The Cooper Basin is an upper Carboniferous-Middle Triassic intracratonic basin in northeastern South Australia and southwestern Queensland. The basin is Australia's premier onshore hydrocarbon producing province and also hosts a range of unconventional gas play types within the Permian Gidgealpa Group, including basin-centred gas and tight gas accumulations, deep dry coal gas associated with the Patchawarra and Toolachee formations, the Murteree and Roseneath shale gas plays and deep coal seam gas in the Weena Trough (e.g. Goldstein et al., 2012). The principal source rocks for these plays are the Permian coals and coaly shales of the Gidgealpa Group (Boreham & Hill, 1998; Deighton et al., 2003; Carr et al., 2016). Mapping the petroleum generation potential of these source rocks, together with describing the resulting fluid composition, is critical for understanding the hydrocarbon prospectivity of the basin.
This study applies petroleum systems analysis to investigate the maturity and generation potential of the Cooper Basin source rocks and is underpinned by a public domain, pseudo-3D petroleum systems model (Hall & Palu, 2016). Over ninety 1D thermal and burial history models were integrated with the 3D geological model and source rock property characteristics to create a regional pseudo-3D petroleum systems model for the basin, to assess generated, expelled and retained volumes of hydrocarbons. The pseudo-3D model was calibrated using present day corrected temperatures and maturity indicators (Ro, Tmax). In addition, lithologies were assigned for key wells and formations and were calibrated using measured velocity, density and thermal conductivity data. The base thermal boundary condition was set as transient heat-flow /fixed temperature at base lithosphere and the top thermal boundary condition was defined by modelled surface temperature changes through time. Crustal thickness and radiogenic heat production properties were from published studies (e.g. Beardsmore, 2004; Meixner et al., 2012; Hall et al., 2015b). Parameters for source rock distribution, amount and quality were added from analysis of log data and source rock geochemical data (Hall et al., 2016) and new Permian source rock kinetics (Mahlstedt et al., 2015).
The modelling outputs quantify both the spatial distribution and total maximum hydrocarbon yield for ten source rock intervals in the basin. A map of total hydrocarbon generation of all Gidgealpa Group source rocks demonstrates the broad extent of the Permian source kitchen across the basin, the distribution of which is consistent with the location of major conventional fields across the basin. Results highlight the variability in burial, thermal and hydrocarbon generation histories for each source rock across the basin. Although source rock maturity varies between depocentres, large areas of the Nappamerri, Patchawarra and Windorah troughs are gas mature. Key variables influencing the thermal history include: higher radiogenic heat production associated with the Big Lake Suite granodiorites, Late Cretaceous uplift and erosion of the Winton Formation and the thermal blanketing effect of the thick Permian coals.
Monte Carlo simulations were used to quantify the uncertainty associated with hydrocarbon yield and to highlight the sensitivity of results to each input parameter. The combined theoretical volume of hydrocarbons generated from all Permian source rocks is mapped, highlighting the broad extent of the source kitchen. The total modelled volume of hydrocarbons generated from the Cooper Basin Permian source rocks is estimated to be ~1,750 BBOe (P50 scenario), however the difference between the P90 (~730 BBOe) and P10 (~4,100 BBOe) scenarios quantifies the large range of uncertainties inherent in the modelling.
The large disparity between the calculated amounts of hydrocarbons expelled and the amounts so far found in reservoirs highlights (a) the large amounts of hydrocarbons which have been lost by leakage and water washing (b) the potential for large volumes to remain within and near the source rocks. The latter are a potential target for production using unconventional technologies.
