Thermal Map from Assessed Proxies (TherMAP): a pilot study to estimate subsurface temperatures for the Australian continent

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1 Proceedings Australian Geothermal Energy Conferences 2013 Brisbane, Australia, November 2013 Thermal Map from Assessed Proxies (TherMAP): a pilot study to estimate subsurface temperatures for the Australian continent M. W. Haynes, E. J. Gerner A.L. Kirkby, P. Petkovic, A. R. Budd and C. Harris-Pascal GPO Box 378, Canberra, ACT, 2601, Australia Marcus.Haynes@ga.gov.au Keywords: Geothermal energy, Australian continent, 3D thermal modelling, Underworld, National Computational Infrastructure ABSTRACT A new approach for developing a 3D temperature map of the Australian continent is currently being developed that relies on combining available proxy data using high-performance computing and large continental-scale datasets. The new modelling approach brings together the current national-scale knowledge contained in datasets collected by Geoscience Australia and others, including AusMoho, OZTemp, OzSeebase, OZCHEM, surface temperature, the Surface Geology of Australia, sedimentary basins thermal conductivity and the National Gravity Map of Australia. Bringing together such a range of datasets provides a geoscientific basis by which to estimate temperature in regions where direct observations are not available. Furthermore, the performance of computing facilities, such as the National Computational Infrastructure, is enabling insights into the nature of Australia s geothermal resources which had not been previously available. This should include developing an understanding of the errors involved in such a study through the quantification of uncertainties. Currently the new approach is being run as a pilot study however, initial results are encouraging. The pilot study has been able to reproduce many observed temperature trends without using direct bore-hole temperature observations as an input into the modelling process. Furthermore, a number of regional areas have now been identified which may warrant further study. 1. INTRODUCTION Previous approaches to estimating subsurface temperatures for the whole of the Australian continent have relied on the extrapolation of direct temperature observations from mineral and petroleum boreholes, well completion reports and/or measured and synthetic heat flow determinations, and interpolation between these points. However, data points are scarce and unevenly distributed across the continent, which leads to the interpolation of observed temperatures across vast distances. The limited depth range of data points also means that shallow observations are often extrapolated to a depth of interest, using limited assumptions about geology at depth. Furthermore, the information available from some well completion reports can often lead to questions of data quality, particularly from older reports where data collection methods can be poorly documented. A robust method for estimating temperatures in areas without direct observations is important because understanding the conditions required to generate anomalous temperatures will help to reduce geothermal exploration risk. There are many geoscientific datasets, with substantial coverage across the Australian continent, which describe the elements that can create a thermal anomaly without providing direct information on subsurface temperatures. However, by leveraging a geothermal systems approach, such data can be combined and used as proxies to estimate temperature at depth. Furthermore, by incorporating all of these sources of information, many layers of data are added to any individual temperature estimate. The development of this new method is enabled due to the large volume of data that is available, and access to high-performance computing on the National Computational Infrastructure (NCI) supercomputer at the Australian National University (ANU). It is expected that such an approach will allow the uncertainty of temperature estimates to be quantitatively described through analysis of the specific data underlying each point. Such a method may also lead to the identification of areas of potential that are currently unidentified due to the lack of availability of temperature data, or due to erroneous observations. 2. METHOD Fourier s law (Eq. 1), at a general level, describes how the temperature gradient of a geothermal system ( T ) can be constrained by two basic parameters; heat flow (q), and thermal conductivity (λ). By combining all of the available data from the various aspects that contribute to these two parameters it is possible to begin to examine the subsurface temperature of an area from a systems perspective. This creates a basis from which to estimate temperature in areas where direct observations are not available. q T (1) From this perspective, six fundamental datasets have been identified as contributing to the heat flow and thermal conductivity of the Australian continent. Thermal conductivity is being constrained from sedimentary thickness, and the thermal conductivity of specific sedimentary basins. Heat flow is being constrained from possible granite shapes and locations, granite heat-production rates, surface temperature, and the depth to Moho. As such these six datasets are currently being prepared for incorporation into a 3D model of the Australian continent, known as the Thermal Map from Assessed Proxies (TherMAP). Details of their inclusion in the current pilot study are discussed below. 1

2 2.1 Datasets Sedimentary thickness The SEEBASE TM reports (OZ SEEBASE TM Study, 2005; De Vries et al., 2006), prepared by FrOGTech, estimate the depth of cover for Proterozoic and Phanerozoic sediments across the Australian continent. For the purposes of this study, the data available from these reports were considered to be the best estimate of depth to basement Sediment thermal conductivity Average thermal conductivity values are currently being determined for individual sedimentary basins throughout Australia (Harris- Pascal, 2013). These values are compiled from the lithological data available in well completion reports and stratigraphic columns, with the specific thermal conductivity of specific rock types taken from published literature. At this stage of the study, thermal conductivity values have not been determined for all the sedimentary basins in Australia. Where an average thermal conductivity has not been determined for a particular basin, a default value of 3 Wm -1 K -1 has been used. It is expected that for future regional studies, a similar method will be used to estimate the average thermal conductivity of individual basin layers Interpreted locations of granites The National Gravity Map of Australia (Bacchin, 2010) has been used to identify the possible locations of buried granites. Rounded, low-density features were extracted from the map, and their proximities to known granite outcrops noted (Petkovic, 2013). For the purposes of the pilot study, all rounded low-gravity anomalies have been modelled as granite. However, for future work it is expected that probabilities will be applied to each of the bodies based on the perceived chance that they correspond to a buried granitic pluton. For bodies that intersect granite outcrop, or are located where wells have been reported to intersect granite, this probability may be fixed to 100%. For bodies that are not located near known granites the probabilities will likely be substantially reduced. Applying such an approach under a Monte Carlo method would help to resolve uncertainties in areas where the location and shape of granites is highly speculative. Furthermore, for detailed regional studies, the inclusion of seismic transects is expected to be of additional benefit in constraining the locations of buried granites Granite heat production rate Variable granite heat production is yet to be incorporated in the TherMAP pilot study. However, the methods by which to do this are currently being developed. For the pilot study, a default granite heat production rate of 5 μwm -3 was used. A default basement heat production rate, of 1.7 μwm -3, was also used. The OZChem dataset (Champion et al., 2007) provides geochemical analyses for specific rock samples across the Australian Continent. This analysis can be used to calculate heat production rates, which are currently being examined at geological-province and crustal-element scales to establish the population statistics of specific granite groups. This is expected to provide a basis by which to estimate the heat production of inferred, buried granite plutons Surface temperature The Bureau of Meteorology provides a map of average daily mean temperatures for the 30-year period This has been combined with estimates of average seafloor temperatures (to a minimum value of 0.5 C), around the Australian coast, to create the fixed surface temperature boundary condition. This boundary condition is spatially variable, both in the sense that it is laterally variable and in that it conforms to the topographic surface across the continent. Software constraints however, only allowed a single constant boundary temperature to be applied to the top of the model voxet. The application of surface temperature was therefore achieved through the use of an artificially-low constant temperature of 0 C being applied to the top of the model. This then allowed the thermal conductivity of individual columns of air and water to be manipulated to create the desired surface temperature Depth of Moho The AusMoho studies (Kennett et al., 2011; Salmon et al., 2012) have been used as an estimate of the depth of the Moho across the Australian continent. These studies used seismic inversion on a variety of different data sources to produce an estimate with broad data coverage across the continent. For the purposes of the pilot study, it was assumed that the Moho corresponds to the 550 C isotherm. Future work will examine this assumption with specific focus to capturing the uncertainties, which are expected to be on the order of several tens-of-degrees. 2.2 Modelling A 3D model of the Australian continent is being prepared using Gocad TM. Data has been projected using the Geoscience Australia Lambert projection (EPSG 3112) for the area bounded by the coordinates , and , The model also extends from 4000 msl vertical to msl depth. At approximately 20 by 20 by 0.4 km resolution, this represents some 6 million voxet cells. Thermal modelling was undertaken using the Underworld software package (Moresi et al., 2007) on the National Computational Infrastructure (NCI) supercomputer at the Australian National University (ANU). Underworld employs a finite-element scheme to resolve the steady-state solution to a geothermal system. Zero-flux, Neumann-type boundaries were imposed on the model for the side boundary conditions. For the top and bottom boundary conditions, it was desirable to set constant temperature, Dirichlet-type boundary conditions to the topographic surface and to the depth of Moho, respectively. However, software constraints prevented the use of spatially variable boundary conditions. To circumvent this, artificially-high thermal conductivity values (~700 Wm -1 K -1 ) were applied to the voxels located above the topographic surface and below the depth to Moho. This allowed the fixed temperatures, at the top and bottom of the model, to be transferred through to these two surfaces, creating pseudo-isotherms. 2

3 3. RESULTS The results of the initial pilot TherMAP study, at 5 km depth, are presented in Figure 1. The results show a patchy distribution of thermal anomalies, with a broad band of low-temperatures stretching from the Northern Territory, through to the southern coastline. The results also seem to indicate the importance of high-heat producing granites in controlling localised temperatures. Notable regional high-temperature anomalies occur beneath the Bowen, Canning, Carnarvon, Clarence-Moreton, Cooper, Hamersley, Perth, and Sydney Basins. For comparison, the interpreted temperatures, at 5 km depth, from the OZTemp study are shown in Figure 2. The temperatures modelled in the TherMAP pilot study are, in general, in the order of one hundred degrees cooler than those modelled using OZTemp (Gerner and Holgate, 2010). However, juxtaposing the distribution of temperature anomalies reveals that many of the broad trends have been captured. Of specific interest though are some of the areas where there is disagreement between the two models. The Yilgarn Craton, and the Perth, Bowen, Hamersley and Clarence-Moreton Basins are all modelled to be relatively warmer in the TherMAP pilot study. The Kimberley, Northern Territory, central Queensland, and Australian Capital Territory regions are all modelled to be relatively cooler. 4. CONCLUSIONS The THERMAP thermal modelling approach is being run as a pilot study; however, initial results are encouraging. The pilot study has been able to reproduce the temperature trends observed in areas that have been heavily constrained by bore-hole observations. This has been achieved without using direct bore-hole temperature observations as an input into the modelling process. Furthermore, a number of areas have now been identified, due to the difference in estimated temperature distributions from previous methods, which may warrant further study. It is too early at this stage to over analyse the differences between the TherMAP and OZTemp studies; the temperature estimates produced from the TherMAP pilot study are still on the order of one hundred degrees cooler than those observed in the OZTemp dataset (Holgate and Gerner, 2010). However, it is expected that with continued refinement of the methods and input datasets that the accuracy of the TherMAP study will improve. Therefore, immediate future work will look to continue to improve the TherMAP methods and datasets, with a focus also on capturing the uncertainties involved in the modelling process. The methods by which to include potential granite bodies in the modelling process are still being tested. However, at this stage the contribution of these bodies to the steady-state thermal solution is limited as only a default heat production rate can be assumed. Further work is required to capture the population statistics of granite heat production rates for specific geological-province and crustal-element zones across the continent. Once these have been appropriately described, iterative modelling, following a Monte Carlo scheme, will better allow the true contributions to be estimated. Improving the basal boundary condition could be achieved through incorporating the probabilistic work undertaken on the Moho surface by Bodin et al. (2012). Alternative methods for constraining the basal temperature, such as the current research into the Curie depth across the continent (Chopping and Kennett, 2013), will also be considered. With the TherMAP model results approximately one hundred degrees cooler than many corresponding OZTemp temperature observations, it is expected that the assumed temperature of the Moho (550 C) may need to be increased in order to systematically increase the heat flow through the model. With such an increase likely to be required, it is therefore expected that the relative importance of granite bodies to localised temperature anomalies will decrease, while the low thermal-conductivity sediments will have a relatively greater impact in controlling subsurface temperatures. An advantage of the TherMAP approach is that it can also be independently validated against existing datasets not currently incorporated into the modelling process. The OZTemp well temperature dataset (Holgate and Gerner, 2010) and the existing heat flow datasets (Kirkby and Gerner, 2010; Jones et al., 2011; Weber et al., 2011; Gerner et al., 2012) provide direct geophysical observations by which to test the modelled solutions. If a reasonable match can be achieved to these independent validation datasets, then this will add some confidence to the predictions of subsurface temperature in areas far from direct observations. The high volumes of data that are currently available, coupled with the performance of computing facilities such as the NCI, are enabling insights into the nature of Australia s geothermal resources which had not been previously available. The synthesis of these broad datasets has allowed the application of a geothermal-systems modelling approach, for the first time, to a continental scale. As the TherMAP approach progresses from the pilot phase, this will provide a geoscientific basis by which to estimate subsurface temperature in regions where direct observations are not available. 5. ACKNOWLEDGEMENTS This research was undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government. The authors would also like to thank David Champion and Rowan Romeyn for their critical review of this paper. This paper is published with permission of the CEO, Geoscience Australia. Commonwealth of Australia (Geoscience Australia)

4 Figure 1: Proof-of-method results for the TherMAP pilot study showing the relative temperature anomalies across the Australian continent at 5 km depth. Warmer colours indicate hotter temperatures, while cooler colours indicate colder temperatures. Sedimentary basin outlines, shown in white are (from west to east): Carnarvon, Perth, Hamersley, Canning, Cooper, Bowen, Sydney and Clarence-Moreton Basins. Figure 2: Interpreted temperatures at 5 km depth from the OZTemp data set (Gerner and Holgate, 2010) showing the relative temperature anomalies across the Australian continent. Warmer colours indicate hotter temperatures, while cooler colours indicate colder temperatures. The colour-ramp is not consistent with that of Figure 1; this figure has been included to show only general trends. OZTemp data points (Holgate and Gerner, 2010) are shown as black points. 4

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