertainties arise from the lack of data. As part of EFTF, Geoscience Australia is working to maximise knowledge of the basin by analysing new data from existing well samples.

Forty one samples from Officer Basin wells, from both Western Australia and South Australia, were analysed in this study (Table 1). Samples were selected from intervals considered to have potential as either conventional or unconventional reservoirs while being representative of lithologies intersected by drillcore throughout the basin. Sampling was largely dependent on core integrity with subsequent testing undertaken by the CSIRO Geomechanics and Geophysics Laboratory in Perth. Tests included:

1. Photographic and 2D X-ray computerised tomography (XCT) scan images of whole core and core plugs;

2. Unconfined compressive strength (UCS) parameters;

3. Stress–strain–time curves for UCS experiments;

4. Tensile strength characteristics;

5. Static elastic properties, Young’s modulus (E) and Poisson’s ratio (v);

6. Dynamic elastic properties, velocity, Vp/Vs ratio, Youngs, Bulk and Shear moduli plus Poisson’s ratio;

7. Gas porosity and permeability; and

8. Grain density.

Table 1.  Samples analysed in this study.

Background, methods and results were provided to Geoscience Australia by CSIRO Energy and are summarised in Bailey et al. (2021). Cylindrical core plugs approximately 25 mm in diameter and 50 mm in length were prepared normal to bedding for UCS testing, and where core was unable to yield a plug suitable for UCS testing, an approximately 25 mm by 10–15 mm disc was extracted for Brazilian tensile strength (BRZ) testing.

The results of the geomechanical and petrophysical analyses are detailed in Bailey et al. (2021), with the geomechanical results discussed herein (Table 1). Discussion of the petrophysical results can be found in Wang et al. (2022), where the authors discuss the results alongside well log interpretation using both conventional and neural network methods.

A total of 47 UCS and BRZ tests were undertaken, including two repeat tests. Static values for UCS, tensile strength, E and v were acquired and are summarised in Fig. 1, with details available in Bailey et al. (2021). Benchtop ultrasonic measurements were also acquired at a single stress point during UCS testing with dynamic rock properties calculated from these data (Fig. 2).

Fig. 1.  Results of compressive testing for: (a) Unconfined compressive strength (UCS); (b) Tensile rock strength; (c) Poisson’s ratio and; (d) Young’s Modulus. Note, Brazilian tensile strength tests were only carried out on a subset of the samples (Table 1). Further details are outlined in Bailey et al. (2021). All plots refer to the legend presented in (a).

Fig. 2.  Crossplots of laboratory measured (static) and calculated values (dynamic) of Young’s modulus and Poisson’s ratio. (a) and (b) are displayed by well and (c) and (d) are displayed by simplified lithology. ‘Sandstone’ includes all sandstones and siltstones, and ‘Shale’ includes all shales, mudstones and claystones. Dynamic values of Young’s modulus and Poisson’s ratio are calculated from benchtop ultrasonic measurements. Further detail on testing and results are in Bailey et al. (2021). The legend in (a) applies to (a) and (b); the legend in (c) applies to (c) and (d).

Crossplots of laboratory measured (static) and ultrasonic derived (dynamic) Young’s modulus and Poisson’s ratio illustrate the relationships between these parameters in the Officer Basin, both by well and by generalised lithology (Fig. 2). Typically, static and dynamic properties differ due to the difference in inelastic effects (e.g. Ciccotti and Mulargia 2004); Davarpanah et al. (2020) attribute the discrepancies between dynamic and static elastic moduli to microcracks and pores. Brotons et al. (2016) note that the ratio between elastic moduli tends to decrease in very stiff and compact rocks, and increases as stiffness and compactness decrease. Generally, static moduli are likely to be slightly lower than dynamic moduli, however, variations of up to an order of magnitude are not uncommon (Ciccotti and Mulargia 2004).

Static and dynamic Young’s modulus (E_stat and E_dyn, respectively) measurements from the Officer Basin demonstrate a clear relationship when plotted against one another, with correlation coefficients (R² values, Fig. 2c) of 0.7858 and 0.7764 for shales and sandstones, respectively (Fig. 2). While there is some scattering of values, there is a clear trend evident in the data. Dynamic values are consistently observed to be higher than static values; the difference between E_stat and E_dyn ranges from 2.92 to 22.63 GPa, with a mean difference of 13.03 GPa and a standard deviation of 4.97 GPa. Approximately, E_stat is on average 40% smaller than E_dyn. Fig. 2c also presents equations for the linear relationship between E_stat and E_dyn specific to each lithological category. Comparing these two methods of estimating E_stat from E_dyn, similar scatter and correlations are observed, implying that both methods approximate E_stat from E_dyn with the same efficiency (Fig. 3). Hence, using a relationship of E_stat = 0.6 × E_dyn will result in a useful approximation. We recommend that where precise understanding of E is required in the Officer Basin, further work be undertaken to locally understand variations in the relationship between static and dynamic Poisson’s ratio.

Fig. 3.  Comparison of two methods of converting dynamic Young’s modulus (E_dyn) into static Young’s modulus (E_stat). Firstly, the equations for the lithology specific linear relationship from Fig. 2c were tried (in red) and secondly, a quick estimate of E_stat = 0.6 × E_dyn (green). Note the very similar correlation coefficients (R² values) for each method.

Generally, the difference between static and dynamic Poisson’s ratio (v_stat and v_dyn, respectively) is very small and so it is often not considered, as there is no direct relationship between dynamic and static Poisson’s ratio. Comparison of v_stat and v_dyn in the Officer Basin (Fig. 2) clearly illustrates that there is no direct relationship between these datasets, with correlation coefficients (R² values, Fig. 2d) of 0.0519 and 0.0014 for shales and sandstones, respectively. However, the difference between v_stat and v_dyn ranges from 0.00 to 0.23, with a mean difference of 0.12 and a standard deviation of 0.06. Hence, while in many instances the difference between v_stat and v_dyn is indeed low, there are also many instances where differences are significant.

Geoscience Australia has acquired new geomechanical rock property data from forty core samples in five legacy stratigraphic and petroleum exploration wells in the Officer Basin as part of the Exploring for the Future program’s Officer–Musgrave Project. Samples cover both the Paleozoic and Neoproterozoic aged intervals within South Australia and Western Australia, and were subjected to unconfined compressive rock strength tests, Brazilian tensile strength tests and laboratory ultrasonic measurements as well as imaging and petrophysical testing. The data used in this study and discussed herein are presented in full, alongside background and relevant methods, in Bailey et al. (2021).

These analyses provide a baseline assessment of potential conventional and unconventional reservoir intervals throughout the basin, as well as providing insights into the relationships that exist between laboratory acquired, or static, and those derived from ultrasonic, or dynamic, elastic rock properties. In the Officer Basin, a firm relationship between static and dynamic Young’s modulus is identified and presented herein, whereas no systematic relationship between static and dynamic Poisson’s ratio was identified.

Accurate characterisation of geomechanical rock properties, such as rock strengths and elastic moduli, through laboratory testing is essential. It allows for the construction of detailed mechanical earth models, enables the designing of drilling programs and placing wells (reducing drilling risk and ensuring wellbore stability), and facilitate the prediction of fracture propagation. These data form foundation datasets that can simplify investment decisions in frontier regions such as the Officer Basin by providing input data that is essential for assessing stress regimes, rock failure, and exploration and development programs.

The data that support this study are available in Geoscience Australia’s catalogue at https://ecat.ga.gov.au/geonetwork/srv/eng/catalog.search#/home.

All authors confirm there are no conflicts of interest.

This research was funded through the Australian Government Exploring for the Future program, led by Geoscience Australia.