Exploring for the Future geomechanics: breaking down barriers to exploration

Keywords: Exploring for the Future, Canning Basin, Kidson Sub-basin Isa Superbasin, South Nicholson region, geomechanics, rock strength, stiffness, velocity, brittleness, mineralogy, porosity, permeability, seal capacity, Young’s modulus, Poisson’s ratio, present-day stress, in situ stress, stress regimes, Barnicarndy 1.

The Australian Government’s Exploring for the Future (EFTF) program launched in 2016 with funding provided to Geoscience Australia to explore Australia’s resource potential and boost investment on northern Australia. In 2020, an additional 4 years of funding, to expand the program nationwide, was announced. To date, the energy component of EFTF aimed to attract industry investment through the delivery of a suite of new pre-competitive geoscience data and knowledge of the oil and gas prospectivity of sedimentary basins across northern Australia. For industry to commit to exploration in frontier regions, additional pre-competitive datasets are needed to adequately evaluate resource potential and recoverability. Provision of these data aids with de-risking an area and gives industry the confidence to initiate exploration activities.

Geomechanical and petrophysical properties of prospective formations form one such dataset. Building an understanding of these properties is key to understanding petroleum systems as well as geological carbon storage and sedimentary geothermal systems; anywhere rock properties can be a key constraint. Geoscience Australia has undertaken geomechanical studies of legacy datasets and new precompetitive data acquired during EFTF. This work includes stress modelling, shale brittleness studies and the acquisition of rock property data through laboratory analysis of samples from existing and new wells.

The energy component of the first phase of EFTF focused on two main frontier regions: the Paleozoic Kidson Sub-basin of Western Australia’s Canning Basin and the Proterozoic South Nicholson region straddling the Queensland/Northern Territory border as described by Jarrett et al. (2020a). Alongside studies of legacy data, significant new regional deep-crustal 2D seismic reflection data were acquired over the Kidson Sub-basin in 2018 and the South Nicholson region in 2017 and 2019 under the EFTF program. These were followed by the drilling of the stratigraphic wells Barnicarndy 1 (previously Waukarlycarly 1) in 2019 (in partnership with the Geological Survey of Western Australia) and Carrara 1 in 2020 (in partnership with the MinEX CRC).

A structural feature of the Lower Ordovician to Lower Cretaceous Canning Basin, the Kidson Sub-basin is a large, underexplored depocentre that possibly hosts a continuation of proven Canning Basin petroleum systems (Carr et al. 2020; Southby et al. 2020). Canning Basin data have been used to provide detail on present-day stresses and provide broad constraints for basin stresses within the Kidson Sub-basin (Bailey et al. 2021). Additionally, conventional and unconventional reservoir rock properties were characterised through laboratory tests undertaken on freshly acquired core samples from Barnicarndy 1 (Jarrett et al. 2020c).

Wireline log data, including wellbore image logs, were interpreted from open-file wells to define the Canning Basin’s stress orientations and magnitudes. An NE-SW regional present-day maximum horizontal stress orientation is interpreted from observed wellbore failure, and is in broad agreement with the Australian Stress Map and previously published earthquake focal mechanism data (Bailey et al. 2021). In the Barnicarndy Graben (previously Waukarlycarly Embayment), maximum horizontal stress orientation is interpreted as ~E-W (Wilson and Thrane 2020) supporting predictions of continent-scale stresses (Rajabi et al. 2017). An overall strike-slip faulting stress regime is interpreted through the basin; however, three distinct stress zones are identified through the studied interval:

1. a reverse to strike-slip faulting stress regime in the <~1.0 km depth range,

2. a strike-slip faulting stress regime from ~1.0 km to ~3.0 km depth, and,

3. a strike-slip to normal faulting regime at > ~3.0 km depth.

Mechanical earth models, built for 15 Canning Basin wells, demonstrate variable present-day stresses within the Canning Basin and highlight the relationship between lithology and stress (Bailey et al. 2021). Significant stress changes are interpreted within and between lithologies, defining discrete mechanical units that form inter- and intraformational stress boundaries. The lithological units likely act as natural barriers to fracture propagation, particularly within those currently targeted for their unconventional resource potential (Bailey et al. 2021).

A similar distribution of stresses to that in the legacy data is observed in the preliminary mechanical earth model constructed for Barnicarndy 1 (Fig. 1); a shallow reverse faulting stress regime is interpreted through to ~1 km depth, with a strike-slip faulting stress regime through the bulk of the well. There are indications that at depths > ~2.5 km, the stress regime transitions towards a normal faulting stress regime; however, the well penetrates underlying basement rocks at ~2.6 km depth where a reverse faulting stress regime is interpreted (Fig. 1).

Fig. 1.  Example new stress data. (a) Preliminary mechanical earth model for the stratigraphic well Barnicarndy 1. Green curve is gamma ray (gAPI), blue curve is minimum horizontal stress magnitude (MPa), red curve is maximum horizontal stress magnitude (MPa) and black curve is vertical stress (MPa); (b) resultant stress–strain curves from unconfined compressive strength tests on shales from (i) and (ii) Barnicarndy 1 in the Canning Basin (Jarrett et al. 2020c) and (iii) and (iv) Amoco DDH 83-1 in the South Nicholson region (Jarrett et al. 2020b); (c) mechanical earth models for the South Nicholson region wells Egilabria 2 and Egilabria 4 (Bailey et al. 2019).

Rock mechanics testing targeted potential reservoir-seal pairs, characterising mechanical and petrophysical properties through unconfined compressive stress (UCS) tests, ultrasonic testing under load (at ~50% peak strength), mercury injection capillary pressure (MICP), broad-ion-beam milling and scanning electron microscopy (BIB-SEM), and gas porosity and permeability tests (CSIRO 2020; Jarrett et al. 2020c). Six Barnicarndy 1 core samples were analysed, with sampling dependent on core integrity (Table 1). Very low Poisson’s ratios imply that these rocks are likely to be brittle, with shales in particular demonstrating brittle behaviour – stress–strain curves demonstrate a pronounced peak strength, followed by fracturing and a resultant load bearing capacity of zero (Fig. 1) (Jarrett et al. 2020c). Measured UCS values are typical for lithology and porosity, although some may be slightly overestimated due to axial splitting mode of failure (Table 1); the uppermost clean sandstone (1127.1 mRT) is porous and permeable (Table 1); hence, it has lower strength than deeper sandstones. The deeper sandstones are tighter, stronger and have higher Young’s moduli, implying that they require more stress to deform than the shallow sandstone (Table 1).

Table 1.  Summary of rock testing results from the Barnicarndy 1 stratigraphic well (Jarrett et al. 2020c)

Further testing was undertaken to understand the diamictite’s seal potential (Table 1). Analysis via MICP and BIB-SEM demonstrates a composition primarily of very poorly sorted quartz, K-feldspar and plagioclase grains that are rarely in contact and are surrounded by a fine grained, extremely tight, matrix (CSIRO 2020). Capillary pressures are estimated from 6888 to 8588 psi (45.4–59.2 MPa), providing a sealing capacity able to contain an ~800 m column of CH₄ or ~600 m of CO₂ (CSIRO 2020).

Jarrett et al. (2020a) defined the South Nicholson region as the extent of the sedimentary package composed of the Paleoproterozoic Isa Superbasin and overlying Mesoproterozoic South Nicholson Group, interpreted as south of the Murphy Inlier. Where present, younger overlying sediments and older underlying successions are included. Though informal, this study uses this terminology as it clearly defines the region of interest. The South Nicholson region is known to contain organic-rich units with the potential to host unconventional gas plays, particularly the River and Lawn supersequences of the northern Lawn Hill Platform that host the Egilabria shale gas prospect (Gorton and Troup 2018).

Interpretation of wellbore failure in image logs acquired in Egilabria 2 and Egilabria 4 reveals an approximately N-S to NNE-SSW maximum horizontal stress orientation, in agreement with predicted continent-scale stresses (Rajabi et al. 2017). Mechanical earth models for Egilabria 2 and Egilabria 4 reveal the present-day state of stress, defining a strike-slip faulting regime in the Egilabria Prospect (Fig. 1) and highlighting the relationship between lithology and stress. Sandstone and carbonate intervals exhibit significantly higher stress magnitudes than shale and siltstone intervals, resulting in localised stress variations (Bailey et al. 2019).

Shale brittleness in the South Nicholson region was analysed in two EFTF studies: Bailey et al. (2019) characterised the Egilabria prospect’s River and Lawn supersequences shales and Jarrett et al. (2019) studied controls over shale brittleness, potential methods for assessing shale brittleness, and how shale brittleness varies spatially and between Isa Superbasin supersequences. Results demonstrate that Isa Superbasin shale brittleness is controlled by increasing quartz and decreasing clay content, and that brittle shales are likely present within each supersequence. Notably, shales in the River and Lawn supersequences are interpreted as having brittle zones potentially favourable for fracture stimulation (Bailey et al. 2019; Jarrett et al. 2019).

Rock property testing was undertaken on legacy South Nicholson region samples (Jarrett et al. 2020b). Mechanical and petrophysical properties were characterised through UCS tests and ultrasonic testing at ~50% peak strength during the UCS test (Jarrett et al. 2020b). Fourteen potential unconventional or conventional reservoir samples were analysed (Table 2). Notably, samples have high Young’s moduli, high UCS and low Poisson’s ratios (Table 2). While shales are potentially brittle due to their low Poisson’s ratio and zero load bearing ability after failure (Fig. 1), all rocks tested are hard and strong (Table 2), and hence, would require significant stresses to deform.

Table 2.  Summary of rock testing results from samples analysed in the South Nicholson region (Jarrett et al. 2020b)

Geoscience Australia has released new geomechanical and petrophysical data within two prospective frontier regions, de-risking exploration through the provision of additional pre-competitive datasets. Analysis and modelling of present-day stresses, shale brittleness studies and newly acquired rock property data from the Proterozoic South Nicholson region and the Paleozoic Canning Basin are presented herein. These data provide geomechanical and petrophysical insights into intervals with identified or potential hydrocarbon prospectivity and allow for extrapolation of rock properties.

As the EFTF program expands in scope to cover more of the Australian continent, Geoscience Australia anticipates expanding these studies to encompass further frontier basins. The delivery of such data contributes to understanding large-scale variations in crustal stresses and local and regional changes in rock properties, lowering barriers to entry and encouraging greenfield exploration in remote, underexplored regions.

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.