A preliminary assessment of regional CO₂ storage potential in the onshore northern Perth Basin

Introduction

A new carbon dioxide (CO₂) geological storage atlas for Western Australia being prepared by the Geological Survey of Western Australia (GSWA) expands the scope of the previous version by 3D-GEO Pty Ltd (2013) to include the State waters of the Northern Carnarvon Basin and the Officer Basin, with updated regional interpretations of the sequestration potential and a focus on providing relevant state-wide datasets. Funded by the Western Australian Ministerial Taskforce on Climate Action, this initiative is aligned with the government’s commitment to establishing a framework for CO₂ sequestration. The project commenced on 1 July 2022 and is ongoing, although initial funding finishes on 30 June 2024.

Building upon the approach set out in National Energy Technology Laboratory (2017), Halland et al. (2014), and Bradshaw et al. (2007), the atlas is focussed on critical temperature and pressure, storage capacity and containment criteria essential for assessing the potential for permanent CO₂ sequestration. To follow this approach the atlas includes state-wide datasets of borehole temperature, well log data, routine core analyses, biostratigraphy and seismic data.

Formation temperature and pressure play a crucial role in screening and defining formations suitable for CO₂ storage as a supercritical fluid. The critical temperature and pressure required for CO₂ to stabilise as a supercritical fluid are generally assumed to be 31.1°C and 10730 psi (Span and Wagner 1996). These conditions are usually met at depths around 800 m below ground level in hydrostatic conditions, but this may vary with geothermal gradient (Bachu 2000, 2002). Our multi-1D modelling of subsurface temperature in the northern Perth Basin incorporates newly collated temperature data, updated regional depth maps, and stratigraphic revisions. This modelling captures lateral temperature variations and enables the creation of temperature maps to identify reservoir units that may fall within the optimum CO₂ temperature–pressure window. Where reservoirs are modelled to fall within this window, isopach maps of associated seal units further highlight prospectivity by indicating where they are of adequate thickness (>50 m, but preferably >200 m), to provide a satisfactory means of containment.

Data availability

Borehole temperature data were collated from open-file reports for 186 wells in the northern Perth Basin. Temperature data contains contextual information required for interpretation, including time since circulation and means of acquisition. Seventy-two wells with bottom-hole temperature (BHT) data from wireline logs have been corrected for the cooling effects of circulating borehole fluids using statistical methods outlined in Beardsmore and Cull (2001), and another 18 have drill stem test (DST) temperature data. Corrected BHT data along with DST temperature data from 35 wells were utilised in the 1D temperature modelling. These temperature data types are likely to provide the best estimates of the true formation temperature (Beardsmore and Cull 2001).

Open-file LAS (Log ASCII standard) files from northern Perth Basin wells have been wrangled via Python code into a unified format with consistent depth references, mnemonics, units, x, y, z coordinates and well names suitable for AI-based analysis. Previously compiled routine core analyses have undergone quality control checks and been reformatted to enhance data accessibility. These datasets have been used to update formation tops and will contribute to reservoir quality assessments.

Depth maps used in the modelling were produced by re-interpreting seismic data and collating existing open-file maps. As part of the mapping process, well velocity survey data have been updated and formation tops revised using biostratigraphic data. As they become available, all datasets will be published via Western Australian Petroleum and Geothermal Information Management System (WAPIMS, https://wapims.dmp.wa.gov.au/wapims) on the Department’s website.

Onshore northern Perth Basin temperature modelling

One-dimensional subsurface temperature models were built for 57 key locations across the northern Perth Basin. These models incorporate well formation tops or were extracted from regional depth maps down to the basement for unpenetrated sections in areas with insufficient well control. Lithologies for each formation in each model are from cuttings or descriptions in well completion reports. The transient heat-flow model uses a fixed temperature of 1330°C at base lithosphere as the bottom boundary condition, and surface temperature as the top boundary condition. Individual 1D models were calibrated to measured borehole temperatures using variable crustal thickness and radiogenic heat production. Heat flow at base sediment section was then calculated, with predictions away from well control based on differences in crustal and sediment thickness, sedimentation rates and lithology (thermal conductivity).

A regional heat flow model for the northern Perth Basin was generated from the 57 1D well models using Zetaware software. Temperature and isothermal depth maps for seven key reservoir horizons were created from the calibrated regional temperature model and newly assembled regional depth maps.

Results

Present-day temperature maps have been created for the following stratigraphic horizons: Top Cadda, Top Cattamarra, Top Lesueur, Top Woodada, Top Dongara/Wagina/Beekeeper, Top Irwin River, and Top Kingia/High Cliff. As an example, a burial depth map of the Top Lesueur (Fig. 1) indicates where this horizon lies between 1000 and 3000 m below ground level. The red contour signifies where the top of the Lesueur Sandstone reaches 31.1°C in the model, whereas the yellow contour indicates where it reaches 1070 psi (assuming hydrostatic conditions). These contours and the temperature map (Fig. 2a) show that the Top Lesueur aligns with the critical temperature–pressure window for CO₂ storage over an extensive area within the basin. By comparison, the Lesueur Sandstone is too shallow for CO₂ storage along the northern and western edges of the basin. Conversely, in the central part of the basin, the formation is deeper than 3000 m, warranting further investigation into reservoir controls on its CO₂ storage capacity.

Fig. 1.

Top Lesueur depth map (metres below ground level), showing the optimum temperature (°C), pressure (psi, hydrostatic conditions) and burial depths (m) for CO₂ storage.

Fig. 2.

(a) Top Lesueur modelled temperature (°C) map showing optimum temperature (modelled), pressure (hydrostatic) and mapped burial depth contours for CO₂ storage; (b) isothermal depth map showing that the critical temperature limit of 31.1°C is modelled at shallower depths in the north and north-west of the basin, where the geothermal gradient is estimated to be higher.

All models for potential reservoir units reveal significant areas within the optimal CO₂ storage window, with the deepest reservoirs reaching the optimum temperature and pressure close to the northern edge of the basin and its north-western onshore extent. Conversely, shallower reservoirs lie within the optimum temperature–pressure window in more central positions towards the Dandaragan Trough. An isothermal depth map shows that the critical temperature limit of 31.1°C is shallower in the north and west of the basin where the modelled geothermal gradient is highest (Fig. 2b).

An isopach map of the overlying Eneabba Formation shows that where the Top Lesueur lies within the optimum CO₂ storage window (Fig. 3), the Eneabba regional seal is >200 m thick over much of the area. The only exception is a small northern area where the formation is 50–200 m thick or less than 50 m thick (Fig. 3, yellow shaded and red shaded polygons, respectively), which may be riskier in terms of ability to provide adequate seal.

Fig. 3.

Eneabba Formation isopach (thickness) map showing areas in green and yellow where the formation is likely to be thick enough to be an adequate seal to the underlying Lesueur Sandstone. Areas shaded red indicate thicknesses <50 m and thus higher sealing risk. Areas where the Lesueur Sandstone lies within the optimum burial depth for CO₂ are shown by cross hatching.

Summary and conclusion

The latest regional temperature model for the northern Perth Basin incorporates stratigraphic revisions, lithology data, borehole temperature data, and new regional depth maps. Initial assessment of the northern Perth Basin through multi-1D temperature modelling, reveals promising CO₂ storage prospectivity, with significant areas identified across various reservoir units falling within the optimum CO₂ storage temperature–pressure window. The lateral variation in temperature associated with lithology changes (thermal conductivity), crustal and sediment thickness, and sedimentation rates across the basin, and in conjunction with seal isopach maps, provide a useful preliminary screening tool for identifying where prospective reservoirs exist.

Future work

Future work will extend this assessment to also include reservoir quality parameters and other basins within state jurisdiction. Prospectivity assessments will be significantly enhanced through the inclusion and integration of open-file petrophysical interpretations, well log interpretation and routine core analyses to define variation of formation reservoir quality within the optimal CO₂ temperature–pressure window. This workflow and sequence of interpretative work continues, with the final CO₂ storage atlas covering the Perth, Carnarvon (including State waters), Canning and Officer basins within state jurisdiction.